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CIHM/ICMH 

Microfiche 

Series. 


CIHM/ICMH 
Collection  de 
microfiches. 


Canadian  Institute  for  Historical  Microreproductions  /  Institut  Canadian  de  microreproductions  historlques 


Technical  and  Bibliographic  Notes/Notes  techniques  et  bibliographlques 


The  institute  has  attempted  to  obtain  the  best 
original  copy  available  for  filming.  Features  of  this 
copy  which  may  be  bibliographically  unique, 
which  may  alter  any  of  the  images  in  the 
reproduction,  or  which  may  significantly  change 
the  usual  method  of  filming,  are  checked  below. 


D 


n 


n 

D 
D 

D 


D 


Coloured  covers/ 
Couverture  de  couleur 


I     I    Covers  damaged/ 


Couverture  endommagde 


Covers  restored  and/or  laminated/ 
Couverture  restaurte  et/ou  peilicui^e 


I      I    Cover  title  missing/ 


Le  titre  de  couverture  manque 


I     I    Coloured  maps/ 


Cartes  g^ographiques  en  couleur 

Coloured  inic  (i.e.  other  than  blue  or  blacic)/ 
Encre  de  couleur  (i.e.  autre  que  bieue  ou  noire) 


Coloured  plates  and/or  illustrations/ 
Planches  et/ou  illustrations  en  couleur 


Bound  with  other  material/ 
Relii  avec  d'autres  documents 

Tight  binding  may  cause  shadows  or  distortion 
along  interior  margin/ 

La  re  liure  serrie  peut  causer  de  I'ombre  ou  de  la 
distortion  le  long  de  la  marge  int^rieure 

Blank  leaves  added  during  restoration  may 
appear  within  thcr  text.  Whenever  possible,  these 
have  been  omitted  from  filming/ 
11  se  peut  que  certaines  pages  blanches  ajouttes 
lors  d'une  restauration  apparaissent  dans  le  texte, 
mais,  lorsque  cela  Atait  possible,  ces  pages  n'ont 
pas  At6  filmAes. 

Additional  comments:/ 
Commentaires  supplAmentaires: 


L'Institut  a  microfilm^  le  meilleur  exempiaire 
qu'il  lui  a  6t6  possible  de  se  procurer.  Les  details 
de  cet  exempiaire  qui  sont  peut-dtre  uniques  du 
point  de  vue  bibliographique,  qui  peuvent  modifier 
une  image  reproduite,  ou  qui  peuvent  exiger  une 
modification  dans  la  methods  normale  de  filmage 
sont  indiquis  ci-dessous. 


I — I  Coloured  pages/ 


D 


Pages  de  couleur 

Pages  damaged/ 
Pages  endommag^es 


□   Pages  restored  and/or  laminated/ 
Pages  restaurtes  et/ou  pellicultes 

0   Pages  discoloured,  stained  or  foxed/ 
Pages  d6color6es.  tachettes  ou  piqui 


Pages  d6color6es.  tachettes  ou  piqutes 

Pages  detached/ 
Pages  d^tachtes 

Showthrough/ 
Transparence 

Quality  of  print  varies/ 
Quality  in^gaie  de  I'impression 

Includes  supplementary  material/ 
Comprend  du  materiel  suppl^mentaire 

Only  edition  available/ 
Seule  Mition  disponibie 


Pages  wholly  or  partially  obscured  by  errata 
slips,  tissues,  etc.,  have  been  refiimed  to 
ensure  the  best  possible  image/ 
Les  pages  totaiement  ou  partiellement 
obscurcies  par  un  feuiilet  d'errata,  une  pelure, 
etc.,  ont  At*  filmtos  A  nouveau  de  fapon  A 
obtenir  ia  meilleure  image  possible. 


Th 
to 


Th 
po 
of 
fill 


Or 
be 

th( 
sic 
oti 
fin 
sic 
or 


Th 
sh 
Til 
wl 

Ml 
dil 
en 
be 

rifl 
re< 
m( 


This  item  is  filmed  at  the  reduction  ratio  checked  below/ 

Ce  document  est  film*  au  taux  de  riduction  indiquA  ci-dessous. 


10X 

14X 

18X 

22X 

26X 

30X 

J 

12X 

16X 

• 

20X 

24X 

28X 

32X 

Th«  copy  filmed  h«r«  has  b««n  reproduced  thanks 
to  the  generosity  of: 

Douglas  Library 
Queen's  University 


L'exemplaire  filmA  fut  reproduit  grAce  A  la 
gAnirosIt*  i^e: 

Douglas  Library 
Queen's  University 


The  images  sppearing  here  are  the  best  quality 
possible  considering  the  condition  and  legibility 
of  the  original  copy  and  In  keeping  with  the 
filming  contract  specifications. 


Les  Images  sulvantes  ont  4H6  reproduites  avec  le 
plus  grand  soin,  compte  tenu  de  la  condition  et 
de  la  nettet*  de  rexemplaire  film6,  et  en 
conformity  evec  les  conditions  du  contrat  de 
filmage. 


Originel  copies  In  printed  paper  covers  ore  filmed 
beginning  with  the  front  cover  and  ending  on 
the  last  page  with  a  printed  or  Illustrated  Impres- 
sion, or  the  back  cover  when  appropriate.  All 
other  original  copies  are  filmed  beginning  on  the 
first  page  wKh  a  printed  or  illustrated  impres- 
sion, end  ending  on  the  last  page  with  a  printed 
or  illustrated  impression. 


Les  exemplalres  origlnaux  dont  la  couverture  en 
papier  est  imprimte  sent  filmAs  en  commenpant 
par  le  premier  plat  et  en  terminant  soit  par  la 
dernlAre  page  qui  comporte  une  empreinte 
d'impression  ou  d'illustrstion,  soit  par  le  second 
plat,  selon  le  cas.  Tous  les  autres  exemplalres 
origlnaux  sent  filmis  en  commenpant  par  la 
premlAre  page  qui  comporte  une  empreinte 
d'impression  ou  d'illustration  et  en  terminant  par 
la  dernlAre  page  qui  comporte  une  telle 
empreinte. 


The  last  recorded  frame  on  each  microfiche 
shall  contain  the  symbol  -^  (meaning  "CON- 
TINUED"), or  the  symbol  V  (meaning  "END"), 
whichever  applies. 


Un  des  symboles  sulvants  apperaftra  sur  la 
dernlAre  image  de  cheque  microfiche,  selon  le 
cas:  le  symbols  — ►  signifle  "A  SUIVRE",  le 
symbols  V  signifle  "FIN  ". 


Meps,  plates,  charts,  etc.,  mey  be  filmed  at 
different  reduction  retios.  Those  too  large  to  be 
entirely  included  in  one  exposure  are  filmed 
beginning  in  the  upper  left  hand  corner,  left  to 
right  and  top  to  bottom,  as  many  frames  as 
required.  The  following  diagrams  illustrate  the 
method: 


Les  cartes,  planches,  tableaux,  etc.,  peuvent  dtre 
filmAs  it  des  taux  de  reduction  diff6rents. 
Lorsque  le  document  est  trop  grand  pour  Atre 
reproduit  en  un  seul  clichA,  11  est  film6  A  partir 
de  Tangle  supArleur  gauche,  de  gauche  i  droite, 
et  de  haut  en  bas,  en  prenent  le  nombre 
d'images  nAcessaire.  Les  diagrammes  sulvants 
illustrent  la  mAthode. 


1 

2 

3 

1 

2 

3 

4 

5 

6 

ZOOLOGY    AND    PHYSIOLOGY. 

BUMPUS'  INVERTEBBATE  ZOOLOGY.    \  Lab- 

oratory  Course.    Revised  Edition.     157  pp.    lamo. 
HOWELL'S  DISSECTION  OP  THE  DOG.    100  pp. 

Advanced  Course,    xvi  +  621 4- 34  pp.    8vo. 
Briefer  Course,    xii  +  877  pp.    l8mo. 
Elementary  Course.    vi  +  261pp.    l2mo. 

"^vifi+YgTp?.  ^°ml  """^  ^^^  ^^^"^^  °*"  Narcotics. 

McMtTBBICH'S    INVERTEBBATE   MOBPHOL- 
OGY.    vii  +  661  pp.    8vo.  -^^.^^^yj^j 

PACKABD'S  ZOOLOGIES.  (American  Science  Series.) 

Advanced  Course.    vii  +  782pp.    8vo. 

Briefer  Course,    vlii  -f  338  pp.    l2mo. 

Elementary  Course.    viii  +  890pp.    12mo. 

PACKARD'S      ENTOMOLOGY     FOB     BEGIN- 
NERS,   xvi  4- 367  pp.    lamo.  ■b'vtjj.^ 

^^C^^D'S     GUIDE      TO     THE    STUDY   OP 
INSECTS,    xii  +  715  pp.    8vo. 

SCUDDEB'S  BUTTEBPLIES.    x+322pp.    lamo. 

SCUDDEB'S  BBIEP  GUIDE  TO  BUTTEBPLIES. 

xi  +  a06  pp.    Vimc. 

SCUDDEB'S    THE   LIFE  OP    A  BUTTEBPLY. 

(For  General  Readers.)    186  pp.    16mo. 

SEDGEWICK  AND  WILSON'S  GENEBAL  BI- 
OJjOO  X .  (American  Science  Series.)  viii  +  193  pp.  8vo. 


a  '^*'® J*"''"^'*®'^'  Educational  Catalogue  with  descriptions 

HENRY  HOLT  &  CO., 
29  West  23d  Street,  New  York. 


A  TEXT-BOOK 


OF 


INVERTEBRATE    MORPHOLOGY 


BY 


J.  PLAYFAIR  McMURRICH,  M.A.,  Ph.D. 

Professor  in  the  University  of  Michigan 


NEW    YORK 

HENRY  HOLT  AND  COMPANY 
1894 


Q.L-:JGZiM\^   . 


Copyright,  1894, 

BY 

Henrv  Holt  &  Co. 


ROBERT  DRUMMOND,    HLECTROTYPKR  AND  PRINTER,   NEW  YORK. 


PREFACE. 

The  Morphology  of  Invertebrate  Animals  may  be  treated 
either  from  the  standpoint  of  Comparative  Anatomy  or  from 
the  zoological  side,  and  either  method  of  treatment  has  much 
to  recommend  it.  In  my  experience,  however,  the  zoological 
method  has  proved  most  satisfactory  for  the  presentation  of 
the  subject  to  students,  inasmuch  as  it  is  necessarily  the 
method  employed  in  the  laboratory,  and  accordingly  in  the 
present  work  that  plan  of  presenting  the  facts  of  morphology 
has  been  followed.  A  bare  statement  of  the  structural 
peculiarities  of  the  various  groups,  however,  is  simply  collect- 
ing the  bricks  and  stones  without  the  mortar  necessary  to 
unite  them  together  into  a  substantial  edifice,  and  where  the 
opportunity  has  presented  itself  attention  has  been  directed 
to  the  comparative  significance  of  various  organs  and  to  the 
affinities  of  the  various  groups. 

A  w^ord  is  perhaps  necessary  in  regard  to  the  classification 
adopted, which  presents  many  radical  changes  from  the  schemes 
usually  employed.  For  the  larger  groups,  following  the  ex- 
ample of  Claus,  the  term  type  has  been  employed,  and  no  less 
than  twelve  of  these  types  are  adopted.  This  increased  num- 
ber has  resulted  from  a  division  of  two  groups  usually  recog- 
nized, namely,  the  Vermes  and  the  Arthropoda.  As  regards  the 
former  it  has  long  been  acknowledged  to  be  a  heterogeneous 
collection,  and  its  retention  is  to  be  regarded  as  a  survival. 
It  is  true  that  the  forms  assigned  to  it  do  present  certain 
phylogenetic  affinities ;  but  if  this  is  to  be  the  reason  for  its 
retention,  then  the  MoUusca  and  Prosopygia  (Molluscoidea) 
should  also  be  assigned  to  it.  It  has  seemed  more  satisfac- 
tory to  retain  the  Mollusca  and  Prosopygia  as  distinct  groups, 
and  to  divide  the  Vermes  into  several  types,  such  as  the 
Platyhelminthes,  Nemathelminthes,  and  Annelida,  each  of  the 
same  rank  as  the  Mollusca,  and  presenting  approximately 
similar  degrees  of  affinity  among  themselves. 

ill 


16799 


IV 


PREFACE. 


As  to  the  Arthropoda,  its  right  to  exist  as  a  group  coordi- 
nate with,  for  instance,  the  Mollusca  has  been  questioned  by 
several  authors.  Undoubtedly  in  this  case  also  many  similar 
structural  features  obtain  among  the  various  members  of  the 
group,  but  embryology  has  indicated  a  probability  of  a  more 
or  less  independent  origin  of  two  Arthropoaan  groups  usually 
regarded  as  closely  related,  namely,  the  Araohnida  and  the 
Tracheata  proper.  Apparently  the  former  have  originated 
from  Crustacean  ancestors,  while,  if  the  supposed  significance 
of  Peripatiis  be  accepted,  the  Tracheates  are  to  be  traced  back 
to  Annelidan  forebears,  and  for  the  purpose  of  calling  the 
attention  of  the  student  to  this  probable  phylogeny  the 
Crustacea,  Arachnida  and  Tracheata  have  been  regarded  as 
distinct  types  coordinate  with  the  Annelida  and  Mollusca. 

A  book  of  this  kind  must  necessarily  be  highly  tinged  with 
the  individual  opinions  of  the  writer,  and  for  these  indulgence 
must  be  craved.  So  far  as  the  facts  are  concerned  every 
care  has  been  taken  that  they  should  be  accurate  and  as 
far  as  possible  up  to  date  with  the  most  recent  investigations. 
Errors  have  no  doubt  crept  in,  a  misfortune  almost  inevitable 
for  the  mass  of  material  which  must  pass  under  consideration 
during  the  progress  of  the  work,  and  for  these  again  indul- 
gence must  be  asked. 

Refraining  from  further  apologies,  the  more  pleasant  duty 
remains  of  thanking  the  many  friends  who  have  so  kindly 
aided  the  work  by  suggestion  or  otherwise,  and  especially 
those  who  have  permitted  the  use  of  figures  taken  from 
special  papers.  A  largo  number  of  the  figures  employed 
are  original  and  the  great  majority  have  been  especially 
drawn  for  this  work,  the  attempt  being  made  to  diagramma- 
tize them  to  a  greater  or  less  extent  for  the  sake  of  clearness. 
In  all  cases  where  figures  have  been  borrowed  the  original 
authorship  has  been  duly  acknowledged. 

Finally,  I  desire  to  make  public  recognition  of  my  indebt- 
edness to  my  wife  for  the  in  valuable  assistance  she  has  ren- 
dered in  many  ways  during  the  progress  of  the  work. 

J.   PlAYFAIB  McMURRICH. 

University  of  Michioan, 
September,  1894. 


TABLE  OF  CONTENTS. 


PAOBS 

Chapter  I.    Protoplasm  and  the  Cell I-12 

Composition  of  Protoplasm,  pp.  1-3.    Structure  of  the  Cell, 
pp.  4-8.    Cell-division,  pp.  9-13.    Literature,  p.  12. 

Chapter  II.    The  Subkingdom  Protozoa 13-40 

The  Class  Rhizopoda,  pp.  14-34.  The  Class  Sporozoa,  pp.  24- 
38.  The  Class  Flagellata,  pp.  88-33.  The  Class  Infusoria, 
pp.  33-38.  Synoptical  Classification,  pp.  38, 39.  Literature, 
pp.  39,  40. 

Chapter  III.    The  Subktngdom  Metazoa 41-63 

Individuality  of  the  Metazoa,  pp.  41-42.  Sexual  Reproduction, 
pp.  43-51.  The  Segmentation  and  Early  Development  of 
the  Ovum,  pp.  51-58.  Non-sexual  Reproduction  of  the 
Metazoa.  pp.  58-60.  Alternation  of  Generations,  pp  (?'»  68. 
Literature,  p.  62. 


Chapter  IV.    Trichoplax,  the  Dicyemid^e  and  the 
Orthonectida 

Trichoplax,  pp.  63,  64.    The  DicyemidsB,  pp.  64,  65.    The  Or- 
thonectida,  pp.  65-67.    Literature,  p.  67. 


63-67 


Chapter  V.    The  Type  Ccelentera 68-119 

The  Subtype  Porifera,  pp.  69  76.  The  Subtype  Cnidaria,  pp. 
76-115.  The  Class  Hydromedusse,  pp.  78-97.  The  Class 
Scyphomedusae,  pp.  97-104.  The  Class  Authozoa.  pp.  104- 
115.  Synoptical  Classification,  pp.  115-117.  Literature, 
pp.  118,  119. 

Chapter  VI.    The  Ctenophora 120-126 

Description  of  the  Group,  pp.  180-126.  Synoptical  Classifica- 
tion, p.  126.    Literature,  p.  126. 


vi 


TABLE  OF  CONTENTS. 


PAGES 

Chapter  VII.    The  Type  Platyhelminthes 127-171 

The  Class  Turbelhiiiii,  pp.  130-143.  The  Class  TreniJitoda,  pp. 
143-152.  The  Class  Cesloda.  pp.  152-161.  The  Class 
Nemertina,  pp.  162-169.  Syuoptical  Classiticatiou,  pp. 
169.  170.     Literature,  pp.  170,  171. 

Chapter  VIII.    The  Type  Nemathelminthes 173-183 

The  Class  Neniatoda,  pp.  173-179.  The  Class  Acauthocephala, 
pp.  179-182.  Synoptical  Clussiflcatiou,  pp.  182,  183.  Lit- 
erature, p.  183. 

Chapter  IX.    The   Order  Echinodera;  the   Class 

CHiETOGNATHA ;      THE      ClASS      RoTIFERA  ;     THE 

Order  Gastrotricha  ;    and  Dinophilus 184-201 

The  Order  Echiuodera,  pp.  184-186.  The  Class  Chojtoguatha, 
pp.  180-189.  The  Class  liotifera,  pp.  189-195.  The  Order 
Gastrotricha,  pp.  195-198.  The  Geuus  Diuophilus,  pp. 
198-200.  Syuoptical  ClassiUcation,  p.  200.  Literature,  p. 
201. 

Chapter  X.    The  Type  Annelida 202-253 

The  Class  Chaelopoda,  pp.  204-227.  The  Class  Hirudinea,  pp. 
228-237.  The  Class  Gephyrea,  pp.  237-243.  The  Class 
Myzostomeae,  pp.  244-246.  The  Class  Phoronidoe,  pp.  247- 
251.  Syuoptical  Classiflcatiou,  pp.  251,  252.  Literature, 
pp.  252,  253. 

Chapter  XI.    The  Type  Prosopygia 254-275 

The  Class  Polyzoa,  pp.  255-268.  The  Class  Brachiopodn,  pp. 
268-274.  Syuoptical  Classification,  p.  274.  Literature,  p. 
275. 

Chapter  XII.     The  Type  Mollusca 276-367 

The  General  Characteristics  of  the  Type,  pp.  276-284.  The 
Class  Ainphineura,  pp.  284-293.  The  Class  Gasteropoda, 
pp.  293-322.  The  Class  Scaphopoda,  pp.  322-326.  The 
Class  Pelecypoda,  pp.  326-340.  The  Class  Cephalopoda, 
pp.  340-362.  The  Affinities  of  the  Mollusca,  pp.  362,  863. 
Syuoptical  Classification,  pp.  363-365.  Literature,  pp. 
865-367. 

Chapter  XIII.    The  Type  Crustacea 368-434 

The  General  Characteristics  of  the  Type,  pp.  868-385.  The 
Class  Entoinostraca,  pp.  385-403.  The  Class  Malacostraca, 
pp.  403-417.  The  Development  and  Affinities  of  the  Crus- 
tacea, pp.  417-423.  Synoptical  Classification,  pp.  423, 424. 
Literature,  pp.  424-427.  The  Order  Xiphosura,  pp.  437- 
484.    Literature,  p.  434. 


TABLE  OF  CONTENTS. 


VU 


Chapteh  XIV.    The  Type  Auachnida 435-4G8 

The  Geueral  Charactuiistios  of  tho  Type.  pp.  435-441.  Descrip- 
tiou  of  the  Various  Orders,  pp.  441-456.  The  Phylogeuy 
of  tlie  Arachiiida,  pp.  456-458.  Synoptical  Clussiticatiou, 
pp.  458,  459.  Literature,  pp.  459,  460.  The  Order  Peuia- 
stomidflB,  pp.  461-463.  The  Order  Pycnogonida,  pp.  483- 
466.  The  Order  Tardigradu,  pp.  466-468.  Literature,  p. 
468. 

Chapter  XV.    The  Type  Tracheata 4G9-530 

The  Geueral  Characteristics  of  the  Type,  pp.  469-474.  The 
Class  Protracheata,  pp.  474-480.  The  Class  Myriiipoda. 
pp.  480-487.  The  Class  Insecta.  pp.  487-533.  The  Phy- 
logeuy  of  the  Tracheata,  pp.  523-525.  Synoptical  Classifl- 
cation,  pp.  5:a5-528.     Literature,  pp.  528-530. 

Chapter  XVI.    The  Type  Echinoderma 531-595 

The  General  Chpracteristics  of  the  Type,  pp.  531-540.  The 
Class  Crinoidea,  pp.  541-551.  The  Class  Asteroidea,  pp. 
552-560.  The  Class  Ophiuroidea,  pp.  561-570.  The  Class 
Echinoidea,  pp.  570-584.  The  Class  Holothuroidea,  pp. 
584-590.  The  Phylogeny  of  the  Echinodernia,  pp.  590- 
592.  Synoptical  Classification,  pp.  592,  593.  Literature 
pp.  593-595. 

Chapter  XVII.    The  Type  Protochordata 59G-G41 

The  Class  Hemichorda,  pp.  596-608.  The  Class  Cephalochorda, 
pp.  608-618.  The  Class  Urochorda,  pp.  618-639.  Synop- 
tical Classification,  pp.  639,  640.    Literature,  pp.  640,  641. 

Index  of  Proper  Names 043-653 

Index  of  Subjects 654-661 


INVERTEBRATE    MORPHOLOGY. 


CHAPTER  I. 


PROTOPLASM  AND  THE  CELL. 


In  the  e^tmiuation  of  organisms  presenting  the  series  of 
phenomena  which  we  term  life,  the  invariable  presence  of  a 
peculiar  emi- fluid  transparent  or  hyaline  substance  becomes 
quickly  apparent.  Whether  the  organism  be  a  plant  or  an 
animal,  whotlier  it  be  of  the  simplest  or  of  the  most  complex 
organization,  it  is  still  composed  of  this  substance,  which  is 
known  as  protoplasm,  and  it  may  be  said  that  so  far  as  our 
knowledge  extends  life  never  exists  except  in  association  with 
this  material.  Protoplasm  is  "  the  physical  basis  of  Life," 
and  it  becomes  of  great  importance  that  its  nature  should  be 
fully  understood,  in  order  that  the  results  of  its  activities, 
Life,  may  become  more  intelligible. 

Much  has  yet  to  be  accomplished,  however,  before  an  accu- 
rate knowledge  of  the  structural  and  chemical  characters  of 
this  substance  is  obtained,  and  indeed  it  is  incorrect  to  regard 
it  as  a  substance,  since  it  is  rather  the  aggregate  of  a  large 
number  of  exceedingly  complex  chemical  compounds,  none 
of  which  are  sufficiently  known.  From  the  very  nature  of 
things  it  is  impossible  at  present  to  get  a  correct  idea  of  these 
substances  and  the  relations  which  they  bear  to  one  another, 
since  our  present  analytical  methods  are  not  capable  of  deter- 
mining and  isolating  them  in  living  protoplasm  and  the  mere 
u.ot  of  subjecting  protoplasm  to  analysis  i^estroys  those  very 
relationships  which  are  the  cause  of  the  vital  manifestations. 


IN  VEHTEDltA  TE  MOliPIIOLOG  V. 


Dead  protoplasm  is  sometliiug  very  difl'erent  from  living  pro- 
toplasm, and  our  present  knowledge  only  imperfectly  extends 
to  this  much-altered  material. 

Furthermore  even  in  the  dead  material  the  chemist  has  to 
deal  not  only  with  the  complex  substances  which  constitute 
protoplasm  proper,  but  also  with  numerous  secondary  prod- 
ucts either  in  the  process  of  being  built  up  into  protoplasmic 
molecules  or  else  resulting  from  the  destruction  of  these 
molecules.  For  both  these  processes  are  continually  going 
on,  the  living  organism  continually  uniting  simple  chemical 
compounds  to  form  new  complex  molecules,  a  process  known 
as  anabolism,  and  resulting  in  growth  ;  and  just  as  continually 
it  is  resolving  into  simpler  compounds  the  complex  mole- 
cules already  formed,  a  process  known  as  catabolism,  and 
resulting  in  the  manifestation  of  energy  in  its  various  forms, 
such  as  heat,  motion,  electricity,  and  even  light.  Growth  and 
the  manifestation  of  energy  are  then  two  most  important 
phenomena  exhibited  by  living  organisms,  standing  in  oppo- 
sition to  one  another  and  determining  the  general  condition 
of  the  organism.  If  anabolic  changes  are  the  more  active, 
then  the  animal  or  plant  grows,  as  we  express  it,  adds  new 
protoplasm  and  increases  in  size  ;  if  the  anabolic  and  catabolic 
changes  are  practically  equal  in  amount,  stability  results  ; 
while  the  preponderance  of  catabolism  leads  to  a  lessening 
of  material,  and  finally  to  what  we  term  death.  These 
changes  constitute  a  cycle  occurring  in  the  life-history  of 
probably  every  organism  and  causing  the  periods  which  we 
denote  as  youth,  maturity,  and  old  age. 

Dead  protoplasm  then,  together  with  the  anabolic  and 
catabolic  constituents  which  .are  inextricably  associated  with 
it,  will  be  found  on  analysis  to  consist  to  a  large  extent  of  the 
chemical  elements  Carbon,  Hydrogen,  Oxygen,  and  Nitrogen, 
together  with  Sulphur  and  Phosphorus,  as  well  as  a  number 
of  substances  present  in  varying  amounts,  such  as  Chlorine, 
Potassium,  Sodium,  Iron,  Calcium,  and  Magnesium.  Exactly 
how  these  various  elements  are  united  together  it  is  difficult 
to  determine,  but  especial  importance  has  been  assigned  to 
the  C,H,  O,  N,  and  S  compounds  which  occur  and  which  form 
a  group  of  chemical  compounds  known  as  Proteids.     Of  such 


PROTOPLASM  AND  THE  CELL. 


3 


compounds  several,  such  as  Albumiu,  Globulii:,  Fibriu,  Flastiu, 
Nucleiu,  have  beeu  isolated  from  protoplasm,  some  beiug 
probably  secondary  products  resultiuj;  from  the  alteration  of 
the  protoplasmic  molecules  proper,  but  others,  such  as 
Plastin  and  Nuclein,  are  especially  constant,  and  seem  to  be 
important  constituents  of  the  protoplasmic  complex. 

Plastin  forms  wlieii  isolated  a  sticky  fibrous  mass,  insoluble  in  concen- 
trated alkaline  solutions  and  unaffected  by  the  peptic  and  tryptic  ferments, 
and  consists  of  C,  H,  O,  N,  S,  and  P.  Nuclein  is  more  especially  charac- 
teristic of  a  special  portion  or  modification  of  protoplasm  termed  the 
nucleus,  of  which  more  will  be  said  hereafter,  and  resembles  plastin  very 
closely,  beinj?,  however,  less  insoluble  than  it,  and  consists  of  the  same 
chemical  elements.  Analyses  of  these  substances,  however,  differ  greatly, 
the  nuclein  from  spermatozoa,  for  instance,  containing  no  sulphur  ;  and  it 
seems  probable  not  only  that  they  differ  materially  according  to  the  source 
from  which  they  are  obtained,  but  also  that  they  are  not  reaHy  chemical 
compounds,  but  a  mixture  of  several  highly  complex  substances. 

With  these  proteids,  then,  there  exist  in  protoplasm  vari- 
ous salts,  such  as  Potassium,  Sodium,  and  Calcium  phosphate, 
Potassium  and  Sodium  chloride,  Magnesium  sulphate,  and 
other  such  salts,  the  exact  significance  of  which  it  is  difficult 
to  estimate.  How  living  protoplasm  differs  chemically  from 
dead  has  not  up  to  the  present  been  accurately  determined. 

As  regards  its  general  structure  prott)plasm  appears  as  a 
moderately  consistent  jelly-like  substance,  usually  colorless 
and  more  or  less  granular  in  appearance.  As  a  rule  the 
peripheral  portion  of  a  mass  of  protoplasm  is  less  granular 
than  the  central,  appearing  therefore  clearer,  and  is  espe- 
cially distinguished  as  the  ectoplasm  from  the  more  opaque 
endoplasm.  Imbedded  in  the  eudoplasm  are  to  be  found 
usually  various  bodies,  the  products  of  the  activities  of  the 
protoplasm,  such  as  large,  clear  spaces  occupietl  by  fluid 
and  known  as  vacuoles,  food-particles  of  various  kinds  in  the 
simpler  organisms,  starch  granules  and  crystals  in  plant-pro- 
toplasm, and  depositions  of  pigment.  One  particular  struc- 
ture, the  nucleus,  however,  seems  to  be  invariably  present, 
occupying  the  central  portion  of  the  mass,  and,  as  will  be 
seen  later,  playing  a  very  important  role  in  the  life  of  the 
protoplasm.  It  is  indeed  a  specially  modified  portion  of  the 
protoplasm  and  cannot,  therefore,  be  placed  iu  the   same 


4 


INVERTEBRATE  MORPHOLOGY. 


category  as  the  vacuoles  and  other  accidental  or  secondaiy 
constituents  which  have  been  mentioned,  and  every  mass  of 
protoplasm  may  be  considered  as  consisting  of  two  essential 
parts,  the  protoplasm  proper  or  cytoplasm  and  the  special 
modification  of  it,  the  nucleus,  which  for  convenience  is 
termed  the  caryoplasm.  Such  a  combination  of  cytoplasm 
aud  caryoplasm  forms  what  is  technically  known  as  a  cell,  and 
all  living  organisms  are  composed  of  one  or  more  such  struc- 
tures, which  are  to  be  regarded  therefore  as  morphological 
units. 

If  the  more  intimate  structure  of  the  cytoplasm  of  such  a 
unit  or  cell  (Fig.  1)  be  examined,  disregarding  the  various 


mn 


Pig.  1.— Diagram  showing  the  Structure  op  an  Animal  Cell. 
c  =  centrosoine.  m  =  microsome. 

cl  =  cytolymph.  nl  =  nucleolus. 

cr  =  chromatin.  nm  =  nuclear  membrane. 

r  =  reticulum. 

secondary  constituents  it  may  enclose,  it  will  be  found  to 
consist  of  a  network  of  exceedingl}'  fine  fibrils,  along  which, 
and  more  especially  at  the  points  where  two  or  more  of  them 
meet,  are  to  be  found  minute  granules  which  stain  deeply  with 
the  ordinary  microscopical  staining  reagents.  The  fibrils 
constitute  the  reticulum  (Fig.  1,  r)  of  the  cytoplasm,  and  the 
granules  are  termed  the  microsomes  (m).  The  reticulum 
seems  to  be  formed  principally  of  the  proteid  substance 
already  mentioned  as  plastiu,  and  its  meshes  are  occupied  by 
a  more  fluid  substance  which  has  been  termed  the  cyto- 
lymph (cl). 


PROTOPLASM  AND  THE  CELL. 


Several  opinions  liave  been  given  in  regard  to  the  structure  of  the  cyto- 
plasm, in  addition  to  that  here  presented,  according  to  which  it  may  be 
compared  to  a  sponge  the  meshes  of  whose  network  are  occupied  by  the 
cytolymph.  According  to  another  view  it  is  composed  of  a  number  of 
fibrils  of  varying  lengths  imbedded  in  a  matrix,  the  fibrils  corresponding  to 
the  reticulum  of  the  reticular  theory  and  the  matrix  to  the  cytolymph. 
According  to  still  another  theory  which  rests  on  the  appearance  produced 
in  the  cytoplasm  by  a  special  method  of  treatment,  there  is  present  a  color- 
less matrix  in  which  are  imbedded  numerous  exceedingly  small  granules 
sometimes  scattered  and  sometimes  united  together  into  chains.  Indeed 
the  upholder  of  this  granular  theory  has  carried  his  view  to  the  extent  of 
regarding  the  granules  as  structural  units  of  which  the  cell  is  composed, 
its  structure  being  comparable  to  that  of  a  zooglcea  of  micrococci.  It 
seems  probable,  however,  that  the  granules  are  to  a  large  extent  secondary 
products  of  the  activities  of  the  cytoplasm  and  have  therefore  but  a  sub- 
ordinate value  in  its  composition.  The  reticular  theory  seems  to  stand 
more  nearly  in  harmony  with  the  majority  of  observations,  though  it 
must  be  admitted  that  some  observers  do  not  seem  to  have  perceived  the 
true  reticulum,  confining  their  attention  to  the  coarser  network  produced 
in  some  cases  by  extensive  vacuolization  of  the  cell. 

An  imitation  of  the  cytoplasm  has  been  recently  obtained  by  the  mix- 
ture of  thickened  olive-oil  with  a  solution  of  potassium  carbonate  or  of 
chloride  of  sodium,  the  watery  solution  taking  the  form  of  polyhedral 
globules  each  surrounded  by  a  thin  layer  of  oil  which  from  its  higher 
refractive  index  gives  the  appearance  of  the  plastin  reticulum  surrounding 
the  cytolymph.  Solid  particles  finely  divided  and  mixed  with  the  oil  tend 
to  collect  at  the  points  where  the  oil-films  of  three  of  the  globules  come 
together,  and  resemble  the  microsomes,  while  it  is  further  noticeable  that 
under  certain  conditions  the  superficial  globules  of  the  emulsion  take  on  a 
columnar  form  and  maybe  compared  with  the  ectoplasm  of  the  cell.  It 
is  possible  that  the  cytoplasm  may  have  this  structure,  in  which  case  the 
reticular  theory  would  require  to  be  modified,  since  there  would  no  longer 
be  a  spongy  structure,  but  rather  an  emulsion  in  which  the  cytolymph  is 
divided  into  a  number  of  globules  each  surrounded  by  a  thin  layer  of 
plastin.  At  present,  however,  the  reticular  theory  seems  to  correspond 
most  accurately  with  the  actual  appearances,  and  therefore  may  be  pro- 
visionally accepted. 

The  caryoplasm  or  nucleus,  as  already  stated,  lies  usually 
about  the  middle  of  the  cytoplasm  and  to  a  certain  extent  re- 
sembles it,  though  it  presents  certain  peculiar  features.  It 
is  usually  round  or  oval,  though  occasionally  it  may  assume 
elongated,  horseshoe-shaped,  moniliform,  or  even  branching 
forms,  and  is  as  a  rule  clearly  marked  off  from  the  cytoplasm  by 
a  membrane  (Fig.  1,  mm)y  which,  however,  at  certain  periods  of 


mm 


6 


INVERTEBRATE  MORPHOLOGY. 


nuclear  activity  seems  to  disappear,  a  new  one  subsequently 
forming.  Traversing  the  space  enclosed  by  the  membrane, 
so  as  to  form  a  network,  are  fibres  which  do  not  stain  very 
deeply  with  the  usual  staining  fluids  and  which  are  composed 
of  a  substance  termed  linin,  which  does  not,  however,  appear 
to  differ  essentially  from  the  plastin  of  the  r  .  ;oplasm.  In- 
deed it  is  not  improbable  that  the  linin  network  is  con- 
tinuous through  the  nuclear  membrane  with  the  plastin 
reticulum  and  that  both  are  identical,  as  is  also  the  caryo- 
lymph  contained  in  the  meshes  of  the  linin  with  the  cyto- 
lymph. 

A  more  characteristic  substance  is  the  chromatin  (Fig. 
1,  cr)^  so  called  from  the  strong  affinities  it  shows  for  many 
staining  fluids,  such  as  carmine,  htematoxylin  solutions,  and 
certain  aniline  stains.  It  seems  to  consist  of  the  substance 
nuclein,  already  alluded  to,  and  in  the  resting  nucleus  forms 
a  reticulum  intimately  associated  with  the  linin  network, 
which  it  usually  to  a  considerable  extent  obscures.  Where 
the  various  strands  of  the  network  meet,  thickenings  of  the 
chromatin  sometimes  occur,  producing  densely  staining 
bodies  {rd)  to  which  the  term  nucleoli  is  given,  though  it  is 
probable  that  bodies  of  a  somewhat  different  composition 
are  also  included  under  this  name ;  for  there  are  usually  to 
be  found  in  the  nucleus,  imbedded  in  the  substance  of  the 
network,  one  or  more  spherical  bodies  whose  chemical  re- 
actions diifer  noticeably  from  those  of  the  chromatin  nucleoli, 
the  substance  of  which  they  are  composed  being  termed 
paranuclein  or  pyrenin. 

There  are  then  in  the  cell  the  following  structural  con- 
stituents : 

(  membrane  (cell-wall), 
I.  Cytoplasm  :    •<  reticulum  (plastin), 
(  ciryolymph. 

r  membrane, 
j  reticulum  (linin), 
II.  Caryoplasm  :  4  caryolymph, 

chromatin  network  (nuclein), 
^  nucleoli  (nuclein  and  paranuclein). 


PROTOPLASM  AND  THE  CELL. 


equently 
smbrane, 
tain  very 
omposed 
r,  appear 
bsm.  In- 
:  is  con- 
)  plastin 
18  caryo- 
;he  cyto- 

itin  (Fig. 
or  many 
ions,  and 
ubstance 
us  forms 
network, 

Where 
gs  of  the 
staining 
ugh  it  is 
iposition 
3ually  to 

of  the 
aical  re- 
nucleoli, 

termed 

ral   con- 


)in). 


In  addition  to  these  there  is,  however,  still  another  body  to 
be  mentioned  which  is  especially  evident  in  cells  which  are 
undergoing  multiplication,  but  Avliich  has  also  been  found  in 
various  resting  cells,  especially  in  lymph-corpuscles,  various 
kinds  of  endothelial  cells,  and  in  pigment-cells.  This  is  the 
structure  known  as  the  centrosome  (Fig.  1,  c).  It  is  usually 
an  exceedingly  small  spherical  body  which  does  not  readily 
stain  with  the  reagents  which  place  the  chromatin  in  evi- 
deuce,  but  has  a  strong  affinity  for  certain  acid  aniline  stains, 
such  as  safraniu,  fuchsin,  or  orange.  Usually  but  a  single 
centrosome  is  present  in  each  cell,  though  occasionally  two  or 
even  more  may  occur,  and  it  is  situated  in  the  cytoplasm  in 
the  neighborhood  of  the  nucleus,  sometimes  resting  in  a 
slight  concavity  on  the  surface  of  that  structure.  Surround- 
ing the  centrosome  there  is  frequently  to  be  seen,  more 
especially  in  dividing  cells,  a  radial  arrangement  of  the  cyto- 
plasmic reticulum,  the  centrosome  being  comparable  to  a 
star  from  which  rays  pass  out  in  all  directions,  whence  the 
term  aster  which  is  applied  to  the  combination  of  the  cen- 
trosome and  the  cytoplasmic  rays. 

The  significance  of  the  centrosome  will  be  seen  later  when  the  phe- 
nomena of  cell-division  are  under  consideration,  but  its  origin  may  be 
inquired  into  at  this  place.  Two  views  are  current  in  regard  to  this 
matter,  according  to  one  of  which  the  centrosome  has  its  origin  in  the 
nucleus  and  at  a  certain  period  of  the  cell's  existence  is  extruded  from  it. 
In  favor  of  this  view  the  intimate  association  of  the  centrosome  and  the 
nucleus  are  pointed  out,  an  association  which  becomes  especially  pro- 
nounced during  cell-division,  the  astral  rays  connected  with  the  cen- 
trosome appearing  to  penetrate  the  nucleus  and  in  fact  to  bring  about  its 
division  into  two  parts.  According  to  the  other  theory,  however,  the  cen- 
trosome is  a  constituent  of  the  cytoplasm  and  in  its  origin  has  nothing  to 
do  with  the  nucleus.  Quite  recently  an  interesting  amplification  of  this 
idea  has  been  suggested  to  the  effect  that  the  centrosome  is  nothing  more 
or  less  than  an  aggregation  of  cytoplasmic  microsomes.  The  astral  rays 
are  cytoplasmic  fibres  converging  from  all  sides,  and  since  microsomes 
occur  along  their  course  an  aggregation  of  these  bodies  might  be  found 
where  the  fibres  meet.  This  idea  cannot  be  discussed  in  detail  here,  but 
it  may  be  pointed  out  that  the  absence  of  a  centrosome  in  cells  which  have 
lost  their  powers  of  reproduction  is  readily  explicable  on  this  idea,  the  ag- 
gregated microsomes  having  scattered  in  such  cells,  and  furthermore 
that  the  theory  explains  certain  peculiar  arrangements  replacing  the 
typical  aster  during  the  division  of  some  cells. 


8 


INVERTEBRATE  MORPUOLOGT. 


Such  a  combination  of  cytoplasm  and  caryoplasm  consti- 
tutes a  morphological  element  capable  of  carrying  on  all  the 
functions  of  life.  It  is  not  only  a  morphological  but  also  a 
physiological  element.  It  is  capable  of  assimilating  the 
necessary  substances  and  building  up  protoplasm  ;  metabol- 
ism and  the  consequent  evolution  of  energy  goes  on  in  it ;  it 
excretes  waste  products  ;  it  is  contractile  and  may  therefore 
be  capable  of  motion ;  it  responds  to  stimuli  of  various  kinds, 
or  in  other  words  it  is  irritable  ;  and,  finally,  it  is  capable 
of  reproduction.  The  question  naturally  arises,  iiowever, 
whether  this  combination  of  the  two  substances  mentioned 
is  essential — whether,  that  is  to  say,  organisms  without  nuclei 
do  not  exist  and  manifest  all  the  phenomena  of  life.  At 
one  time  the  existence  of  unicellular  organisms  destitute  of 
a  nucleus  was  recognized,  the  term  cytode  being  applied  to 
them  to  distinguish  them  from  nucleated  cells.  Within  re- 
cent years,  however,  a  growing  skepticism  has  come  into 
existence  as  to  the  non-nucleate  character  of  these  organ- 
isms, the  recent  improvements  of  the  microscope  and  the 
application  of  modern  staining  reagents  having  revealed  the 
existence  of  nuclei  in  many  of  the  forms  at  one  time  regarded 
as  typical  cytodes.  It  would  perhaps  be  going  too  far  to 
state  that  cytodes  do  not  exist,  but  the  evidence  at  hand  indi- 
cates that  their  existence  is  highly  problematical. 

This  conclusion  is  strengthened  by  the  results  which  have 
been  obtained  from  the  observations  of  artificially  produced 
cytodes.  Some  of  the  larger  unicellular  organisms  have 
been  cut  into  fragments  some  of  which  can  be  definitely 
shown  to  be  destitute  of  nuclear  or  caryoplasmatic  substance. 
In  such  cases  it  is  found  that  the  nucleated  fragments  if 
l)laced  under  proper  conditions  will  regenerate  and  airvj  on 
their  existence  as  before,  while  the  cytode  fragments,  though 
manifesting  signs  of  life  for  a  considerable  length  of  time, 
will  not  regenerate  and  do  not  possess  the  power  of  repro- 
duction. The  nucleus  seems  to  possess  a  marked  regulating 
or  coordinating  action  upon  the  cytoplasm,  coordinating  the 
anabolic  and  catabolic  activities  upon  which  the  continuan^ie 
of  life  depends. 

It  would  be  beyond  the  scope  of  the  present  work  to  enter 


B 


PROTOPLASM  AND  THE  CELL. 


9 


iuto  a  discussion  of  the  various  forms  of  physiological  activity 
of  the  cell,  but  one  of  its  physiological  functions,  reproduc- 
tion, must  receive  special  attention  in  couuection  with  the 
remarkable  structural  changes  which  accompany  it.  Since 
the  disproval  of  the  doctrine  of  spontaneous  generation  the 
epigrammatic  statement  Omnis  cellula  e  celluld  has  been  the 
Avatchword  of  modern  histology  and  embryology,  and  today 
it  finds  its  complement  in  a  corresponding  epigram,  Omnis 
nucleus  e  nucleo.  Every  cell  at  present  in  existence  miay  be 
assumed  to  have  descended  from  some  previously  existing 
cell,  and  the  nucleus  it  contains  to  be  a  portion  of  the  nucleus 
of  the  ancestral  cell.  New  cells  arise  by  the  division  of 
previously  existing  cells,  and  each  division  of  the  cytoplasm 
is  accompanied  by  a  division  of  the  nucleus.  Not  but  that 
under  certain  conditions  a  division  of  the  nucleus  may 
occur  without  a  corresponding  division  of  the  cytoplasm, 
multinucleated  cells  thus  arising,  and  conversely  a  division 
of  the  cytoplasm  may  possibly  in  certain  cases  be  inaugu- 
rated without  entailing  a  division  of  the  caryoplasm  ;  but,  as 
might  be  expected  from  the  relation  which  exists  between 
the  nucleus  and  the  cytoplasm,  the  division  of  the  latter 
is  usually  preceded  by  a  division  of  the  caryoplasm. 

This  latter  process  may  take  place  in  two  ways.  It  may 
begin  as  a  simple  constriction  of  the  nucleus  which,  becoming 
deeper  and  deeper,  tiually  separates  off  a  portion  of  it,  a  divis- 
ion of  the  cytoplasm  in  a  similar  manner  then  occurring,  so 
that  each  of  the  new  cells  thus  formed  contains  a  portion  of 
the  original  nucleus.  This  method  of  nuclear  division,  which 
is  rather  rare,  occurs  for  instance  in  the  embryonic  mem- 
branes of  the  Scorpion  and  is  termed  direct  or  amitotic  divis- 
ion, to  distinguish  it  from  the  more  usual  indirect  or  mitotic 
method  which  is  accompanied  by  a  series  of  complicated 
phenomena  to  which  the  general  term  hiryokinesis  or  mitosis 
is  applied. 

Starting  with  a  typical  cell,  consisting  of  the  various  parts 
mentioned  above,  the  karyokinetic  phenomena  may  be  re- 
garded as  affecting  two  constituents,  i.e.  the  centrosome  and 
the  nuclear  chromatin.  The  centrosome  which  lies  at  one 
pole  of  the  nucleus  first  divides,  the  two  resulting  portions 


10 


IN  VERTEBRA  TE  MORPHOLOG  Y. 


gradually  separating  from  one  another  (Fig.  2,  A)  until  they 
lie  at  opposite  poles  of  the  nucleus,  usually  taking  up  a  posi- 
tion ninety  degrees  distant  from  the  point  at  which  the  origi- 
nal centrosome  lay.  During  this  process  the  radiating  tihi- 
meuts  which  surround  the  centrosome  become  especially 
distinct  and  may  be  divided  into  two  portions,  those  which 
come  in  contact  with  the  nucleus  and  which  from  their 
appearance  in  later  stages  are  termed  the  spindle-Jibres,  and 


Pig.  2. — Diagram  showing  the  Phenomena  of  Cell-division. 

A,  sepanilion  of  the  ceutrosomes  ;  chromatin  iu  skein-stage. 

B,  fully  formed  spindle  ;  chromatin  loops  formed. 

C,  longitudinal  division  of  the  chromatin  loops. 

J),  separation  of  chromatin  loops  and  commencement  of  the  division  of  the 
cytophism. 

those  which  radiate  outwards  and  are  lost  iu  the  cytoplasmic 
network  and  form  the  aster.  In  the  meantime,  however,  im- 
portant changes  have  been  taking  place  within  the  nucleus. 
The  chromatin  substance,  which  originally  was  scattered  in 
a  reticulum,  begins  to  arrange  itself  in  a  band  (Fig.  2,  A) 
which  with  many  turns  traverses  the  nuclear  substance,  the 
nucleoli  which  Avere  present  at  the  same  time  gradually  van- 
ishing.    This  stage  of  the  process  is  termed  the  skein  stage. 


PROTOPLASM  AND   TUB  CELL. 


11 


The  spindle-fibres  of  the  centrosome  then  appear  to  penetrate 
through  the  nuclear  membrane,  which  sooner  or  later  disap- 
pears, and  by  thoir  growth  push  the  chromatin  skein  towards 
the  equator  of  the  nucleus,  the  skein  at  the  same  time  break- 
ing into  a  number  of  fragments,  termed  chromomnies.  The 
number  of  these  chromosomes  is  practically  constant  for  the 
cells  of  any  species  of  animal,  and  though  there  is  consider- 
able variation  in  different  species,  yet  in  the  majority  of  ob- 
served cases  the  number  belongs  either  to  the  series  2,  4,  8, 
1(),  32,  or  to  that  of  6,  12,  24.  They  vary  considerably  in  size 
in  different  forms,  being  in  some  cases  V-shaped  or  in  others 
dumbbell-shaped,  and  arrange  themselves  finally  in  a  more  or 
less  definite  ring  surrounding  the  equator  of  the  nucleus.  At 
this  stage,  which  is  known  as  the  equatorial-plate  stfige,  the 
appearance  presented  in  Figure  2,  B,  is  found.  At  each  pole 
of  the  nucleus  is  a  centrosome  surrounded  by  the  a^:tral  rays 
and  with  the  spindle-fibres  extending  towards  and  coming 
in  contact  Avith  the  chromosomes  lying  at  the  equator  of  the 
nucleus,  and  to  *the  entire  complex  the  term  cunphiasler  is 
sometimes  applied. 

In  the  next  stage  tlie  V-shaped  chromosomes,  to  take  this 
as  a  typical  shape,  which  are  arranged  with  the  apex  of  the 
V  towards  the  nuclear  axis,  divide  longitudinally.  Assuming 
that  there  were  originally  six  chromosomes  in  the  equatorial 
plate,  as  the  result  of  the  division  there  are  now  twelve  ar- 
ranged in  pairs  (Fig.  2,  C).  One  of  each  pair  now  proceeds 
to  move  towards  one  of  the  poles  of  the  nucleus  .and  the  other 
to  the  other,  so  that  eventually  near  each  pole  there  is  a 
group  of  six  chromosomes,  and  between  the  two  groups  there 
may  be  seen  stretched  a  number  of  connecting  fibres  identical 
in  appearance  with  the  original  spindle-fibres,  while  in  some 
cases  at  the  equator  of  the  egg  there  is  to  be  seen  on  these 
fibres  a  number  of  darkly  staining  dots  which  may  be  termed 
the  intermediate  bodies  (Fig.  2,  D).  At  about  this  stage  the 
cytoplasm  begins  to  divide,  the  plane  of  its  division  passing 
through  the  equator  of  the  nucleus,  and  there  are  thus  formed 
two  cells,  each  containing  a  nucleus  composed  of  six  chromo- 
somes and  a  centrosome.  The  chromosomes  now  begin  to 
become  irregular  in  shape,  they  gradually  fuse  and  are  finally 


19 


IN  VERTEBRA  TE  MORPUOLOG  7. 


scattered  in  the  form  of  a  chromatic  reticulum  through  the 
substauce  of  the  nucleus,  which  thus  passes  again  iuto  the 
resting  stage,  developing  a  new  nuclear  membrane. 

Our  knowledge  of  many  of  the  details  of  karyokinesis  is  yet  imperfect, 
and  especially  is  this  the  case  with  regard  to  the  mode  in  which  the  cen- 
trosome  exerts  its  influence.  It  has  been  regarded  as  a  simple  centre  of 
attraction,  similar  to  the  pole  of  a  magnet,  but  the  spindle-fibres  seem  to 
be  more  than  passive  in  the  phenomena.  A  comparison  of  the  centrosome 
with  an  aggregation  of  microsomes  has  already  been  referred  to,  and  if 
this  idea  be  extended  some  light  may  be  thrown  upon  the  spindle-fibres. 
They  would  then  naturally  be  regarded  as  reticular  fibres,  i.e.  fibres  of 
plastin  to  which  a  certain  amount  of  contractility  and  extensibility  may  be 
ascribed.  During  the  earlier  stages  of  karyokinesis  their  extensibility  is 
more  manifest,  and  extending  into  the  nucleus  they  compress  its  chromatic 
substance,  the  contractility  manifesting  itself  later  and  determining  the 
migration  of  the  chromatin  loops  or  chromosomes  towards  the  poles  of  the 
nucleus.  Furthermore,  since  the  linin  reticulum  of  the  nucleus  is  probably 
continuous  with  the  plastin  reticulum  of  the  cytoplasm,  it  is  conceivable 
that  the  activities  of  the  centrosomes  may  call  out  in  it  changes  of  contrac- 
tion or  extension  which  may  suffice  to  bring  about  the  characteristic  skein 
formation  of  the  chromatin  and  the  subsequent  fragmentation  of  the  skein 
into  the  chromosomes,  as  well  as  the  formation  of  the  connective  fibres  in 
later  stages,  the  intermediate  bodies  upon  these  being  regarded  as  micro- 
somes. These  views,  which  have  been  but  recently  suggested,  require  con- 
firmation, however ;  if  true  they  afford  a  new  basis  from  which  to  attack  the 
problems  involved  in  the  phenomena  of  karyokinesis,  and  even  at  present 
throw  no  little  light  upon  the  structural  details  associated  with  the  process. 
It  must  be  mentioned,  however,  that  certain  recent  observations  have  been 
held  to  prove  that  the  centrosome  has  a  nuclear  origin,  and  for  the  present 
the  important  question  of  its  significance  must  be  considered  as  open. 


LITERATURE. 

0.  Hertwig.     Die  Zelle  und  die  Gewebe.     Jena,  1893. 

W.  Flemming.     Zellsubstam,  Kern  und  Zelltheilung.    Leipzig,  1883. 

0.  Bfltschli.     Untersuchungen  ilber  mikroskopiscfie  Schdume  und  daa  Proto- 

plasma.     Leipzig,  1893. 
C.  Rabl.     Ueber  Zelltheilung.    Morpholog.  Jahrbach,  x.  1884. 
G.  Platner.    Beitrdge  zur  Kenntniss  der  Zelle  und  ihrer  TheUung.    Archiv  fttr 

mikrosk.  Anatomie,  xxxiii.  1889. 
M.  Heidenhain.     Uber  Kern  und  Protoplasma.     Leipzig,  1893. 
H.  P.  Johnson.    Amitosis  in  the  Embryonal  Envelopes  of  the  Scorpion.    Bulletin 

of  the  Museum  of  Comp.  Zoology,  xxii.  1893. 


8UBKINQD0M  PROTOZOA. 


13 


CHAPTER  II. 


SUBKINGDOM  PROTOZOA. 


A  SIMPLE  cell,  as  has  already  beeu  stated,  possesses  the 
power  of  performing  all  the  functions  of  life,  and  conse- 
quently the  existence  of  unicellular  organisms  is  possible. 
Such  organisms,  together  with  those  which  consist  of  a 
number  of  cells  grouped  together,  each  cell,  however,  retain- 
ing to  a  greater  or  less  extent  its  own  individuality,  are 
grouped  together  in  a  subkiugdom  and  are  collectively 
termed  Pkotozoa.  In  its  simplest  form  a  Protozoou  may 
show  but  little  diiferentiation  of  its  protoplasm,  but  in  the 
majority  of  cases  various  portions  of  the  cell-substance  take 
upon  themselves  special  functions,  and  in  accordance  with 
this  physiological  differentiation  undergo  various  structural 
modifications.  Locomotor  and  prehensile  structures  of  vari- 
ous forms  may  be  developed,  excretory  pulsating  vacuoles,  a 
permanent  mouth  and  pharynx,  special  contractile  bands, 
and  even  pigment  spots  presumably  connected  with  light 
absorption  may  occur,  and  in  addition  the  power  of  secreting 
horny,  calcareous,  or  siliceous  skeletons,  serving  either  as 
protective  or  supportive  structures,  is  frequently  present.  A 
high  degree  of  complexity  may  therefore  occur  in  a  unicel- 
lular organism,  a  complexity  produced  by  a  differentiation  of 
various  portions  of  the  protoplasm  composing  the  individual. 
For  the  most  part  the  organisms  are  simple,  but  occasionally 
they  associate  together  to  form  colonies.  The  individuals  of 
the  colonies  are  as  a  rule  all  alike,  each  carrying  on  all  the 
functions  of  existence  for  itself,  and  there  is  no  division  of 
labor  among  the  various  individuals.  The  complexity  which 
exists  is  individual  and  not  colonial.  A  few  forms,  however, 
such  as  Volvox,  do  present  a  certain  amount  of  colonial  differ- 
entiation ;  all  the  cells  composing  the  colony  are  not  perfectly 
identical  physiologically,  some  becoming,  for  instance,  spe- 


14 


IN  VEliTKlilU  TE  MOIWUOLOG  Y. 


cialized  for  reprocluctivo  purposes,  while  the  rest  take  but 
little  part  in  this  process.  Such  a  colouy  presents  iudica- 
tious  of  a  passage  towards  a  higher  grade  of  individuality, 
some  of  the  various  cell-individuals  merging  to  a  certain  ox- 
tent  their  individualities  in  that  of  the  entire  colony,  and 
becoming  somewhat  dependent  for  existence  on  the  coopera- 
tion of  their  fellows.  This  dependence,  however,  never 
reaches  a  high  degree  of  development  in  the  Protozoa  and  is 
for  the  most  part  entirely  absent.  It  is  in  this  respect  that 
colonial  Protozoa  difter  from  the  higher  organisms,  but  the 
difference  is  one  of  degree,  not  of  kind. 

Four  well-marked  classes  may  be  distinguished   among 
the  Protozoa : 

I.  CI.  llhizopoda. 
IT.  CI.  Si)()r()Z()a. 
III.  CI.  riagellata. 
ly.  Cl,  Infusoria. 


I.  Class  Rhizopoda. 

The  simplest  liliizopods  present  an  approach  to  the  least 
complicated  condition  uutler  Avhich  protoplasm  is  known  to 
us.  They  are  simj)ly  small  masses  of  protoplasm,  more  or 
less  granular  towards  the  centre,  clearer  towards  the  periph- 


cv 


Fig.  Z.—AmcRba  protem  (after  Ghubkr). 
cv  =  contractile  vacuole.  n  =  nucleus.  pa  =  pseudopodium. 

ery,  and  continually  alter  their  shape  by  pushing  out  lobe- 
or  thread-like   processes  known  as  pseiidopodia  (Fig.  3,  joa). 


SUBKINODOM  J'liOTOZOA. 


If) 


By  throwing;  out  such  a  procons  iiiul  tiowing  after  it,  as  it 
were,  locomotiou  in  ])erf()riiuH],  which  from  a  well-kuown 
genus  of  the  chiss  is  toniied  onuyhohf.  Food  is  simply  tMi- 
gulfed  by  the  protoplasm  tiowing  around  it,  after  it  has  come 
iu  C(mtact  with  a  pseudopodium,  and  the  digestion  of  the 
food-substance  takes  jdaco  within  the  protoplasm,  bc'iiig  thus 
intracellular.  T^udigestible  material  is  discarded  at  any 
part  of  the  body ;  respiration  and  excretion  are  carried  on  by 
the  general  surface ;  and  reproduction  is  limited  to  the  sim- 
ple process  of  division. 

It  is  rare,  however,  to  find  such  a  simple  condition  as 
this;  veu  among  the  simpler  forms  a  certain  differentiation 
of  the  protoplasm  exists,  and  it  is  doubtful  if  it  is  really 
absent  in  any  of  the  forms  known  to  us.  The  structural  dif- 
ferentiations most  usually  occurring  are  the  nucleus  (Fig.  3,  n) 
and  the  contractile  vacuole  (Fig.  3,  cv).  The  former,  as  was 
noticed  iu  the  preceding  chapter,  is  of  great  importance  to 
the  cell,  and  it  is  questionable  whether  it  is  really  absent 
even  in  those  Rhizopods  iu  which  it  has  not  yet  been  dis- 
covered. It  is  presumable,  of  course,  that  it  is  a  structure 
which  has  gradually  become  elaborated,  that  has  evolved, 
and  that  in  the  simplest  conceivable  organism  it  may  have 
been  undifferentiated,  but  whether  such  an  organism  now 
exists  is  questionable.  The  contractile  vacuole  is  excretory 
in  its  function,  fluid  containing  products  of  metabolism  iu 
solution  accumulating  at  one  or  more  definite  regions  of  the 
protoplasm  to  form  it,  and  being  by  the  sudden  and  rhythmi- 
cal contraction  of  the  surrounding  protoplasm  periodically 
expelled  from  the  body. 

Various  degrees  of  complexity  are,  however,  found  among 
the  Rhizopods,  the  higher  forms  presenting  a  considerable 
degree  of  differentiation  both  iu  structure  and  in  the  modes  of 
reproduction,  and  three  orders  based  upon  structural  charac- 
teristics may  be  distinguished. 

1.  Order  Foraminifera. 

The  Foraminifera  contains  the  simpler  members  of  the 
class.  In  the  genus  Amoeba  (Fig.  3)  are  organisms  presenting 
the  simple  characters  above  alluded  to,  being  simple  naked 


16 


IN  VERTEBRATE  MORPHOLOQ  T. 


Fig.  4. — Arcella  mitrata 

(after  Leidy). 


masses  of  protoplasm  containing  a  nucleus  and  a  contractile 
vesicle  and  presenting  a  slight  differentiation  into  a  peripheral 
more  transparent  ectoplasm  and  a  central  more  granular 
endoplasm  in  which  the  nucleus  is  imbedded.  The  pseudo- 
podia  are  as  a  rule  blunt  lobose  processes,  though  in  some 

species  they  are  more  or  less  fila- 
mentous and  may  even  be  some- 
what permanent.  The  majority  of 
forms,  however,  secrete  a  protective 
shell  of  varying  composition  and 
complexity.  In  Arcella  (Fig-  4)  it 
is  chitinous  and  smooth,  and  len- 
ticular in  shape,  completely  sur- 
rounding the  protoplasm,  the  pseu- 
dopodia  projecting  from  the  cir- 
cular opening  on  the  flat  surface ; 
in  Euglypha  it  is  similar  in  composition,  but  sculptured  on 
the  convex  surface  ;  in  Diffiugia  the  shell  is  flask-shaped  and 
composed  of  particles  of  sand  .and  similar  foreign  bodies 
cemented  together,  while  in  a  large  number  of  forms,  es- 
pecially those  which  are  marine  in  habitat,  the  shell  is 
calcareous  in  composition. 

It  is  in  these  forms  with  calcareous  shells  that  the  great- 
est complexity  of  structure  occurs.  In  some,  such  as  Gromia, 
the  shell  is  simple  and  flask-shaped,  the  protoplasm  pro- 
truding from  the  mouth  of  the  shell  and  covering  its  entire 
surface  as  a  delicate  layer,  from  which  the  long,  slender,  and 
frequently  anastomosing  pseudopodia  take  their  origin.  Al- 
though the  pseudopodia  are  practically  permanent  in  form 
their  protoplasm  is  continually  changing,  currents  streaming 
from  the  body  towards  the  tips  of  the  pseudopodia  and  re- 
turning again  to  the  central  mass,  a  constant  circulation  being 
thus  maintained,  and  food-particles  caught  by  the  delicate 
pseudopodia  conveyed  to  the  central  mass,  there  to  be  di- 
gested. A  simple  shell  is,  however,  comparatively  rare 
among  these  calcareous  forms ;  more  frequently  it  consists  of 
several  chambers,  as  in  Miliola,  the  chambers  varying  in  size, 
the  first-formed  one  being  the  smallest,  and,  in  addition,  in 
very  many  forms  the  shell  is  perforated  by  minute  pores 


SUBKINODOM  PROTOZOA. 


17 


through  which  the  pseudopodia  are  emitted.  The  successive 
chambers  are  arranged  in  various  ways,  sometimes  end  to  end 
as  in  Nodosaria,  sometimes  alternately  on  opposite  sides  of  an 
axis  as  in  Textidaria,  sometimes  as  a  spiral  as  in  Glohigerina^ 
sometimes  as  a  helix  as  in  Rotalia  (Fig.  5),  and  sometimes 
more  or  less  irregularly  as  in  Acervularia. 


■'■:  i£ 


^  '        ^    y  y   '•       .  /y->    "■•'til    ^  '    II'.:   til      'N      »  .  V"-.     \      «     •  ■,  •■■^  • 

////.  J'/hr.,  hi:  !  i    'l  '.T'      ••-,  \\\\-    ^  \V\ 


Fig.  5. — Rotalia  venaia  (after  ?I.  Schultze  from  Hatschek). 

Notwithstanding  the  complexity  of  the  shell,  however,  the 
protoplasm  retains  throughout  the  order  its  simple  structure, 
and  though  in  the  more  complicated  forms  the  single  nucleus 
may  be  replaced  by  several,  yet  beyond  this  they  present  no 
more  marked  differentiation  than  is  found  in  the  simpler 
genera. 

2.  Order  Heliozoa. 

In  the  second  order,  the  Heliozoa,  the  pseudopodia  are 
slender  as  in  the  calcareous  Forarainifera  and  are  permanent 
and  somewhat  rigid,  the  central  protoplasm  of  each  one 
being  differentiated  into  an  elastic  axial  support.  The  ani- 
mals are  usually  globular  in  shape,  the  slender  pseudopodia 
radiating  out  from  the  central  mass,  an  appearance  being 
thus  produced  which  is  suftitient  cause  for  the  popular  term 
"  sun-animalcule  "  which  is  applied  to  several  of  the  genera, 
such  as  Actinophrys  and  Adinosphfprinm  (Fig.  6).     Currents 


b^ 


18 


IJ^ VEKTEBHATE  MOHPHOLOO  Y. 


i 


of  protoplasm  traverse  the  pseiidopodia  as  in  the  Foramiuifera 
and  carry  the  food-particles  to  the  body  proper.  This  has  a 
delicate  ectoplasm  and  a  central  endoplasm  which  is  fre- 
quently highly  vacuolated  and  contains  one  or  more  nuclei 


Fig.  6. — Actinospharium  Eichliornii  (after  Leidy), 
cv  and  cv'  =  coutnictile  vacuoles.  //=  iugesled  food, 

ef=  t'gested  food.  j)s  =  pseiidopodium. 

and  contractile  vacuoles.  In  some  forms  also  a  skeleton  is 
developed ;  it  reaches  its  most  perfect  form  in  the  stalked 
ClathruUna,  in  which  it  consists  of  a  delicate  fenestrated 
siliceous  sphere. 

3.  Order  Radiolaria. 

The  Radiolaria  are  exclusively  marine  and  are  the  most 
complicated  of  all  the  Rhizopods.  Their  pseudopodia  re- 
semble closely  those  of  the  Heliozoa,  being  slender  and  pos- 
sessing an  axial  support.  The  bodj'^  varies  in  shape  somewhat 
in  accordance  with  the  shape  of  the  siliceous  shell  with  which 
almost  all  the  forms  are  provided.  In  those  forms  in  which 
the  shell  is  simplest,  as  in  Thalassicolla  (Fig.  7),  where  it  is  in 
reality  absent,  the  body  is  spherical  and  is  clearly  diflferen- 
tiated  into  two  regions,  not,  however,  corresponding  to  the 
ectoplasm  and  endoplasm  of  other  Rhizopods.     The  centre  of 


SUBKINQDOM  PROTOZOA. 


19 


the  body  is  occupied  by  a  spherical  mass  surrounded  by  a 
firEi  chitiuous  covering  and  forming  the  central  capsule.  This 
contains  usually  many  nuclei  as  well  as  vacuoles,  oil-globules, 
and  in  some  cases  crystals  and  pigment-granules.  The  wall 
of  the  capsule  is  probably  comparable  to  the  shell  of  the 
Foraminifera,  being  perforated  as  in  those  forms  by  minute 
pores  through  which  the  intracapsular  protoplasm  becomes 


^-■11     III    I:' 


Fig.  7. — ThalnasicoUa  pelagica  (after  Haeckkl  from  Hatschek). 

continuous  with  the  extracapsular.  This  latter  portion  on 
this  supposition,  notwithstanding  its  greater  relative  thick- 
ness, is  equivalent  to  that  portion  of  the  protoplasm  of  the 
Foraminifera  which  is  outside  the  shell  and  from  which  the 
pseudopodia  arise.  It  is  usually  richly  vacuolated  and  pig- 
mented, but  contains  no  nuclei ;  the  axial  supports  of  the 
pseudopodia  traverse  it  and  take  their  origin  from  the  inner 
layers  which  immediately  surround  the  central  capsule  and 
are  more  homogeneous  than  the  outer  portions. 

The  shell  is  very  various  in  form  in  the  different  genera, 
reaching  a  high  degree  of  differentiation  in  some  forms,  such 
as  Helioaphcera  (Fig.  8),  where  it  consists  of  a  fenestrated 
globe  traversed  by  radiating  spines.  Its  greatest  simplicity 
is  seen  in  Sphcerozoum,  in  which  it  is  represented  by  scattered 


20 


INVERTEBRATE  MORPHOLOGY. 


I    I 


I 


spicules,  while  in  Thalassicolla,  already  alluded  to  (Fig.  7),  it 
is  entirely  absent.  As  stated,  it  is  usually  siliceous  in  char- 
acter, though  in  Acanthomelra  it  is  composed  of  a  peculiar 
horny  material  termed  acanthin. 

Scattered  through  the  protoplasm  of  the  Eadiolarians 
there  are  usually  to  be  seen  numbers  of  small  yellowish 
bodies  long  known  as  the  "yellow  cells."  They  are  not  con- 
stant, however,  individuals  of  any  species  frequently  being 
destitute  of  them,  a  peculiarity  due  to  the  "yellow  cells" 
not  being  really  constituent   parts  of  the  Radiolarian,  but 


Fig.  8. — Heliosphcera  actinota  "after  Haeckel  from  IIatschkk). 

foreign  bodies,  in  fact  unicellular  plants,  for  which  the  term 
Zooxantliellce  has  been  proposed.  They  cannot  be  consid- 
ered parasites,  since  they  do  not  appear  to  exist  at  the  ex- 
pense of  the  host,  but,  on  the  contrary,  their  presence  seems 
actually  to  be  beneficial.  Mutual  benefits  are  conferred  by 
the  plant  and  the  Radiolarian,  the  coexistence  constituting 
an  example  of  the  phenomenon  known  as  Symbiosis. 

Reproduction  in  the  lihizopods. — Throughout  all  the  groups 
the  simplest  form  of  reproduction,  fission,  is  probably  preva- 
lent (Fig.  9),  though  it  is  not  yet  definitely  known  to  occur  in 


aUBKINQDOM  PROTOZOA. 


21 


the  marine  Foramiuifera  nor  among  the  Kadiolaria.  In  the 
fresh-water  Foraminifera  and  Heliozoa  it  is,  however,  the 
usual  method  in  genera  both  with  and  without  shells. 
Where  the  shell  is  thin  it  may  be  divided  duriDg  the  process, 
but  where  it  is  thicker  the  protoplasm  divides  within  it,  one 
of  the  new  individuals  retaining  the  old  shell,  while  the  other 
wanders  forth  and  constructs  a  new  house  for  itself.  This  is 
the  case,  for  instance,  in  Arcella,  in  which  the  wandering  indi- 


FiG.  9.— Division  of  Ammba  (after  Schulze). 

vidual  protrudes  from  the  mouth  of  the  parent  shell  until  it 
forms  its  new  shell,  only  separating  when  this  is  accom- 
plished. 

Colonies,  produced  by  repeated  divisions  and  the  imper- 
fect separation  of  the  forms  so  produced,  are  occasionally 
formed,  but  they  are  simply  aggregations  of  similar  individ- 
uals, no  diflferentiation  or  individualization  of  the  colony  as  a 
whole  occurring.  Among  the  fresh-water  Rhizopods  this  is 
the  case  with  Microgromia,  a  shelled  form,  numerous  individ- 


• 


22 


Ilf  VEHTEBRATE  MOBPUOLOQ  Y. 


uala  of  which  may  remain  in  connection  with  one  another  by 
means  of  their  profusely-branching  pseudopodia.  Colonies 
of  ActinopTirys  are  also  formed  in  a  similar  manner,  and  among 
the  Radiolaria  the  forms  with  rudimentary  shells — such  as 
Sphcerozoum,  produce,  apparently  by  the  division  of  the  cen- 
tral capsule,  numerous  individuals  which  remain  in  contact. 

A  modification  of  fission  known  as  budding  or  gemmation 
also  occurs  in  some  forms.  It  differs  from  fission  only  in 
that  the  products  of  the  division  differ  in  size,  so  that  it  is 
possible  to  regard  the  larger  individual  as  the  parent  and 
the  one  or  more  smaller  ones  formed  from  it  by  budding  as 
the  progeny.  The  process  is,  however,  fundamentally  the 
same  as  fission  and  is  a  derivative  of  that  process.  In  Arcella 
bud-like  processes  arise  from  the  periphery  of  the  parent 
protoplasm,  separate,  and  assume  amoeboid  movement  leav- 
ing the  shell  in  an  ^mcefta-like  condition,  and  it  seems  prob- 
able that  the  marine  Foraminifera  and  certain  Heliozoa  re- 
produce in  a  similar  manner. 

Spore-formation  also  oc  ars,  the  parent  protoplasm  break- 
ing up  more  or  less  completely  into  a  number  of  small  por- 
tions termed  spores,  which  later  increase  in  size  and  assume 
the  characters  of  the  parent.  This  process  is  sometimes  pre- 
ceded by  encystment,  a  phenomenon  not,  however,  in  its  origin 
connected  with  reproduction.  It  is  more  prevalent  among 
fresh-water  than  among  marine  forms,  and  seems  to  have 
been  originally  developed  as  a  protection  from  injurious  ex- 
ternal conditions,  such  as  the  drying  up  of  the  pools  in  which 
the  organisms  live.  When  about  to  encyst  an  Amoeba,  for 
instance,  withdraws  its  pseudopodia  and  assumes  a  spherical 
shape,  and  then  secretes  a  more  or  less  dense  chitinous  case 
or  cyst  which  completely  encloses  it.  In  virtue  of  the  resist- 
ent  and  non-conductive  nature  of  the  cyst  the  organism  may, 
while  in  this  state,  suffer  uninjured  prolonged  exposure  to 
conditions  which  would  quickly  entail  the  death  of  the  non- 
encysted  individual,  and  on  the  return  of  favorable  condi- 
tions may  leave  the  cyst  and  reassume  its  active  life.  Occa- 
sionally, too,  encystment  may  occur  as  the  result  of  good 
nutrition,  an  individual  which  has  engulfed  a  number  of 
diatoms,  for  instance,  secreting  a  cyst  around  itself  within 


8UDKINGD0M  PROTOZOA. 


23 


which  it  remaius  uutil  the  food-matter  has  been  thoroughly 
digested,  when  the  cyst  is  tlirowu  off  together  with  the  empty 
diatom  shells  aud  the  auiinal  again  becomes  active. 

Plentiful  nutrition  aud  reproduction  by  division  (including 
under  this  term  the  various  modifications  of  fission)  are 
related  to  a  certain  extent,  and  it  is  easy  to  understand  why 
the  two  processes  of  encystment  and  spore-formation  should 
be  associated  together.  The  Heliozoan  Vampyrella  (Fig. 
10,  A)  feeds  in  its  active  condition  on  diatoms,  aud  especially 
on  a  stalked  form,  Gompltonema.  After  having  digested  the 
contents  of  the  diatom  frustules  which  it  engulfs  it  pushes 


.      ^.4cvijl 


■•'^.v 


Fig.  \S).— Vampyrella  (from  Haeckel  after  Butschli). 

A.  Vampyrella  feeding  upon  the  stalked  diatom  Gompltonema. 

B.  Vampyrella  encysted  upon  the  stalk  of  the  diatom. 

them  aside  and  encysts  itself  upon  the  stalk  previously  occu- 
pied by  them.  Within  the  cyst  the  animal  divides  into  four 
spores  (Fig.  10,  B),  each  of  which  escapin'g  from  the  cyst 
becomes  a  new  Vampyrella. 

Among  the  Radiolaria  spore-formation  seems  to  be  the 
most  usual  method  of  reproduction,  and  a  complication  occurs 
among  them  in  that  spores  of  two  kinds  may  be  formed.  In 
some  cases  the  spores,  which  are  formed  from  the  intracap- 
sular protoplasm,  are  all  equal  in  size  (isospores),  while  in 
others  some  of  the  spores  may  be  large  (macrospores)  and 
others  small  (microspores).  Both  macrospores  and  micro- 
spores may  be  formed  in  the  same  individual,  or  each  indi- 
vidual may  produce  only  one  of  the  two  forms.  In  such  cases 
it  is  easy  to  determine  whether  one  has  to  do  with  macro- 
spores or  isospores,  which  closely  resemble  each  other  in  size. 


24 


IN  VERTEBRA  TE  MORPUOLOG  Y. 


I   -!i 


V\ 


|)    ! 


from  the  fact  that  the  iaospores  are  spherical  in  shape  and 
each  possesses  r.  peculiar  whetstoue-like  crystal,  wanting  in 
the  macrospores.  All  the  spores  are  provided  with  single 
whip-like  processes,  flagella,  by  which  they  are  propelled 
through  the  water  when  set  free  from  the  parent. 

The  various  processes  so  far  mentioned  concern  a  single 
individual  only  and  are  therefore  non-sexual.  Whether  sex- 
ual reproduction,  the  union  of  two  individuals  (conjugation), 
occurs  among  the  Rhizopods  is  uncertain,  although  the  fusion 
of  two  individuals  preceding  spi/re-formation  has  been  ob- 
served in  several  instances.  That  the  fusion,  however,  is  the 
predisposing  cause  of  the  spore-formation  seems  probable, 
but  cannot  be  positively  asserted  until  the  behavior  of  the 
nuclei  of  the  two  fused  individuals  is  ascertained.  It  seems 
exceedingly  probable,  also,  that  the  macrospores  and  micro- 
spores of  the  Radiolaria  are  sexual  cells,  their  further  de- 
velopment depending  on  the  conjugation  of  a  micro-  with  a 
macrospore,  but  the  fate  of  these  spores  has  not  as  yet  been 
ascertained,  and  their  conjugation  can  only  be  imagined  from 
analogy  with  other  forms. 

II.  Class  Sfobozoa. 

The  Sporozoa,  which  constitute  the  second  class  of  Proto- 
zoa, are  all  parasitic,  living  in  the  cavities,  cells,  or  tissues  of 
other  animals  and  deriving  their  nutrition  from  their  hosts. 
At  present  much  is  lackint;  to  an  adequate  knowledge  of  the 
various  members  of  the  group,  but  at  least  three  orders  are 
to  be  recognized. 

1.  Order  Oregarinida. 

The  Gregarinida  include  some  of  the  largest  Sporozoa, 
and  are  parasitic  either  in  the  body-cavity,  intestine,  or 
organs  of  various  Invertebrata  (especially  in  Annelids  and 
Tracheata),  or  in  the  cells  especially  of  Vertebrated  Animals, 
these  intracellular  parasites  being  .usually  known  as  the 
Coccidia  in  contradistinction  to  the  former,  the  Gregarinida 
proper.  The  members  of  both  groups  show  a  marked  dilier- 
entiation  of  their  protoplasm  into  ectoplasm  and  endoplasm, 


8UBKINGD0M  PROTOZOA. 


25 


a  relatively  large  nucleus  lying  in  the  latter,  and  none  are 
known  to  possess  pseuilopotlia.     Indeed  in 
many    Gregarinida    a    well-marked    cuticle 
covers  the  exterior  of  the  body  (Fig.  11), 
sometimes  distinctly  striated  or  occasionally 
tuberculated.     The  Coccidia  and  many  Gre- 
garinida show  little  difl'erentiation  beyond 
what  has  been  mentioned,  but  the  Gregari- 
nida which  inhabit  Tracheate  hosts  usually 
present  the  appearance  of  being  composed 
of  two  cells,  owing  to  the  anterior  portion 
of  the  body  being  separated  by  a  partition 
of  ectoplasm  from  the  posterior  part,  and  in 
addition  to  this  the  anterior  moiety  in  some 
cases  is  furnished   with  hooks,  bristles,  or 
fiuger-like  processes  (Fig.  11)  of  use  in  fixing  ^^«;  il--f/i'P^<"%»- 
the  animal  to  the  walls  ot  the  cavity  m  which      (after  scunkidek). 
it  lives.     Even  in  these  cases,  however,  but 
a  single  nucleus  is   present  and   the   organism   is   unicellu- 
lar. 

Reproduction  is  carried  on  by  spore-formation,  preceded 
in  some  cases  by  conjugation  (Fig.  12),  but  simple  division 
or  gemmation  is  not  known  to  occv-,  apparent  instances  of 
division  being  more  probably  cases  of  conjugation.  In  spore- 
formation,  preceded  or  not  by  conjugation,  the  animal  as- 
sumes a  spherical  shape  and  forms  a  cyst  about  itself,  the 
greater  portion  of  the  protoplasm  splitting  up  into  usually 
a  number  of  nucleated  spores,  a  small  portion  of  it,  how- 
ever, remaining  undivided  {t'esiducd  body)  (Fig.  12).  When 
mature  the  spores  are  usually  spindle-  or  boat-shaped  and 
have  received  the  name  of  pseudonaviceUce.  They  do  not, 
however,  develop  directly  into  Gregarines,  but  their  proto- 
plasmic contents  break  up  into  2,  8,  or  more  crescentic 
spores  (Fig.  12),  a  residual  body  being  again  formed  as  in 
the  formation  of  pseudonavicella3.  The  further  history  of 
these  crescentic  spores  is  not  thoroughly  known,  but  in  some 
cases  {Pot'ospora  from  the  intestine  of  the  lobster)  each 
seems  to  become  converted  into  an  amoeboid  structure  which 
later  elongates  to  an  actively  moving  thread-like  organism, 


26 


IN  VKli TKBHA TE  MOUI'UOLOG  Y. 


I  ' 


;» 


i 


the  pseudqfilaria,  and  this,  gradually  losiug  its  motility,  de- 
velops into  the  adult  form. 


Pig.  13.— Repkoduction  of  Greoarine  (from  Hbbtwiq). 

1.  Clepsidrina  blattarum  in  conjugation;  ck  =  ectosarc,  en  =  endosarc.cji  = 

ciiticula,  pm  =  anterior  portion,  dm  =  posterior  portion,  7i  =  nucleus. 

2.  Cysts  in    transformation    into   pseudonavicellue;    pn  =  pseudonavicelliE; 

rk  =  residual  protoplasm. 

3.  A,  a  pseudonavicella  strongly  magnified;  B,  the  same  divided  into  spores, 

sk;  n  —  nucleus,  rk  =  residual  protoplasm. 


2.  Order  Myxosporidia. 

The  Myxosporidia  are  found  almost  exclusively  parasitic 
in  Fishes,  affecting  principally  the  skin,  but  also  occurring 
in  the  internal  organs,  such  as  kidneys,  spleen,  and  urinary 
bladder.  They  consist  of  irregularly-shaped  masses  of  pro- 
toplasm, sometimes  reaching  a  length  0.1  mm.,  but  usually 
falling  considerably  short  of  this  size.  Frequently  they  are 
enclosed  in  cysts  developed  from  the  tissues  of  the  host,  but 


SUBKINGDOM  PROTOZOA. 


27 


when  not  so  euclosetl  Heem  to  possess  the  power  of  sh)w 
uuKL'boid  movement.  The  eudophism  is  usually  well  ditler- 
entiated  from  the  ectf)phism  and  contains  in  the  adult  condi- 
tion a  larf,'e  number  of  minute  nuclei. 

lieproductiou  by  division  is  not  known  to  occur,  spore- 
formation  beinf?  the  only  method  as  yet  observed.  In  the 
Myxosporidium  occurriu*,'  in  the  uiiuary  bladder  of  the 
Pike  the  protoplasm  breaks  up  into  a  number  of  spherical 
masses  each  containing  a  number  of  nuclei.  The  fate  of 
all  of  these  masses  is  not  known,  but  some,  containinj^  only 
six  nuclei,  form  a  wall  about  themselves  and  divide  into 
two  portions  each  of  which  contains  three  nuclei.  These 
trinucleated  bodies  elouj^ate,  develop  a  wall,  and  become 
pseudonavicella-like  spores,  one  of  the  three  nuclei  per- 
sistinj^  as  the  spore-nucleus,  while  the  other  two,  situated 
at  the  extremities  of  the  spore,  seem  to  give  rise  to  a  sac-like 
structure  containing  within  its  interior  a  spirally  rolled  lila- 
meut  which  is  emitted  when  the  spore  is  subjected  to  press- 
ure and  probably  serves  for  the  fixation  of  the  spore  to  the 
body  of  a  host.  The  further  history  of  the  spores  is  not 
thoroughly  known,  but  it  seems  probable  that  the  contents 
escape  as  amoeboid  masses  which  develop  into  adult  Mj-xo- 
sporidia. 

In  many  respects  the  Myxosporidi.'v  resemble  closely  the  Gregarinida, 
but  the  possibility  of  their  being  in  reality  not  of  an  animal  but  of  a 
jjlant  nature  must  not  be  overlooked.  By  some  authors  their  nearest  re- 
lations have  been  found  in  the  Myxoraycetous  and  Chytridiaceous  fungi,  a 
view  which  certainly  has  not  a  little  to  recommend  it. 


3.  Order  Sarcosporidia. 

The  Sarcosporidia  are,  with  a  single  exception,  parasites 
in  the  muscle-tissue  of  warm-blooded  animals,  especially 
of  Mammalia,  being  found  in  the  interior  of  the  jjrimitive 
fibrils  of  the  striated  muscles,  whose  contents  they  more  or 
less  destroy. 

They  form  somewhat  elongated  sacs  1-2  mm.  in  length, 
the  wall  of  the  sac  being  formed  of  a  distinct  membrane 
which  has  the  appearance  of  being  covered  with  fine  bristles. 
The   contents  of  the  sac  consist  of  a  protoplasmic   ground- 


28 


IN  VERTEDHA  TE  MORVUOLOG  Y. 


! 


■  !    i 
'    i 


subBtance  in  which  n  large  number  of  nuclei  are  imbedded, 
sometimes  aggre^^ated  into  masses  each  of  which  is  sur< 
rounded  by  a  delicate  membrane.  It  seems  probable  that 
these  masses  represent  a  process  of  spore-formation,  but  as 
yet  nothing  is  known  regarding  the  further  development  of 
the  spores. 

III.  Class  Flaoellata. 

The  Flagellates  are  characterized  by  the  possession  of 
one  or  more  long  filamentous  processes  of  protoplasm,  known 
iin  flagellar  which,  by  whip-like  movements,  propel  the  organ- 
isms through  the  water  in  which  they  live,  and  at  the  same 
time  by  the  production  of  currents  in  the  water  bring  food- 
particles  within  their  reach.  Some  forms  possess  pseudo- 
podia  in  addition  to  the  flagella,  which  are  indeed  simply  at- 
tenuated and  mobile  pseudopodia,  but  the  majority  have  a 
more  or  less  permanent  body-form.  This  in  many  species 
is  accompanied  by  the  formation  at  the  exterior  of  the  body 
of  a  skin  or  cuticle  which  in  some  cases,  as  in  the  Dino- 
fiagellata,  may  assume  a  sufficient  density  and  thickness  to 
entitle  it  to  be  termed  a  shell. 

1.  Order  Autoflagellata. 

In  the  Autoflagellata  the  body  is  usually  more  or  less 
oval,  and  while  in  many  forms  it  is  naked  and  capable  of 
changing  form  (Fig.  13,  A),  yet  in  others  special  cuticular  in- 
vestments may  be  present,  taking  the  form  in  some  cases  of 
a  simple  cuticular  covering,  as  in  Euglena  (Fig.  13,  £),  in 
others  forming  a  stalk  by  which  the  organism  is  attached 
to  a  foreigu  body ;  in  some  forms,  as  in  Codosiga  (Fig.  13,  6'), 
a  cuticular  collar  surrounding  the  base  of  the  flagellum  is 
present,  wbile  in  others,  such  as  Dinobryon,  a  cup  is  formed, 
within  which  the  organism  lives. 

Usually  but  one  or  two  whip-like  flagella  are  present, 
though  occasionally  a  larger  number  (6  or  8)  may  occur,  and 
in  some  instances  one  or  more  may  assume  a  firmer  character 
and  serve  for  fixation  of  the  organism.  All  forms  possess 
a  single  nucleus  and  a  contractile  vacuole.     In  the  simpler 


8UDKIN0D0M  PROTOZOA. 


29 


forms,  such  as  Motms,  iu  which  no  cuticle  is  deveh)pe(l,  uo 
special  luouth-oritice  is  present,  though  the  in- 
gestion of  food  tiikes  place  at  a  more  or  less 
deMuitely  localized  region  at  the  base  of  the 
Hagellum,  the  food-particles  drawn  to  the 
organism  by  the  currents  established  by  the 
ilagellum  usually  impinging  at  this  point; 
where,  however,  a  detinito  cuticle  or  shell  is 
developed  a  definite  mouth  occurs,  and  in 
some  cases,  as  Euglena  (Fig.  13,  B),  this  leads 
into  a  distinct  tubular  pharynx  projecting  some 
distance  into  the  interior.  No  hollow  digestive 
tract  is,  however,  present,  but  the  food-parti- 
cles, after  traversing  the  gullet,  are  received 
directly  into  the  protoplasm  of  the  body,  and 
are  digested  there  as  in  Amceha.  A  localized 
egestive  region,  situated  usually  towards  the 
posterior  end  of  the  body,  has  been  ascer- 
tained to  occur  in  some  species,  but  in  no 
instance  is  it  a  permanent  orifice,  as  is  the 
case  with  the  mouth.  In  addition  to  the  nucleus,  contractile 
vacuole,  and  food-particles,  other  definitely  organized  particles, 
such  as  starch-like  granules  and  pigment-granules,  may  be 
imbedded  iu  the  protoplasm.  In  Euglena  the  pigment  is 
green  and  resembles  plant-chlorophyll,  probably  too  possess- 
ing a  similar  function.  A  red  pigment-spot  (stigma)  is  rise 
present  iu  this  and  other  genera  at  the  base  of  the  flagellum 
and  is  supposed  to  be  concerned  in  light-perception. 

The  typical  Flagellate  is  a  free-swimming  single  organism, 
but  many  forms  are  fixed,  developing  a  stalk  by  which  they 
are  fastened  to  foreign  bodies  ;  the  stalk  may  be  very  much 
branched,  each  terminal  branch  supporting  an  individual,  the 
whole  thus  forming  a  colony,  without,  however,  any  differ- 
entiation among  the  individuals.  Free-swimming  colonies 
also  exist,  such  for  example  as  Volvox^  in  which  a  large 
number  of  individuals  are  grouped  together  to  form  a  spheri- 
cal hollow  colony.  Each  individual  contains  chlorophyll- 
granules  and  a  red  stigma,  and  is  provided  with  two  fla- 
gella  by  the  action  of  which  the  entire  colony  is  propelled 


Fig.  13. 

A,  Oikomonaa 
(after  Bt    '<('hli). 

B,  Eugle  mt  acua. 
G,  Codosiyu 
(after  BuTBi  "     '. 


V\ 


m 


HO 


INVEliTEIiUATB  MOIiPlIOLOG Y. 


II 


a 


through  the  water  with  a  rotatory  motion.  The  rotation  is 
around  a  definite  axis,  one  portion  of  the  spherical  coh)ny 
always  being  in  front  in  progression,  and  it  is  notieeab)e  that 
the  stigmata  of  the  individuals  of  this  anterior  hemisphere 
are  slightly  larger  than  those  of  the  colls  of  the  posterior 
hemisphere,  a  slight  differentiation  of  the  individuals  being 
thus  present. 

2.  Order  Dinoflagellata. 

The  Dinoflagellata  are  distinguished  from  the  members  of 
the  i)receding  order  by  the  almost  general  occurrence  of  .« 
rather  dense  shell  c()mi)()sed  of  plates  of  a  substance  resem- 
bling closely  vegetable  cellulose.     Some  of  the  forms,  such  as 

Cemtinm  (Fig.  14),  ])resent  a  rather 
bizarre  shape  on  account  of  the  shell 
being  prolonged  into  horns,  and  in  the 
majority  the  shell-plates  are  delicately 
scul])tured,  while  around  the  equator 
of  the  shell  runs  a  furrow,  and  from 
an  opening  in  the  line  of  the  furrow 
two  tlagella  protrude,  one  of  which 
possesses  the  ordinary  whip-like  char- 
acter, while  the  other  lies  in  the  fur- 
row and  in  some  cases  has  tlie  form 
Fig.  W.—Ctintium  tripos  of  a  delicate  undulating  band.  Chlo- 
(a combination  of  two  flRures  rophylKUke  ])igment  is  almost  invari- 

ably  present,  as  is  also  the  red  stigma. 
Peculiar  cysts  are  also  present  in  the  ])rotoplasm  of  many 
forms,  consisting  of  a  hollow  capsule  having  rolled  uj)  within 
it  a  hollow  thread,  which  on  occasion  may  be  ra])idly  evagi- 
iiated  and  no  doubt  has  u  protective  function,  resembling  very 
closely  in  its  structure  the  nematocysts  of  the  Caileuterates. 

3.  Order  Cystoflagellata. 

The  order  of  the  Cystoflagellata  includes  only  two  -,  era, 
NodUuca  and  Leptodiscus.  The  latter  is  a  somewhat  disk-like 
structure  nearly  2  mm.  in  diameter,  while  Noctilma  (Fig.  15) 
is  almost  globular  with  a  slight  depression  at  one  point  where 


SUIiKINODOM  riiOTOZOA. 


'.n 


-\-tx 


Ful.    15.  —  Nociilnca  miliaria 
(utter  Ui'TsciiLi). 

t  =  tentacle.         n  =  muk'iis. 


the  flagella  are  situated,  and  at  the  bottou  of  which  is  situated 
the  in()uth-opeuiu<:,.  A^odiluca  has  the  form  of  a  c^'st,  pos- 
sessing au  external  thin  luembraue- 
like  outer  wall,  to  whieh  branehinj^ 
strands  of  piot()i)lasni  extend  from 
the  central  mass  eoiitainin"^  the 
nucleus  and  lying  sli«;litly  below  the 
depression  which  contains  the  fia- 
fjfella.  These  are  two  in  number, 
one  beinj^  short  and  whip-like,  while 
the  other,  usually  known  as  the 
"  tentacle  "  (Fij^.  15,  /),  is  a  hi<^hly 
contractile,  somewhat  Hattened,  and, 
relatively  to  the  Haj^ellum,  thick  })ro- 
cess  of  the  internal  protoplasm. 
This  structure  is  unreprcsenttnl  in  Lepfodi'scus,  Avhich  other- 
wise closely  resembles  Nodilncu. 

Nocliluca  is  of  consi(U!r;il)l(!  pliysiolojijieal  interest,  sineo  it  is  one  of  tlio 
forms  to  wliich  tlie  i)iiosi)lior(*s(!eiiee  of  tlio  oeeiin  is  due.  Tlio  cause  of  tiio 
li^lit  and  its  cluiracter  are,  however,  as  y? '  unknown. 

Reprodiivt'um  in  the  Fhau'lhttit. — The  most  frequent  m(;thod 
of  reproduction  in  all  the  orders  of  the  Flagellates  is  simple 
division,  either  transverse  or  longitudinal.  Encystment,  fol- 
lowed or  not  as  the  case  may  be  by  spore-formation,  is  also 
common,  and  when  accompanied  by  spore-formation  may  be 
preceded  by  the  conjugation  and  fusion  of  two  individuals. 
lu  Cercomonaa  the  spores  are  exceedingly  abundant  and  small, 
presenting  the  appearance  of  minute  granules  even  under  the 
hij^hest  powers  of  the  microscope,  but  in  other  forms,  as 
O/damydomoiuvi,  the  spores  are  larger  and  much  fewer  in  num- 
ber, being  only  4:  or  8  in  this  particular  case.  An  interesting 
moditication  occurs  in  closely-related  species  (Fig.  10),  some 
individuals  of  which  divide  into  a  number  of  snnill  spores 
{microspores),  while  others  undergo  a  more  restricted  division 
and  give  rise  to  a  few  aarge  s|)ores  [tuacrospores).  The  latter 
develop  directly  into  the  adult  forms,  but  the  micros])ore8 
show  a  tendency  to  conjugate  in  pairs  before  undergoing 
further  development.  This  ditt'erentiation  of  two  kinds  of 
spores  is  carried  still  farther  in  ()Lher  forms  where  neither 


i1 


32 


INVERTEBRATE  MORPHOLOGY. 


macro-  nor  microspores  develop  directly  but  further  develop- 
ment is  contiugeut  upon  the  conjugation  of  a  micro-  with  a 
macrospore. 

In  this  respect  considerable  interest  attaches  to  Volvox ; 
certain  cells,  usually  those  situated  in  the  posterior  hemi- 
sphere, enlarge  and  project  into  the  interior  cavity,  dividing 


Fig.  16. — 1.  Phacotus  lenticiUaria ;   2.  MACRosPORFh  Ayr  Microspores  op 

THE  SAME  Species  (after  ButschliX 

8ch  =  sbell.  n  =  nucleus. 

when  they  have  reached  their  full  growth  into  a  number  of 
cells  which  arrange  themselves  in  a  hollow  sphere  forming 
daughter  colonies  in  the  interior  of  the  parent.  In  addition 
to  this  a  sexual  process  occurs  ushered  in  by  certain  indi- 
viduals gradually  enlarging,  and  leaving  their  position  at  the 
surface  of  the  colony.  In  the  interior  some  of  them  continue 
to  enlarge,  forming  ova  (macrospores),  while  others  divide 
frequently,  forming  packets  of  elongated  cells  furnishod  with 
flagella;  these  may  be  termed  apermatozoa  (microspores K  Tiie 
ova  develop  into  colonies  similar  to  the  parent  aft«  i*  -oujuga- 
tion  with  spermatozoa.  Since  many  of  the  cells  of  flie  ].;  oeiifc 
colony  do  not  participate  in  this  reproductive  act,  but  tlism- 
tegrate  and  die  on  the  development  of  the  daughter  colonies, 
it  is  clear  that  we  have  in  this  form  a  rather  marked  differ- 
entiation of  the  individuals  of  the  colony,  the  individualities 
of  the  constituent  cells  being  to  a  slight  extent  merged  in  the 
individuality  of  the  colony. 

In  Not'tilum  in  addition  to  simple  division  a  process  of  i  ;» iKluctiou 
occurs  which  partakes  of  the  character  of  budding.     It  is  app.t  v'ltiy  pro 


SUBKINGDOM  PROTOZOA. 


33 


ceded  by  the  conjugation  of  two  individuals,  the  combined  central  proto- 
plasms coming  to  tlie  surface  of  tlie  cyst  where  tliey  form  a  protuberance. 
Repeated  division  of  the  tiucleus  into  2,  4,  S,  etc.,  up  to  256  or  more  now 
takes  place  accompanied  by  only  a  partial  division  of  tlie  protoplasm,  so 
that  the  surface  of  the  protuberance  is  covered  by  a  large  number  of  bud- 
like structures.  Eventually  these  separate,  develop  a  tiagellum,  and  take 
on  the  character  of  motile  spores.  Their  further  development  into  the 
adult  Noctiluca  has,  however,  not  yet  been  followed. 


IV.  Class  Infusoria. 

The  Infusoria  are  the  most  highly  specialized  of  all  the 
Protozoa,  showing  a  differeutiatiou  of  the  protoplasm  uuat- 
taiued  by  other  members  of  the  group.  They  are  character- 
ized by  the  possession  during  the  whole  or  part  of  their  lives 
of  numerous  delicate  short  motile  hair-like  processes  termed 
cilia  by  means  of  which  locomotion  is  performed  and  food 
procured.  In  one  of  the  orders  into  which  the  class  may  be 
divided,  the  Ciliata,  these  structures  are  present  during  the 
adult  life  of  the  organisms,  while  in  the  other,  the  Suctoria, 
though  present  in  the  young  stages  they  are  replaced  later  by 
immovable  processes  of  the  body,  which  extract  the  nourish- 
ment from  the  food-particles  which  come  into  contact  with 
them. 


1.  Order  Ciliata. 

The  Ciliata  are  for  the  most  part  free-swimming  organ- 
isms, though  some,  e.g.  Vorttcella  (Fig.  17,  C),  adhere  to  foreign 
bodies  by  means  of  a  stalk,  similar  to  that  found  in  Flagel- 
lates, and  colonial  stalked  forms  also  occur  as  in  that  class. 
In  these  stalked  forms  the  body  is  enveloped  in  a  chitinous 
case,  of  which  the  stalk  is  a  prolongation,  the  surface  oppo- 
site the  stalk  being,  however,  left  naked  and  being  surrounded 
by  cilia  which  are  absent  on  the  portions  of  the  body  pro- 
tected by  the  chitin  {Peritrichous  forms,  Fig.  17,  C).  In  the 
free-swimming  forms,  however,  the  cilia  are  more  universally 
distributed,  covering  either  the  entire  surface  {Holotrichous 
forms.  Fig.  17,  A)  or  else  one  surface  of  the  flattened  body, 
some  of  them  in  this  case  being  modified  into  stout  movable 


84 


IN  VEUTEBliA  TE  MORPHOLOQ  Y. 


■  \ 


bristles  upon  which  the  animal  creeps  {Hypotrichous  forms, 

A  definitely  localized  mouth-opeuing  is  always  present, 
situated  frequently  at  the  extremity  of  a  peristoraial  groove 
and  leading  into  a  gullet  of  variable  extent,  usually  lined  by 
cilia,  though  sometimes  furnished  with  a  chitinous  support 


Fig.  17.— a,  Paramoecium  ;  B,  Stentor;  C,  Vm'tieella  ;  D,Euplotes. 
cv   =  contractile  vacuole.  n  =  uucleus. 


m   =  mouth. 
my  =  myophaiie. 


«'  =  micronucleus. 
tr  =■  trichocyst. 


(ChUodon).  There  is,  however,  no  special  digestive  tract,  the 
food-particles  after  traversing  the  gullet  being  received  into 
the  body-protoplasm,  where  they  are  digested.  Usually  there 
is  a  localized  ogestive  region,  and  in  a  few  cases  there  is  a 
definite  anal  opening.  The  food  is  procured  as  in  the  Flag- 
ellates by  the  currents  set  uj)  in  the  water  by  the  cilia  carry- 


8UBKIN0D0M  PROTOZOA. 


35 


w  forms, 

present, 
1  groove 
ined  by 
support 


es. 


i,  the 
iuto 
lere 
in  a 
"lag- 
rry- 


iug  minute  organisms  to  the  neighborhood  of  the  mouth,  the 
cilia  surrounding  this  opening  directing  them  to  the  gullet. 

The  body-protoplasm  is  usually  very  granular  in  its  cen- 
tr.al  part,  and  tilled  with  food-vacuoles  and  products  of  diges- 
tion. Pigment-granules  are  sometimes  i)resent  and  may  con- 
sist of  Chlorophyll,  as  in  Sfetifor,  and  one  or  more  excretory 
contractile  vacuoles  are  always  present.  The  nucleus  is  usu- 
ally single,  though  occasionally  two  are  present,  and  in  the 
genus  Opalina,  which  occurs  in  the  intestine  of  the  Frog,  they 
are  numerous  in  the  adult  condition.  When  single  the  nu- 
cleus may  be  very  large  and  either  spherical,  elongated, 
horseshoe-shaped  as  in  Vorticella  (Fig.  17,  c),  moniliform  as  in 
)Stento7'  (Fig.  17,  B),  or  otherwise  shaped.  In  addition  to  the 
nucleus  there  are  one  or  two  minute  structures  usually  to  be 
found  in  its  vicinity  which  play  an  important  part  in  repro- 
duction and  are  known  as  microniiclei  (Fig.  17,  A,  n).  Other 
differentiations  of  the  protoplasm  are  also  found  in  certain 
forms,  as,  for  instance,  special  bands  differentiated  so  as  to  be 
specially  contractile  and  therefore  corresponding  in  function 
to  the  muscles  of  the  higher  animals,  and  hence  termed  myo- 
phanes.  In  Vorticella  a  more  striking  differentiation  of  spe- 
cially contractile  protoplasm  occurs  (Fig.  17,  C,my) ;  runniL^- 
in  an  open  spiral  through  the  centre  of  the  supporting  stalk 
of  this  organism  is  a  strong  myophane  terminating  above  in 
the  protoplasm  of  the  animal.  When  the  latter  is  stimulated 
the  myophane  contracts,  coiling  the  stalk  iuto  a  close  spiral 
and  withdraAviug  the  animal  from  the  source  of  irritation.  In 
some  of  the  Holotricha,  such  as  Pm'amcecium,  numerous  mi- 
nute  rod-like  structures  occur  imbedded  in  the  protoplasm 
near  the  surface  of  the  body  (Fig.  17,  A,  tr).  They  are  appar- 
ently defensive  in  function,  since  when  stimulated  they  sud- 
denly, as  if  by  an  explosive  action,  become  transformed  into 
long  threads  or  needle-like  structures  projecting  beyond  the 
cilia.     These  trichocyds  also  occur  in  some  Flagellates. 

2.  Order  Suctoria. 

The   Suctoria  lack  the  active  movements  of  the  Ciliata, 
being  destitute  in  the  adult  stage  of  cilia,  and  many  of  the 


36 


INVERTEBRATE  MORPHOLOGY. 


s 


Fig.  Vd.—Acineta 
grandis  (after 
Saville-Kent). 


forms,  e.g.  Acineta  (Fig.  18),  are  attached  to  foreign  bodies  by 
a  stalk.  They  do  not  possess  any  mouth,  but  a  number  of 
simple  or  branched  stiff  processes  project 
from  the  body  which  serve  for  the  prehension 
and  digestion  of  the  organisms  upon  which 
they  feed.  A  contractile  vacuole,  nucleus,  and 
micronucleus  are  always  present,  the  nucleus 
having  sometimes  a  very  complicated  shape. 
It  seems  pretty  clear  that  they  have  been  de- 
rived from  the  Ciliata,  since  in  their  young 
stages  they  are  free-swimming  ciliated  struc- 
tures ;  the  tentacular  processes  have  been 
compared  to  the  pseudopodia  of  the  Rhizo- 
pods,  but  good  reasons  for  such  an  homology 
do  not  exist,  and  it  is  more  probable  that  they 
are  structures  peculiar  to  the  group. 

The  Reproduction  of  the  Infusoria. — In  the 
Infusoria  the  reproductive  processes  reach  a 
much  higher  grade  of  complication  than  occurs 
in  other  Protozoa,  though  the  simple  pro- 
cesses of  fission  and  spore-formation  likewise  occur.  The 
former  occurs  in  the  majority  of  forms,  and  may  be  the  only 
mode  of  reproduction  occurring  throughout  a  number  of  gen- 
erations. Long-continued  fission  seems,  however,  to  lead  in 
iuany  cases  to  structural  and  physiological  derangements, 
unless  the  process  of  conjugation  be  interposed. 

Encystment  is  also  of  frequent  occurrence  and  may  occur 
under  various  conditions.  In  Colpoda,  in  which  the  process 
has  been  most  thoroughly  studied,  encystment  may  or  mr  y 
not  be  followed  by  reproduction.  In  the  latter  case  the  cyst, 
a  resting  cyst,  is  perfectly  closed,  and  the  walls  are  thick  and 
resisteut  so  as  to  withstand  unfavorable  conditions,  such  as 
insufficient  aeration  or  dryness.  When  reproduction  is  asso- 
ciated with  encystment  it  may  be  eithi^r  fission  or  spore-forma- 
tion. The  division  cyst  is  thin-walled  and  is  not  completely 
closed,  and  within  it  the  animal  undergoes  division  into  two 
or  four  parts.  In  spore-formation  a  thin  cyst  is  first  formed, 
within  which  the  animal  slowly  rotates,  at  the  same  time 
gradually  growing  smaller  by  the  expulsion  of  fluid.     Finally 


1    ■! 


8UBKIN0D0M  PROTOZOA. 


37 


it  contracts  to  a  rouud  mass  aud  surrounds  itself  with  a  sec- 
ond cyst  within  the  first.  At  the  surface  of  the  encysted 
animal  from  eight  to  thirty  minute  spherical  and  highly  re- 
fractive bodies  ajjpear  Avhich  are  the  spores,  and  by  the 
bursting  of  the  cyst  they,  with  the  remains  of  the  protoplasm 
in  which  they  arose,  escape  to  the  exterior  and  soon  begin  to 
develop.  Losing  its  spherical  shape  each  spore  becomes 
amceboid ;  then,  urawiug  in  all  the  pseudopodia  but  one, 
which  elongates  and  becomes  a  flagellum,  it  passes  from  the 
Ehi?;opod  to  the  Flagellate  stage ;  and  finally  the  flagellum  is 
withdrawn,  cilia  appear,  and  the  animal  gradually  assumes 
the  adult  form.  Spore-development  somewhat  similar  to  this 
has  been  observed  also  in  Vorticdla,  and  special  interest  at- 
taches to  it  as  probably  indicating  the  line  of  descent  of  the 
Infusoria. 

Conjugation  is  a  frequent  process  among  the  Infusoria, 
where  it  seems  to  have  a  rejuvenating  rather  than  a  strictly 
reproductive  function.  If  prevented,  and  fission  goes  on 
through  a  number  of  generations,  marked  degeneration  en- 
sues ;  while  if  it  be  allowed,  the  same  number  of  generations 
may  be  produced  Avithout  any  signs  of  degeneration.  The 
process  consists  of  a  renewal  of  the  nuclei  and  micronuclei  of 
the  conjugating  forms,  and  the  process  as  it  occurs  in  Colpid- 
ium  colpoda  may  be  described  thus.  Two  individuals  come 
into  contact  by  the  anterior  portions  of  their  body,  actual 
fusion  of  the  two  protoplasms  taking  place  at  the  point  of 
contact.  The  micrcmucleus  in  each  individual  then  enlarges 
and  divides,  the  two  thus  formed  subsequently  dividing  again, 
so  that  each  of  the  conjugating  individuals  contains  four 
micronuclei  and  one  nucleus.  One  of  the  four  micronuclei 
iu  each  individual  now  divides,  and  one  of  the  two  thus 
formed  (the  male  pronucleus)  crosses  over  to  the  other  indi- 
vidual and  unites  with  the  other  product  of  the  division,  the 
female  pronucleus,  there  being  thus  a  mutual  interchange  of 
micronuclei.  The  individuals  now  separate  and  resume  their 
independent  existences,  and  a  rearrangement  of  the  nuclear 
structures  accompanied  by  fission  takes  place.  The  three 
micronuclei  which  did  not  take  part  in  the  formation  of  the 
pronuclei  of  conjugation  degenerate,  as  does  also  the  original 


liii 


38 


IN  VERTEBRA  TE  MORPIIOLOQ  Y. 


nucleus.  The  conjugation  micronucleus,  formed  by  the  fusion 
of  the  male  and  female  pronuclei,  divides  twice,  forming  four 
micronuclei,  and  this  is  followed  by  a  fission  of  the  entire 
Infusorian,  each  of  the  daughter  forms  so  produced  possessing 
two  micronuclei.  One  of  these,  enlarging,  becomes  the  new 
nucleus,  while  the  other  remains  as  the  micronucleus.  This 
complicated  process  may  perhaps  be  better  followed  in  the 
accompanying  diagram  (Fig.  19). 


Fig.  19.— Diagram  to  Illustbate  the  Behavior  op  the  Nuclei  and 
Micronuclei  during  Conjugation  in  Infusoria  (after  Maupas). 

In  the  majority  of  forms  the  conjugation  is  a  temporary 
process,  the  two  individuals  separating  after  the  exchange  of 
pronuclei.  In  Vorticella,  however,  a  permanent  fusion  occurs. 
By  repeated  longitudinal  fission  a  Vorticella  becomes  divided 
into  a  number  of  small  individuals  which  leave  their  stalks 
and  swim  about  freely  in  the  water.  Should  one  of  them 
come  into  contact  with  a  large  individual  a  complete  and 
permanent  fusion  of  the  small  with  the  large  one  occurs. 

SUBKINGDOM  PROTOZOA. 

I.  Class  Rhizopoda. — Protozoa,  with  lobe-like  or  filamentous  pseudopodia. 
1.  Order  Foraminifera. — Pseudopodia  without  axial  support;  shell 
when  present  horny  or  calcareous, 
(a)  Shell  absent.     Amceha. 
(6)  Shell  horny.     Arcella,  Euglypha. 

(c)  Shell  of  foreign  particles  cemented  together.    Diffltigia. 

(d)  Shell  calcareous,  imperforate.     Gromia. 

(e)  Shell  calcareous,  perforate.    Miliola,  Nodosaria,  Textu- 

laria,  Globigerina,  Eotula,  Acertndaria. 


8UBK1NGD0M  PROTOZOA. 


3d 


2.  Order  //e/^2c»f/.— Psoiulopodia  slender,  with  axial  support;  shell  if 

present  siliceous  ;  no  central  capsule. 

(a)  Shell  wanting.    Actinophrys,  Actinosphcerium,   Vampy- 

rella,  Jncivf/roDi  hi. 
(ft)  Shell  present.     (Jlathnilina. 

3.  Order  iiac/ /«/«/m.—P.scudopodia  slender  with  axial  support;  shell 

usually  present  and  siliceous   (rarely  horny) ;  central  capsule 
present. 

(a)  Shell  wanting.     Thnlasskolla,  Sphcrrozoon. 

(b)  Shell  siliceous.     Actiuomma,  Heliosphicra. 

(c)  Shell  horny.     Amnthometra. 

II.  Class  Sporozoa.— Parasitic  ;  without  pseudopodia,  fiagella  or  cilia. 

1.  Order  Gr€(jarinUla.~-l^a.vix^\i\c  in  cavities  of  the  body  especially  of 

Invertebra<^f)s  or  in  the  cells  especially  of  Vertebrates. 

2.  Order  Myxosporiuia.—ViWix^itxyi  usually  in  the  skin,  sometimes  in 

internal  organs  of  fishes. 

3.  Order  ^'a/-cospo/7V/m.— Parasitic  in  the  muscle-fibres  of  Mammalia. 

III.  Class  Flagellata.  Provided  with  one  or  more  flagella. 

1.  Order  Autoflagellata.—'^ [ihowt  shell,  protoplasm  not  especially 

vacuolated. 

(a)  Without  collar.— J/o/m5,   Cercomonas,  Chlamydomonas, 
Euglena,  Volvox. 

(b)  With  coWin-.—Codosiga,  Dinobryon. 

2.  Order  Dino/layellata.— With  shell  composed  of  cellulose.     Cera- 

Hum. 

3.  Order  Cystqftagellata.— Without  shell,  protoplasm  highly  vacuo- 

lated, marine.    NoctiUtca,  Leptodiscus. 

IV.  Class  INFUSOKIA.— Provided  with  cilia  or  immovable  processes. 

1.  Order  C/Zmto.— Provided  with  cilia  in  adult  stage. 

(a)  Cilia  of  nearly  uniform  length  all  over  the  body  (Holo- 
triclia).  Faramcecium,  Colpoda,  Colpidium,  Chilodon, 
Opalina. 

(6)  Cilia  around  anterior  end  of  body  longer  than  the  rest 
(Heterotricha).     Stentor. 

(c)  Cilia  limited  to  anterior  end  of  body  (Peritricha).    Vorti- 

cella. 

(d)  Cilia  or  setae  only  on  ventral  surface  of  the  body  {Hypo- 
tricha).     Stylonychia. 

2,  Order  Suctoria.— With  cilia  only  in  the  young  stages,  in  the  adult 

with  immovable  processes.     Podophrya,  Acineta. 


LITERATURE. 

0.  Biitsoh.'i.     Protozoa.    Brnnn'sKlassen  u.Ordnungen  desThierreichs.    Leip- 
zig u,  Heidelberg,  1883-87. 
W.  S.  Kent.     A  Manual  of  the  Infusoria.     London,  1880-82. 


!■    :  I 


40 


IN  VKliVEBltATE  MOHPUOLOGY. 


TJ.  S.  Geological  Sur- 


J.  Leldy.     Freah-imtn-  Jihizopoda  of  North  America. 
vey  of  the  Territories,  xii.  1879. 

*•  ^T^^^-  .?'"»*^^«'*^^'*  ««'•  Organimtiou  and  syatematisehen  Stellung  der 

I*oramniferen.    Jenaische  Zeitscbr.,  x.  1«76. 
E.  Haeckel.     JJie  liadiulaneu.     Miie  Monographie.     Berlin   1863-88 
K.  Brandt     m  Kolonie-bildenden  Radiolanen  {Sphatrozo'eu)  des  Oolfes  von 

Neapel     Fauna  u.  Flora  de«  Go)  fes  von  Neapei.    Monographie.  xiii.  1885. 
*.  Biein.     JJer  Organwnua  der  Infuaionathiere.    Leipzifj,  1859-79. 
E.  Maupas.     La  rajeuntasement  ka/ryogamique  chez  lea  Viliea.    Archives  de  Zool 

experimeutale,  2"'«  Ser.  vii.  1889. 
L.  Ehumbler     Die    verachiedenen    Oyatenbildungen    and  die   Entwicklungage- 

achichte  der  holotnclien  Infmorungattung  Colpoda.      Zeitschr    f Ur  wis- 

sensch.  Zoologie,  XLvi.  1888. 

*"  ^1*876^*"     ^'^'^  P''<^f>'rya  gemmipara,  etc.    Morpholog.  Jahrbuch.   i. 


I 


sur- 
der 


aUBKlNUDOM  METAZOA. 


41 


CHAPTER  III. 

SUBKINGDOM  METAZOA. 

The  Metazoa  are  equivalent  to  colonies  of  Protozoa,  the 
individual  cells  of  which  have  differentiated  in  various  direc- 
tions, some  being  more  especially  contractile,  others  nutritive, 
others  irritable,  others  reproductive,  etc.,  instead  of  each  one 
for  itself  performing  equally  all  the  functions  necessary  for 
existence.  A  physiological  division  of  labor  of  a  more  or  less 
perfect  kind  is  introduced  among  the  individuals  composing 
the  colony,  and  the  welfare  of  each  individual  becomes  de- 
pendent upon  the  proper  performance  by  its  colleagues  of 
their  special  functions ;  in  short,  the  individualities  of  the 
component  cells  are  merged  in  the  higher  individuality  of 
the  whole  organism. 

Phj'siologically  a  Metazoon  is  equivalent  to  a  Protozoon, 
but  morphologically  it  is  the  equivalent  of  a  large  number  of 
them.  Each  is  physiologically  an  individual,  but  morpholog- 
ically the  Metazoon  is  a  colony  of  Protozoan  individuals.  To 
harmonize  the  physiological  and  morphological  conceptions 
of  an  individual  it  is  necessary  to  recognize  several  grades 
of  morphological  individuality  of  which  the  cell  may  be  as- 
sumed to  be  the  lowest.  In  the  Metazoa  the  physiological 
differentiations  of  the  cell-individuals  are  accompanied  by 
structural  differentiations,  so  that  it  is  possible,  as  a  rule,  to 
determine  from  its  structure  what  the  function  of  a  cell  may 
be  ;  aggregates  of  similar  cells  are  termed  tissues  or  tissue-indi- 
viduals,  and  as  the  simplest  Metazoa  are  complexes  of  various 
tissues,  such  a  complex  f  ;vi  is  the  third  grade  of  individu- 
ality and  may  be  termed  an  Organ-indt  'idual.  A  complex  of 
organ-individuals  united  to  form  a  phy^! jlogical  unit  consti- 
tutes an  individual  of  the  third  grade,  the  Metamere-individual, 
while  the  fourth  grade,  the  Cormtis,  is  formed  by  a  similar 
union  of  a  number  of  metaraeres,  as,  for  instance,  in  the 
Earthworm,  each  joint  or  segment  of  which  is  a  metamere. 


m 


42 


INVKUTEBHATE  MOHPUOLOOY. 


• 


It  has  beeu  pointed  out  that  the  Flagellate  Volvox  presents 
a  tendency  towards  a  higher  individuality,  being  somewhat 
higher  than  a  mere  colony  of  cell-individuals  and  yet  not 
quite  reaching  the  dignity  of  an  organ-individual ;  similarly 
intermediate  conditions  between  the  other  grades  may  occur. 
In  certain  worms,  for  instance,  considerable  iude})endence  of 
the  constituent  metameres  exists,  any  one  of  them,  when  de- 
tached, being  capable  of  carrying  ou  an  indei)endent  exist- 
ence,  and  of  developing  into  an  organism  similar  to  that  of 
which  it  was  originally  a  part.  In  the  Earl  rm  the  depend- 
ence of  the  various  segments  or  metameres  upon  one  another 
is  greater  than  this,  but  in  it,  too,  a  certain  amount  of  inde- 
pendence is  shown  by  the  power  it  possesses  of  regenerating 
lost  metameres.  In  other  cormi,  as,  for  instance,  in  the 
Lobster,  the  interdependence  of  the  component  metameres 
proceeds  still  farther,  and  a  differentiation  of  the  various  meta- 
meres occurs,  a  process  carried  to  its  greatest  extent  in  the 
higher  Vertebrates.  A  physiological  division  of  labor  among 
the  metameres  develops,  some  of  them  losing,  for  instance, 
their  excretory  organs,  while  in  others  these  organs  lose  tlieir 
excretory  functions  and  serve  as  ducts  by  '^'hicli  the  repro- 
ductive elements  may  pass  to  the  exterio  The  subordina- 
tion of  the  metameres  proceeds  most  r^*^  liy  and  is  most 
complete  at  the  anterior  extremity  of  the  organism,  leading 
to  the  formation  of  a  head  bearing  highly  developed  sense- 
organs  and  containing  a  complex  nervous  system,  which  rep- 
resents originally  distinct  metamere  nervous  systems,  now 
fused  and  destitute  of  all  independence. 

Sexual  Repn'oduction  in  the  Metazoa. — In  cell-individuals  it 
has  been  seen  that  fission  is  the  most  frequent  and  simplest 
mode  of  reproduction  ;  in  the  Metazoa  this  method  and  its 
modification,  budding,  also  occurs,  but,  as  a  rule,  only  in 
forms  of  a  low  grade  of  individuality  or  in  a  transition  stage 
between  a  lower  and  a  higher  grade.  In  organ-individuals 
it  is  of  frequent  occurrence,  the  imperfect  separation  of  the 
individuals  so  produced  leading,  in  many  cases,  to  the  forma- 
tion of  colonies,  and  in  cormi  in  which  the  integration  of  the 
constituent  metameres  is  but  slight  it  also  occurs. 

In  the  Protozoa  cell-division  naturally  entails  reproduc- 


tiUBKIN(WOM  METAZOA. 


4*3 


Its 
[at 
ot 

ir. 


tioii,  but  iu  orj^'.iu-iiulividujils  reproductiou  of  the  constitu- 
eut  cell-iudividuiils  Ih  uot  iiec«;sHarily  coin>iM-tetl  Avitli  the 
leproductiou  of  the  entire  iudividiisil,  l)Ut  iiiiix  .siiui)ly  iueveasG 
the  uumber  of  h)\ver-f»rade  individuals  of  which  it  is  coiii- 
posed.  Siniihirly  multiplication  of  the  orj^au-iiidividuul.s  of  a 
luetamere,  or  of  the  nietaniere-individuals  of  a  eormus  may 
occur  without  j)ro(lucing  reproduction  of  the  whole ;  it  is 
simply  f»rowth.  From  growth  to  reproduction  by  budding 
the  path  is  short,  and  various  intermediate  stages  counecting 
the  two  processes  can  be  found.  Hence  reproduction  has  beeti 
aptly  defined  as  "discontinuous  growth,"  though  p(!rhaps  it 
would  be  even  more  apt  to  define  growth  as  reproduction  wifli- 
out  disamiinuiftf,  growth  in  a  Metazoou  depending  on  the 
reproduction  of  the  lower-grade  individuals  of  which  it  is 
composed. 

It  is  possible  to  carry  this  idea  still  further  back  and  refer  the  <j;ro\vth 
of  a  cell  to  the  reproduction  of  the  constituent  elements,  plasonies,  of  which, 
it  may  be  imagined,  it  is  composed.  In  the  simplest  cells  tlio  various 
forms  of  plasomes  are  distributed  throughout  the  cell,  l)Ut  in  the  higlier 
Protozoa,  for  instance,  an  aggregation  of  similar  i)lasomcs  occurs,  giving 
rise  to  such  struc  ires  as  the  myophanes.  In  a  similar  manner  in  the 
lower  Metazoa,  although  a  division  of  labor  and  structural  ditTerentiation 
has  taken  place  among  the  constituent  cells,  yet  the  cells  possessing  similar 
functions,  as,  for  instance,  the  nerve-cells,  are  more  or  less  irregularly 
scattered  throughout  the  body,  only  becoming  aggregated  in  the  higher 
forms  into  distinct  tissues,  and  giving  rise  to  the  most  perfect  type  of  an 
organ-individual.  Likewise  iu  a  metamere-individual  a  multiplication  of 
the  organs  leads  to  a  transition  form  with  discretely  arranged  parts,  the 
definite  aggregation  of  which  produces  a  cormus,  composed  in  the  simpler 
forms  of  distinct  metameres,  which  become  more  and  more  integrated  and 
subordinated  to  the  individuality  of  the  cormus  in  higher  tyi)es  of  that 
grade  of  individual. 

According  to  this  view  the  segmentation  or  metamerism  of  the  higher 
Metazoa  is  the  result  of  the  multiplication  and  subsequent  integration  of 
the  organ-individuals  of  an  ancestral  metamere-individual,  and  explains 
the  occurrence  of  imperfect  metamerism  in  certain  forms  of  that  grade  of 
individuality  (Turbellarfa).  Some  authors  have  considered  metamerism  to 
have  arisen  by  the  reproduction  by  budding  of  an  ancestral  metamere,  an 
idea  which  fails  to  explain  satisfactorily  the  condition  just  referred  to. 
The  view  presented  here  considers  metamerism  to  be  the  result  of  growth. 
It  has  not  arisen  by  the  reproduction  of  the  metamere,  but  by  that  of  its 
organs,  just  as  a  typical  organ-individual  has  arisen  by  the  reproduction 
and  integration  of  its  constituent  cell-individuals. 


iiii» 


illi 


44 


INVEHTEDRATE  MORPHOLOGY. 


!■: 


As  a  mode  of  reproduction  in  the  Metazoa  division  plays 
but  a  secondary  part,  the  sexual  process  being  the  character- 
istic method.  Attention  has  already  been  called  to  the  par- 
tial specialization  in  Volvox  of  reproductive  cells  which  serve 
to  perjietuate  the  species,  the  remaining  cells  of  the  colony 
perishing.  This  condition  is  a  premonition  of  the  more  per- 
fect specialization  found  in  the  Metazoa  of  reproductive  or  germ 
cells  and  non-reproductive  or  somatic  cells,  the  latter  serving 
for  the  nutrition  and  protection  of  the  germ- cells,  to  which 
the  perpetuation  of  the  species  is  entrusted.  Comparatively 
early  in  the  development  of  an  individual  certain  cells  differ- 
entiate from  the  others,  not  undergoing  like  them  a  physi- 
ological and  structural  specialization,  but  retaining  a  general- 
ized  character.  These  are  the  germ-cells  usually  grouped 
together  to  form  the  reproductive  organs. 

In  describing  the  methods  of  reproduction  occurring  in  the 
Flagellata,  the  manner  of  the  development  of  sexual  repro- 
duction was  indicated.  It  appears  to  have  been  originally  a 
more  or  less  accidental  fusion  of  two  similar  cells  or  spores, 
and  from  being  accidental  this  fusion  gradually  became  the 
rule  on  account  of  the  greater  vitality  which  the  conjugate  in- 
dividual  possessed  over  cells  which  did  not  conjugate.  The 
next  step  was  the  differentiation  of  microspores  and  macro- 
spores,  which  reaches  a  high  development  in  Volvox,  where 
it  is  associated  also  with  a  differentiation  into  somatic  and 
germ  cells.  In  the  Metazoa  both  these  differentiations  are 
carried  to  a  higher  degree,  the  macrospores  being  known 
as  ova  and  the  microspores  as  spermatozoa,  while  the  aggre- 
gates of  these  cells  are  termed  respectively  ovaries  and  testes. 

In  a  young  embryo  a  mass  of  germ-cells  which  is  to  give 
rise  to  spermatozoa  cannot  be  distinguished  from  one  which 
is  destined  to  be  converted  into  ova.  Fundamentally  both 
are  the  same,  and  occasionally  a  portion  of  a  mass  of  germ- 
cells  may  be  differentiated  into  ova,  while  the  rest  of  it  devel- 
ops into  spermatozoa.  This  has  not  unfrequently  been  seen 
in  fishes  in  which  there  is  normally  a  separation  of  the  sexual 
elements  in  distinct  individuals,  and  throws  considerable 
light  upon  the  occurrence  of  forms  which  normally  possess 
both  elements.     This  condition  of  hermaphroditism,  which  oc- 


SUBKINQDOM  METAZOA. 


4*^ 
1 


g 


curs  in  many  parasitic  forms  and  in  certain  sponges,  Flat- 
worms,  Mollusks,  and  Crustacea,  seems  to  have  been  second- 
arily acquired.  It  is  probable  that  the  ancestral  Metazoa 
were  unisexual,  possessing  reproductive  elements  of  only  one 
kind,  a  supposition  borne  out  by  the  frequent  association  of 
hermaphroditism  with  a  parusitic  or  sessile  mode  of  life,  such 
conditions  being  what  may  be  termed  abnormal,  and  usually 
accompanied  by  marked  structural  characters  which  are  to 
be  regarded  as  secondary  mo  liticatious.  On  the  other  hand, 
it  is  noticeable  that  the  lowest  free  Metazoa  (such  as  the  free- 
swimming  Cnidaria)  are  unisexual. 

An  ovum  is  a  single  cell,  and  in  its  typical  form  consists 
of  a  mass  of  protoplasm  containing  a  nucleus,  and  may  or 
may  not  be  surrounded  by  a  membrane. 
Seldom,  however,  does  such  a  simple  ovum 
occur;  usually  more  or  less  yolk,  consisting 
of  fatty  and  albuminous  globules,  is  distrib- 
uted throughout  the  protoplasm,  and  fre- 
quently the  amount  of  yolk  far  overbalances 
the  amount  of  protoplasm.  Other  structures, 
such  as  albumen  and  one  or  more  enveloping 
membranes,  may  be  added,  the  ova  of  diflferent 
species  differing  greatly  in  this  respect. 
Among  the  lower  forms  the  ova  are  usually 
extruded  freely  from  the  body  of  the  parent, 
but  in  many  of  the  liigher  Metazoa  they  are 
enclosed  within  protective  cases  (cocoons),  as 
in  the  Earthworm,  or  imbedded  in  jelly-like 
masses,  as  in  the  common  Pond-snails. 

In  the  ovary  of  a  young  individual  all  the  Fio.2(i.-()vakial 
germ-cells  are  alike,  asid  all  are  potentially  Tuhk  <»k  a  Uke- 
reproductive  cells;  very  frequently,  however,  "'','  ^"'^'"  '"' 
many  of  the  i)!;imitive  germ-cells  relinquish  „  _  .rermiiml  le- 
their  reproductive  function  and  serve  as  pur-  o  =  ova.  [gion. 
veyors  of  nutrition  to  certain  of  their  com-  t^*  =niaturo  ovum. 
rades  which  enlarge  and  become  mature  ova.  'i~^^„y?*^  ^' 
This  18  well  seen  in  insects,  m  which  each  ovary 
(Fig.  20)  consists  of  a  number  of  tubes  tapering  to  a  point  at 
QUO  end,  while  at  the  other  they  open  into  a  common  duct, 


il 


m 


I 


46 


INVERTEBRATE  MORPHOLOGY. 


i 


•  I 


I 


I 


the  oviduct,  leading  to  the  exterior.  At  the  tip  of  each  tube 
the  primitive  germ-cells  (Fig.  20,  g)  are  located,  and  lower 
down  ova  (o)  in  various  stages  of  development  towards  matu- 
rity are  to  be  found,  each  surrounded  by  a  number  of  small 
undeveloped  germ-cells,  known  as  follicle-cells  {/),  whose  func- 
tion it  is  to  transfer  food-yolk  {y)  to  the  growing  ovum.  As 
the  latter  approaches  maturity  the  follicle-cells  secrete  around 
it  a  thick,  sometimes  highly  sculptured  shell  and  finally 
degenerate. 

As  a  rule,  conjugation  with  a  spermatozoon,  i.e.  fertiliza- 
tion, is  necessary  as  an  antecedent  to  further  development. 
Before  this  takes  place,  however,  certain  modifications  of  the 
ovum  are  necessary,  the  pheno-^^ona  which  accompany  them 
being  known  as  the  maturatio,    /  the  ovum.     In  this  process 


A  B 

Fig.  21.— Diaohams  illustuatino  the  ^Iatukation  of  tiir  Ovum. 

A  =  foriuutioii  of  the  Hist  polar  globule  (pg). 

B  —  fonnatiou  of  llie  second  polar  globule  aud  eutiauce  of  tlie  sperm- nucleus 
(«/>)■ 

(I'ig.  21,  A)  the  nucleus  approaches  the  surface  of  the  ovum 
aud  there  undergoes  a  karyokiuetic  division  which  is  peculiar 
in  that  in  the  equatorial-plate  stage  twice  as  many  chromo- 
somes are  formed  as  are  typical  for  the  species.  These  do  not 
undergo  longitudinal  division,  and  by  the  karyokinesis  their 
number  is  reduced  to  the  typical  number,  a  small  cell,  the 
polar  globule  (pg),  being  separated  from  the  ovum  with  lialf 
the  chromosomes,  while  the  others  are  retained  witliin  the 
ovum.  Tht  nucleus  of  the  ovum,  instead  of  now  returning 
to  the  resting  stage,  divides  again  (Fig.  21,  B),  a  second  polar 
globule  being  formed    and   receiving  half  the  chromosomes 


SUBKINUDOM  METAZOA. 


47 


which  remain,  so  that  the  nucleus  of  the  ovum  now  possesses 
only  Jialf  the  number  of  chromosomes  which  are  character- 
istic for  the  species.  At  the  time  of  the  formation  of  the 
second  polar  globule  the  first  frequently  divides  without  its 
nucleus  passing  into  a  resting  stage,  so  that  as  the  result  of 
this  maturation  process  four  cells  have  been  formed,  three  of 
which  are  small,  while  the  third  is  relatively  very  large  and 
will  alone  undergo  further  development.  When  these  divi- 
sions have  been  completed  and  the  chromosomes  have  been 
reduced  to  one-half  their  proper  number  the  nucleus  of  the 
ovum  passes  into  the  resting  stage,  migrates  back  towards 
the  centre  of  the  ovum,  and  is  ready  for  conjugation  with  the 
nucleus  of  a  spermatozoon. 

The  spermatozoa  are  always  much  smaller  than  the  ova, 
and  are,  as  a  rule,  capable  of  active  motion,  though  in  certain 
Crustacea,  for  instance,  they  lack  this  power.  The  ova  and 
spermatozoa  have  specialized  in  opposite  directions  in  this 
respect.  The  ova  of  the  Metazoa  are  specialized  as  the 
nutritive  cells  of  conjugation,  possessing  abundant  protoplasm 
and  usually  a  considerable  amount  of  yolk  for  the  nutrition 
of  the  young  embryo.  They  consequently  have  lost  their 
motility,  and  in  order  that  conjugation  may  be  made  prob- 
able the  spermatozoa  lack  all  unnecessary  material  which 
would  interfere  with  their  motility,  no  yolk  being  stored  up 
and  the  protoplasm  even  being  reduced  to  the  smallest 
amount  consistent  with  the  development  of  a  locomotor 
organ.  The  nuclei,  as  will  be  seen  later,  are  essential  ele- 
ments in  conjugation,  and  the  spermatozoa  are  to  all  intents 
locomotor  nuclei,  the  ova  supplying  the  protoplasmic  nidus 
necessary  for  the  growth  and  division  of  the  nucleus  formed 
by  conjugation. 

In  their  typical  form  spermatozoa  are  composed  of  a 
globular  or  pyriforra  head  consisting  of  a  nucleus  surrounded 
by  a  small  amount  of  protoplasm,  and  a  long  filamentous  tail 
continuous  with  the  protoplasm  and  frequently  provided  with 
a  delicate  fringe-like  membrane  (Fig.  22,  F).  By  the  rapid 
whipping  movements  of  the  tail  the  organism  is  propelled 
through  the  water,  or  other  fluid  in  which  it  may  find  itself, 
and  so  may  come  into  contact  with  an  ovum. 


I 


!«.  f! 


48 


INVERTEBRATE  MORPUOLOGY. 


'■ 


The  transformation  of  the  germ-cells  present  in  an  em 
bryo  into   spermatozoa  is   usually  a  somewhat  complicated 
process.     In  the    Kound-worm  Ascaris,   in  which  it  retains 
somewhat   primitive  characters,  the  process   closely  resem- 
bles what  takes  place  during  the  maturation  of  the  ovum. 


Pig.  22. — Diaquams  to  ilt^ustrate  the  Matukation  op  the  Spkum-cell. 

A  =  division  of  the  spermogoue. 
B  =  division  of  the  two  speruiocytes. 
C  =  the  four  spermatids. 
D,  E  —  conversiou  of  a  spermatid  into  a  spermatozoon. 
F  —  fully  developed  spermatozoon. 

The  embry^'iic  germ-cells  {spermatogones,  Fig.  22,  A)  undergo 
karyokinetic  division,  the  number  of  chromosomes  being,  as 
in  the  ovum  in  the  division  which  results  in  the  formation  of 
the  first  polar  globule,  twice  that  which  is  characteristic  for 
the  species.  They  do  not  undergo  longitudinal  division,  and 
one  half  of  them  passes  into  one  of  the  daughter  cells  {sper- 
niatocyfes)  and  the  other  half  into  the  other,  so  that  tliese  two 
cells  possess  the  number  of  chromosomes  characteristic  for 
the  species.  A  division  of  these  daughter  cells  (Fig.  22,  B) 
immediately  takes  place  without  a  return  to  the  resting  stage, 
and  unaccompanied  by  a  longitudinal  division  of  the  chromo- 
somes, so  that  four  cells  {spermatids,  Fig.  22,  6')  are  formed, 
each  of  which  contains  only  half  the  typical  number  of  chro- 
mosomes, and  each  one  of  these  cells  becomes  a  spermato- 
zoou.    This   process  is  comparable    step  by  step  with    the 


8UBKIN0D0M  METAZOA. 


49 


maturation  of  the  ovum  and  seems  to  indicate  that  the  polar 
globules  are  to  be  regarded  as  abortive  ova. 

The  conversion  of  the  spermatids  into  spermatozoa  is 
simply  a  differentiation  of  structures  already  present.  In 
the  air-breathing  MoUusca,  for  instance,  the  spermatids  consist 
of  a  mass  of  cytoplasm  containing  a  nucleus,  in  close  proxim- 
ity to  which  may  be  found  the  centrosome,  while  an  irregular 
mass  of  filaments  represents  the  remains  of  the  spiudle-tila- 
ments.  In  the  differentiation  which  follows  (Fig.  22,  D,  E, 
and  F)  the  nucleus  elongates  and  its  chromatin-tilaments 
fuse  to  form  a  homogeneous  mass ;  the  cytoplasm  likewise 
elongates,  and  in  it  appears  an  axial  filament  which  latii-  will 
form  the  tail-filament.  The  origin  of  this  filament  is  doubt- 
ful, some  authors  maintaining  that  it  is  a  differentiation  of 
the  cytoplasm,  while  others  believe  it  to  be  a  prolongation  of 
the  nuclear  substance ;  but,  however  that  may  be,  the  spiral 
friuge  which  surrounds  the  axial  filament  is  certainly  the 
remains  of  the  cytoplasm  of  the  spermatid.  The  rem.aius  of 
the  spindle-filaments  disappear,  while  the  centrosome  prob- 
ably persists  as  a  structure  lying  behind  the  head  and  termed 
the  "  Mittelstuck." 

In  some  cases,  as  the  insect  Pyrrhocoris  and  tlie  crustacean  Diaptomns, 
the  doubUng  of  the  chromosomes  previous  to  division  into  spermatocytes 
does  not  take  place.  In  Pyrrhocoris  twenty-four  chromosomes  are  typi- 
cally present  and  twelve  of  these  pass  into  each  of  the  spermatocytes,  and 
in  the  division  of  these  to  form  the  spermatids  each  of  the  twelve  chromo- 
somes divides  so  that  each  sj)ermatid  possesses  half  the  typical  number. 
In  Diaptomus  the  same  result  is  brought  about  somewhat  differently. 
The  spermatogones  possess  eight  chromosomes  which  assume  a  dumbbell 
shape  and  divide  transversely,  so  that  each  spermatocyte  has  the.  typical 
number  of  chromosomes ;  the  spermatocytes  divide  without  passing 
through  a  resting  stage,  and  each  spermatid  thus  contains  four  chromo- 
somes, i.e.  half  the  typical  number. 

Fertilization  of  the  Ovum. — So  soon  as  the  formation  of  the 
polar  globules  has  been  completed,  the  nucleus  of  the  ovum 
migrates  towards  the  centre  of  the  protoplasm  and  is  the 
female  pronudeiis  (Fig.  23,  fp)  of  conjugation.  The  penetra- 
tion of  the  spermatozoon  may  occur  at  any  portion  of  the  sur- 
face of  the  ovum  and  may  take  place  before,  during  (Fig.  21, 
B,  s'p),  or  after  the  formaticm  of  the  polar  globules,  a  single 


!!  I. 


i!f 


S  ! 


n 


'1 


i 


60 


INVERTEBRATE  MORPHOLOGY. 


spenuatozoon,  as  a  rule,  in  healthy  ova,  peuetratiug  and  tak- 
in^  part  in  the  conjugation,  though  apparently  in  some  cases 
polyspermy,  or  the  penetration  of  several  spermatozoa,  may 
occur.  The  head  of  the  spermatozoon  comes  into  contact 
with  tlie  protoplasm  of  the  ovum,  which  in  some  cases  rises 
up  to  meet  it,  and  is  rapidly  engulfed.     The  tail  likewise  of 


^2^ 


Fig.  23. —Diagrams  to  Illustrate  the  Phenomena  op  Fertilization. 

A,  the  approximation  of  tlie  male  aud  female  nuclei. 

B,  division  of  the  centrosomes. 

C,  rotation  of  tlie  centrosomes. 

D,  fusion  of  the  centrosomes  and  nuclei,  and  formation  of  the  segmentation 

spindle. 

cc  =  compound  centrosome.  oc  =  ovum  centrosome. 

/;)  =  female  nucleus.  sc  =  sperm  centrosome. 

mp  =  male  nucleus.  an  =  segmentation  nucleus. 

the  spermatozoon  is  taken  into  the  ovum  and  seems  to  be  com- 
pletely absorbed,  the  head  alone  being  visible  in  later  stages  ; 
it  constitutes  the  male  promicleits  (Fig.  23,  mp)  aud  moves 
towards  the  centre  of  the  egg  until  it  comes  into  contact  with 
tlie  female  pronucleus,  without,  however,  fusing  with  it.  A 
spindle  now  makes  its  appearance,  aud  the  two  pronuclei  pass 
through  the  various  karyokinetic  stages,  forming  equatorial 
plates  each  >vith  half  the  typical  number  of  chromosomes, 


SUBKINUDOM  METAZOA. 


61 


wliicli  divide  longitudinally  iu  the  usual  mauuer,  one  half  the 
chromosomes  of  each  nucleus  passing  towards  one  of  the  cen- 
trosomes.  The  ovUin  then  divides  into  two  cells  and  the 
compound  nucleus  of  each  passes  into  the  resting  stage,  the 
chromosomes  now  uniting  to  form  a  single  chromatic  net- 
work. 

It  will  be  seen  from  this  that  the  conjugation  or  fertiliza- 
tion process  consists  of  the  union  of  two  distinct  nuclei,  whose 
complete  fusion  does  not  necessarily  occur  until  after  the  first 
division  or  segmentation  of  the  ovum. 

A  conjugation  of  centrosomes  to  form  those  of  the  first 
segmentation-spindle  also  occurs.  A  centrosome  accompanies 
each  of  the  conjugating  nuclei  (Fig.  23,  A),  and  before  the 
formation  of  the  spindle  each  divides  into  two  (Fig.  23,  B,  oc 
and  sc),  which  conjugate  in  pairs  (Fig,  23,  C  and  D),  forming 
the  centrosomes  of  the  spindle,  each  of  which  thus  contains 
elements  of  both  the  original  centrosomes. 

Furthermore,  in  some  cases  at  least,  it  is  possible  to  dis- 
tinguish the  nuclear  elements  derived  from  the  male  and 
female  pronuclei  respectively  in  stages  later  than  the  first 
segmentation,  owing  to  a  slightly  different  behavior  to  certain 
staining  reagents  which  characterizes  them.  The  pronuclei 
undergo  a  morphological  fusion  during  the  first  cleavage  of 
the  ovum,  but  a  physiological  differentiation  persists. 

Segmentation  and  Early  DeueJopnient  of  the  Ovum. — The 
development  of  the  ovum  into  the  embryo  consists  in  its  divi- 
sion into  a  number  of  cells,  which  gradually  undergo  a  phy- 
siological and  morphological  differentiation  resulting  in  the 
formation  of  tissues,  organs,  etc.  These  divisions  constitute 
the  segmentation  of  the  ovum. 

The  first  division  has  alrefidy  been  described ;  it  bears  a 
definite  relation  to  the  formation  of  the  polar  globules,  the 
plane  of  the  division  passing  through  the  point  at  which  they 
were  separated  from  the  ovum.  Considering  this  point  to 
represent  one  pole  of  the  ovum,  the  first  division  is  meridio- 
nal, and  the  second  division  likewise,  though  its  plane  is  at 
right  angles  to  that  of  the  first  division  (Fig.  24,  A).  The 
third  division  is,  on  the  other  hand,  equatorial,  its  plane  cutting 
the  planes  of  previous  divisions  at  right  angles  (Fig.  24,  B). 


mw-i 


I 


rrrr 


52 


INVEHTEBliATE  MOHPUOLOGY. 


Eight  segmeiitutiuu-cells  are  thus  formed  which  remain  in 
contact  with  each  other  and  enclose  a  small  cavity,  the  seg- 
mentation-cavity or  bhistocail.  The  further  division  of  the 
cells  (Fig.  24,  (J)  results  in  the  formation  of  an  oval  or  spheri- 
cal organism  (Fig.  24,  JJ)  which  may  be  compared  to  Volvox, 
consisting  of  a  single  layer  of  cells  enclosing  a  more  or  less 
voluminous  blastocoel.  This  embryonic  stage  is  known  as 
the  hlosttda.  In  its  simplest  form  it  shows  no  special  diflfer- 
entiation  into  tissues,  its  cells  being  uniformly  ciliated,  and 


Flli.  24  — DlAGKAMS  ILLUSTRATING  THE  SEGMENTATION  OP  THE  OvUM. 

A,  fuur-celled  stuge. 

B,  eit>bt-ct!lled  stuge  of  a  telolecithal  ovum. 

C,  sixteen-celled  stage. 

D,  blastula. 

Tile  uiTows  iudicate  the  mode  of  division. 

the  organism  free-swimming,  moving  through  the  water  with 
a  rotatory  movement  about  a  definite  axis,  one  and  the  same 
end  of  which  is  always  anterior.  In  many  blastulas,  however, 
especially  in  those  which  for  one  reason  or  another  are  not 
free-swimming,  an  early  dilferentiation  of  the  cells  takes  place, 
especially  at  the  extremity  which  is  posterior  in  the  free- 
swimming  forms  or  which  corresponds  to  that  pole  in  the 
non-motile  embryos.  These  posterior  cells  are  usually  some- 
what larger  than  those  at  the  anterior  pole,  and  if  much  food- 
yolk  is  present  in  the  embryo  it  is  especially  concentrated  in 


SUDKINGDOM  METAZOA. 


m 


these  cells,  which  in  the  later  developineut  will  assume  the 
vegetative  fuuctious  of  the  organism. 

In  many  ova  the  processes  just  described  are  moditicd  to  a  greater  or 
less  extent,  but  from  the  frequency  of  their  occurrence  they  must  be  re- 
garded as  fundamental  and  tlie  modifications  as  secondary. 

Ova  which  contain  but  little  yolk  usually  follow  more  or  loss  closely 
the  typical  processes,  but  where  the  yolk  is  abundant,  being  an  inert  sub- 
stance, it  acts  as  a  drag  upon  the  protoplasmic  activity  and  produces  modi- 
fication of  the  segmentation-processes.  Two  methods  of  arrangement  of 
the  yolk  may  be  recognized :  («)  it  may  be  aggregated  more  or  less  com- 
pletely at  one  pole  of  the  ovum,  such  ova  being  termed  teloledthal,  or  (6) 
it  may  be  distributed  in  the  meshes  of  a  protoplasmic  network,  a  small 
quantity  of  yolkless  protoplasm  being  concentrated  around  the  nucleus  of 
the  ovum,  while  another  portion  of  it  forms  a  thin  peripheral  layer  sur- 
rounding the  yolk,  this  arrangement  being  termed  centrolecithal. 

In  telolecithal  ova  the  third  segmentation-division  results  in  the  forma- 
tion of  four  cells  containing  very  little  yolk  at  one  pole  of  the  ovum,  while 
nearly  all  the  yolk  is  concentrated  in  the  four  cells  at  the  other  pole  (Fig. 
24,  B).  This  arrangement,  which  occurs  in  many  Mollusca,  constitutes 
what  is  termed  a  total  ii'regular  segmentation,  in  which,  owing  to  the  large 
size  of  the  yolk-containing  vegetative  cells,  the  blastocoel  is  usually  com- 
paratively small.  In  the  Squids  the  amount  of  yolk  present  at  the  vegetative 
pole  is  very  great  and  the  protoplasm  of  the  ovum  collects  upon  its  surface, 
there  undergoing  division  and  producing  a  plate  of  cells,  the  blastoderm, 
which  by  further  division  gradually  extends  and  finally  encloses  the  inert 
yolk.  This  partial  segmentation  is  the  result  of  the  presence  of  a  very 
large  quantity  of  yolk  and  its  telolecithal  arrangement,  and  necessarily 
obscures  greatly  the  blastula  stage. 

In  centrolecithal  ova  which  occur  in  Crustacea  and  Insects,  the  division 
of  the  nucleus  is  accompanied  by  a  division  of  the  central  yolkless  proto- 
plasm only,  the  yolk-containing  reticulum  and  the  peripheral  layer  not  tak- 
ing part  in  the  process.  As  the  divisions  continue  the  nuclei  gradually 
approach  the  surface  and  finally  come  to  lie  in  the  peiipheral  protoplasm, 
which  then  takes  part  in  the  division,  a  greater  or  less  portion  of  the  inert 
undiviJed  yolk  occupying  the  blastocoel  of  the  resulting  blastula.  Many 
intermediate  gradations  occur  between  such  a  typical  centrolecithal  and  a 
total  regular  segmentation,  from  which  both  the  centrolecithal  and  telo- 
lecithal methods  are  to  be  derived. 

The  blastula  is  a  single  layer  of  cells  surrounding  a  large 
blastocoel  in  typical  cases,  and  is  a  stage  quickly  passed  over 
in  the  Metazoa.  It  is  succeeded  by  a  stage  in  which  the  em- 
bryo consists  of  a  double-walled  sac  open  at  one  end,  the  gas- 
trvla  (Fig.  25).     This  is  most  frequently  produced  from  the 


I 


tffa 


64 


INVEHTEBHATK  MORPUOLOGY. 


Fig.  25. — Diagram  of  a  Gastuula. 


bliistulu  by  the  pusliiupj  in  or  invagination  of  the  cells  of  oue 
pole  (the  posterior  in  free-swimmiug  blastulas)  into  the  blas- 
tocoel,  which  thus  becomes  more  or  less  ])erfectly  obliterated. 

The  cavity  liued  by  the  iuvagi- 
uated  cells  is  the  primitive  di- 
gestive tract  or  archenteron,  its 
opening  to  the  exterior  being 
the  gastrula  mouth  or  blastopore. 
The  gastrula  is  a  two-layered 
organism  or  is  diplohlastic,  and 
the  cell-layers  of  which  it  is 
composed  are  the  primitive  germ- 
layers.  The  outer  layer  in  the 
higher  Metazoa  gives  rise  to  the 
integument,  nervous  system,  and 
sense-organs  of  the  adult  and 
is  known  as  the  ectoderm^  while 
the  inner  one,  from  which  the  digestive  tract  and  its  glands, 
such  as  the  liver,  will  develop,  is  termed  the  emioderm. 

Just  as  the  presence  of  yolk  in  the  ovum  may  modify  the 
segmentation,  so  too  it  may  produce  decided  moditications  in 
the  formation  of  the  gastrula.  The  method  just  described, 
which  occurs  in  embryos  containing  little  food-yolk,  is  distin- 
guished as  embolic  from  the  epibolic  method  occurring  in  telo- 
lecithal  ova  which  undergo  a  markedly  irregular  segmentation. 
In  sucli  ova,  as  has  been  stated,  one  pole  is  occupied  by  inert 
yolk-laden  spherules,  while  at  the  other  are  almost  yolkless 
active  cells.  These  latter  divide  rapidly  and  extend  as  a  cap 
over  the  yolk-laden  cells  and  finally  completely  enclose  them. 
The  result  is  practically  the  same  as  in  the  embolic  method, 
the  yolk-laden  endoderm  cells  being  enclosed  within  the  yolk- 
less ectoderm. 

Among  the  lower  Metazoa  especially,  another  method  oc- 
curs by  which  the  diploblastic  embryo  is  formed.  Instead  of 
certain  cells  invaginatiug,  each  cell  of  the  blastula  divides  in 
a  plane  parallel  to  the  surface  of  the  organism,  one  of  the  two 
cells  thus  produced  becoming  ectoderm,  while  the  other  is  a 
portion  of  the  endoderm.  A  diploblastic  closed  sac  thus  re- 
sults, the  blastopore  appearing  later  and  placing  the  archeu- 


aiUKJAunOM  METAZOA. 


55 


terou,  which  iii  this  ease  is  ideuticul  with  the  blastocael,  iu 
commuuicatiou  with  the  exterior.  This  process  is  ku(.'.":i  as 
dclamination  (Fi^.  2(),  A). 

A  third  method  also  exists,  occurring  like  delamiuatiou  in 
its  most  typical  form  umoug  the  lower  Metazoa.  This  is  the 
immigration  method  (Fig.  20,  Ji),  certain  cells  of  the  blastula 
leaving  their  position  at  the  surface  and  passing  into  the 
blastocoel.  Here  they  undergo  division,  and,  by  the  addition 
of  other  cells  by  immigration,  the  blastocad  gradually  be- 
comes tilled  Up  and  a  solid  organism,  consisting  of  an  exter- 
nal layer  of  cells  surrounding  a  central  more  or  less  solid 


Fig.  26.— Diaguam  Illtjstuating  the  Fohmation  of  the  Dipi.obi.astic 
Stage  (A)  by  Dei.amination,  (/?)  by  Immiokation. 

mass,  results.  This  is  known  as  the  parenchymeUa  or  sterrula. 
Later  a  cavity  appears  in  the  centre  of  the  solid  mass,  whose 
cells  gradually  are  pushed  towards  the  periphery,  wliere  they 
form  eventually  a  singhi  layer,  the  endoderm.  Finally  a  blas- 
topore is  formed  and  the  embryo  becomes  a  gastrula. 

It  does  not  seem  easy  to  bring  the  delamination  and  invagination 
methods  of  gastrulation  into  direct  relation  witli  eacli  other,  or  to  derive 
one  from  the  other,  but  it  is  probable  that  both  must  be  referred  back  to 
the  immigration  method.  In  typical  cases  of  immigration  the  cells  which 
migrate  are  situated  irregularly  at  any  part  of  the  blastula,  but  frequently, 
especially  in  free-swimming  blastulas,  the  migrating  cells  are  all  located  at 
the  posterior  extrem'ty.  If  iu  such  cases  of  polar  immigration  the  migrat- 
ing cells  were  to  pass  into  the  blastoccel  en  masse  instead  of  individually, 
invagination  would  result.  On  the  other  hand,  if  a  considerable  amount 
of  yolk  were  present  in  all  the  cells  of  a  blastula,  it  might  happen  that,  in- 
stead of  migrating,  the  cell  might  undergo  division,  cutting  off  the  yolk- 
containing  protoplasm  from  the  yolkless,  delamination  thus  taking  place. 


66 


IN  VERTEBRA  TB  MOHPUOLOG  Y. 


The  fact  that  in  some  cases  both  immigration  and  delamination  may  occur 
simultaneously,  leading  to  the  formation  of  a  sterrula,  bears  out  the  idea 
that  the  latter  process  has  arisen  from  the  former. 

Furthermore,  it  may  be  pointed  out  that  the  occurrence  of  immigration 
in  such  colonial  Flagellates  as  Votoox  indicates  the  primitive  character  of 
immigration  in  the  Metazoan  blastulas,  as  well  as  the  manner  in  which 
diploblastic  organisms  have  arisen  from  the  more  primitive  single- layered 
organisms. 

It  is  only  in  the  lowest  Metazoa,  however,  that  the  adult 
organism  is  diploblastic.     In  all  others  a  triploblastic  (Fig. 

dm 


sm 


spitt 


bm 


vm. 


Fig.  27. — Diagrammatic  Tuansverse  Section   of  an   Earthworm  to 

snow   TIIK   TuiPLOltLASTIC   CONDITION. 


bm  =  basement  membrane. 

C  =  ca'loin. 
dm  —  dor.sjil  mesentery. 

ec  =  ectoderm. 


en  =  endodenn. 
S7n  =  soiniitio  mesoderm. 
spin  =  spliinchuic  niesoi'frm 
xm  =  veutial  meseuter}'. 


27)  condition  supervenes  during  embryonic  life,  by  the  devel- 
opment of  a  third  layer,  primitively  separated  from  the  eudo- 
derm,  and  occupying  the  space  which  may  remain  between 
the  two  primitive  layers.  This  is  the  secondary  germ-layer 
or  mesoderm.  From  it  there  arise  the  muscular,  excretor}', 
circulatory,  and  reproductive  systems  in  the  triploblastic  ani- 
mals, the  first  and  last  of  these  bein<r  derived  in  diploblastic 
forms  from  either  one  or  both  of  the  iniiii  ''v  layers,  while 
the  excretory  and  circulator}'  systei   s  a  e  not  differentiated. 

The  manner  of  formation  of  i  ^'smlerm  i     the  embryo 

varies  greatly.  In  some  cases  it  ..  ses  as  bilateial  pouch-like 
outgrowths  of  the  archenteron,  Avhicii  la  er  form  closed  sacks 
completely  surrounding  the  digestive  tract,  the  sack  of  either 


SUnh'lNUJJOM  MKTAZOA. 


tn 


side  comiug  into  couttict  abuve  uud  below,  the  united  wullu 
forming  the  doisiil  and  ventral  mefienteries  which  suspend  the 
intiistino  (Fig.  27,  <tm  und  bm).  That  wall  of  each  sack  which 
surrounds  the  digestive  tract  is  termed  the  splanchnic  layer  of 
tlie  mosoderni  (Fig.  27,  s/>m),  while  that  lying  immediately 
below  the  ectoderm  is  the  somatic  layer  (sni),  and  the  enclosed 
cavity  is  the  coelom  (C)  or  body-cavity.  In  other  cases  the 
j)r()toplasm  destined  to  give  rise  to  the  mesoderm  segregates 
into  a  small  number  of  cells,  or  sometimes  even  into  a  single 
cell,  at  an  early  period  of  the  development,  frequently  while 
the  embryo  is  still  in  what  may  be  considered  the  blastula 
stage.  Tliese  cells,  known  as  mesohlasts,  give  rise  by  repeated 
division  in  one  direction,  and  by  the  subsequent  division  of 
the  daughter  cells  so  formed,  to  bands  of  mesodermic  tissue 
extending  along  the  ventral  surface  of  the  embryo  (see  Fig. 
105),  and  later  growing  dorsally  so  as  to  enclose  the  diges- 
tive tract.  The  coelom  forms  by  the  hollowing  out  of  the 
mesodermic  bands,  and  when  fully  developed  presents  the 
same  appearance  as  in  the  former  case. 

In  many  animals,  such  as  some  Turbellarian  worms,  a 
well-developed  ccelom  is  not  present,  the  only  traces  of  it 
being  minute  scattered  cavities  in  a  mass  of  mesodermic  tissue 
which  fills  up  the  space  between  the  eudoderm  and  ectoderm. 
A  strict  demarcation  of  this  form  of  coelom  (schizocoel)  from 
the  other  variety  {enterocoel)  does  not,  however,  exist,  grada- 
tions occurring  in  various  groups  of  animals  and  both  varie- 
ties sometimes  being  coexistent  in  the  same  form,  as  for 
instance  in  bivalve  Mollusca,  where  the  pericardial  cavity  is 
to  be  regarded  as  an  enterocoel,  while  the  spaces  existing  else- 
where in  the  mesoderm  are  schizoccjels. 

If  the  conditions  which  exist  in  the  lowest  triploblastic  animals  known 
to  us,  tlie  Turbellarian  worms,  tiirow  any  light  upon  the  origin  of  the  meso- 
derm, it  would  seem  that  primui.'^ly  it  was  a  solid  tissue,  not  completely 
marked  otT  from  thi;  eudoderm,  and  thai  ci,.:^  coelom  that  it  contained  was 
of  the  nature  of  a  schizocoel.  From  this  c<  adition  it  became  more  and 
more  differentiated  from  the  eudoderm  proper,  and  either  tended  to  appear 
as  a  separate  germ-layer  at  an  early  stage  of  development  in  the  form  of 
the  mesoblasts,  or  was  delayed  in  its  development  until  after  the  formation 
of  the  primitive  digestive  tract,  from  wliieh  it  then  separated  in  the  meso- 
dermic pouches.     According  to  this  view  the   mesoderm  is  a  secondary 


■ 


58 


INVEliTEBRATE  MORPHOLOGY. 


i:  y 


/»*\ 


derivative  of  the  eiulodenn,  and  the  endoderm  of  the  diploblastic  organ- 
isms is  equivalent  to  the  endoderm  pins  mesoderm  of  the  triplobhistic 
forms.  Tiie  apparent  derivation  of  the  mesoderm  from  the  ectoderm  in 
some  of  tiie  hitter  (o.^.  AnntlhUi)  is  to  be  rcjLjai'ded  as  resulting  from  the 
ju'ecocious  segregation  of  the;  mesoderm  at  an  early  period  of  development 
and  is  not  to  be  regarded  as  indicating  its  original  derivation. 

Non-scxunl  Jieprod notion  in  the  Metazoa. — Reproduction  by 
division  and  by  budding,  though  phiying  by  no  means  so  im- 
portant a  part  as  iu  the  Protozoa,  is  neverthe- 
less   of   frequent   occurrence  in  the  Metazoa, 
im        especially  in  certain  groups.     In  certain  Tiir- 
bellarian  worms  (Microdonia)  division  is  the 
usual  mode  of  reproduction,  replacing  almost 
completely  the  sexual  method,  aiid,  the  indi- 
viduals so  produced  remaining  in  connection 
'm         with    one    another,   longitudinal    chains    are 
produced,  consisting  of  individuals  iu  various 
degrees   of  separation   (Fig.    28).     In   certain 
Annelids   also  (Naididtr)   division   frequently 
fin»"        takes  place,  occasionally  each  metamere  being 
capable  of  developing  into  a  new  animal,  as  in 
Ctenodribis. 

Budding,   however,    is  a  rather  more   fre- 
quent method  and  is  characteristic  of  certain 
groups,  such  as  the  Hydroids,  Anthozoa,  and 
Fig.    28.  —  D  I  a-  Bryozoa.     In  some  cases,  as   in   Hydra    and 
OUA.M    SHOWING  ^^^^^^   medusiB,  the   buds   separate   from   the 
I'uoDJCTioN    OF  P'1'1"^"*  '>'i^d   lead   an   mdepeudent  existence  ; 
ATuitiJKM-AKiAN  but  frequently  the  separation  is  not  complete, 
WouM   Microsto-  resulting    iu    the   formation    of   colonies    the 
^'"*  individual    components    of  which  are   in  or- 
ganic connection  with   each  other.     In   such 
colonies   a   physiological   division  of   labor  among   the  C(m- 
stituent  individuals    may   take    ])lace,    as    in    the    Hydroid 
Hydractinia  (see  ]).  87)  where  some  of  the  individuals  devote 
themselves   to    the    nutrition   of    the   colony,   others   to   its 
reproduction,    and   others   again    to  the   protection  of   their 
weaker  companions.      The  assend)lages   produced   by   bud- 
ding  may    assume  very   complicated   shapes,    though   occa- 


ma        (after 
Graff). 


SUBKINGDOM  METAZOA. 


69 


sionally  linear  colouies  are  formed  which  are  with  difliculty 
to  be  distinguished  from  those  formed  by  division.  Indeed 
a  definite  distinction  between  budding  and  division  is  not 
possible,  thouf^h  where  an  alternation  of  older  and  younj^er 
individuals  occurs  in  a  linear  colony  division  is  indicated, 
while  in  one  produced  by  budding  there  is  a  regular  succes- 
sion of  gradually  older  inilividuals  from  before  backwards. 

Closely  related  to  budding  is  the  power  of  regeneration 
of  parts.  The  higher  Crustacea  possess  an  extraordinary 
power  of  regenerating  lost  limbs,  and  provision  is  present  in 
crabs  and  the  lobster  for  the  self-amputation  of  a  lind)  when 
such  a  mutilation  seems  to  be  demanded  by  the  exigencies 
of  the  situation.  In  the  lower  forms,  however,  the  extent  to 
which  such  regeneration  may  be  carried  is  much  greater,  ex- 
tending even  to  the  reproduction  of  the  whole  by  a  compaia- 
tively  snjall  part.  A  Starfish  is  not  only  able  to  regenerate 
an  arm  which  has  been  accidentally  lost,  but  from  an  arm  and 
a  portion  of  the  disk  all  the  missing  parts  may  be  developed  ; 
and  Hydra  or  a  Sponge  may  be  divided  into  ?'.  large  nunjber 
of  pieces  each  of  which  is  capable  of  developing  into  an  entire 
animal.  Such  phenomena,  as  well  as  budding  and  division, 
depend  either  upon  a  low  degree  of  diti'erentiation  of  the 
tissues,  as  in  such  a  form  as  a  Sponge  or  in  Hydra^  or  el.-^e  to 
the  persistence  of  a  certain  amount  of  tissue  in  an  embryonic 
or  undifferentiated  condition.  In  a  Brvozoan  bud,  for  in- 
stance,  as  its  tissues  gradually  differentiate  into  the  julult 
condition,  a  number  of  cells  lag  behind  and  do  not  take  part 
in  the  differentiation,  and  later  give  rise  to  a  new  bud  ;  and 
similarly  in  the  Annelid  worms  the  tissues  of  a  regenerating 
part  show  an  appearance  and  mode  of  differentiation  similar 
to  what  they  present  in  the  development  from  tlie  ovum. 
Conversely,  the  greater  the  degree  of  differentiation  and  in- 
tegration of  the  tissues  and  organs  of  an  animal  the  less 
is  the  power  of  regenerating  lost  parts  or  of  reproducing  by 
budding. 

As  a  general  rule  ova  are  incapable  of  developing  into  the 
adult  form  unless  fertilized  by  a  spermatozoon.  In  a  number 
of  forms,  however,  a  develo])ment  of  unfertilized  ova  occurs 
constituting   a   mode    of    reproduction   known    as    purfheno- 


T 


■w 


«?  « 


I 


60 


IN VEHTEBliA TE  MOliPlIOLOG  Y. 


1/ 

I 

II 


•'^ 


genesis.     Examples  of  this  plienomeuon  are  to  be  met  with  in 
luseets,  a   familiar   oue   beiuj^   the   commou   Hive   Bee,  the 
queens   of   which   speeieKS    deposit    large    numbers   of   eggs, 
those  last  deposited,  Avhich  give  rise  to  drones,  being  unfer- 
tilized and  developing  })artlienogeuetically.     In  certain  flies 
(Cecidnmyi(i)    this    jjartheuogonetic   development    of   the  ova 
may  occur  while  the  insect  is  still  in  the  larval  or  maggot 
stage,  a  jjlienomeuon  which  is  known  as  pa'dogenesis  (Fig.  29). 
Alternation  of  Generations. — The  majority  of  forms  which 
possess   the   power  of    non-sexual  reproduction   also    repro- 
duce by  the  sexual  method,  no  definite  relation 
existing,  however,  between  the  two  processes. 
In  some  cases,  however,  a  definite  relation  is 
established,  the   one  method  succeeding  the 
other  with  rhythmic  regularity,  the  individuals 
also  which  reproduce  sexually  dift'ering  materi- 
ally in  form  and  organization  from  those  which 
gave   rise  to  them   by  a  non-sexual  method  ; 
such  a  condition  of  affairs  is   termed  Alter- 
nation  of    Generations,   a  generation    of    in- 
dividuals  reproducing  only   by  a   non-sexual 
method  alternating  Avith  a  second  generation 
reproducing   exclusively  or   almost  so  in  the 
sexual    manner.      Typical  examples    of    this 
process    are    afforded    by  the   DiscomedussB, 
in   many   of   which    the  individual    produced 
by  the  development  of  the  ovum    is   a  fixed, 
Fig    '>9  —  p^d    cylindrical  organism  of  simple  structure,  known 
GENKTic     Cecklo-^^  '**  Pf^yPf  possessiug  the  jjower  of  non-sexual 
wym  Lauva  (after  reproduction   (see   Fig.  55).      By  a   series   of 
pacienstbchkr  from  trausverse  divisions  it    gives  rise  to  a  linear 

Hatschek).  ...  .         . 

colony  of  individuals  which  in  the  course  of 
development  assume  a  form  very  different  from  that  of  the 
parent  polyp,  becoming  more  com])licated  in  structure,  more 
highly  organized,  and  free-swimming.  These  organisms, 
known  as  Medusre,  are  the  sexual  generation,  })roducing  sper- 
matozoa and  ova,  the  latter  after  fertilization  developing  a 
non-sexual  generation,  a  polyp,  with  which  the  cycle  begins 
again. 


8UBKINGD0M  METAZOA. 


61 


Schematically  such  an  arrangement  may  be  represented  thus,  A  repre- 
senting the  non-sexual  and  B  the  sexual  generation  : 

/B — A,  etc. 

■^^R— A,  etc. 
\H— A,  etc. 

Among  the  Hydromedusae,  in  wliich  group  alternation  of  generations 
likewise  occurs,  the  process  is  usually  complicated  by  a  number  of  non- 
sexual generations  succeeding  one  another  before  the  intervention  of  the 
Medusa,  thus : 

,A'/A" 
A< 


. /"  \ A"  =  B  =  A, 


etc. 
etc. 


And  in  some  cases  the  succession  is  still  further  complicated  by  non-sexual 
reproduction  on  the  part  of  the  medusa,  thus  : 


/A\A" 
I' 


A,  etc. 


\, 


"\B 


< 


A,  etc. 

A,  etc. 

„  _  T./B'  =  A,  etc. 


A"  =  B 

II 


\B'  =  A,  etc. 


A,  etc. 


But  wch  complications  do  not  interfere  with  the  general  alternation 
which  invariably  occurs  in  such  forms  before  the  completion  of  the  repro- 
ductive cycle. 

Such  a  phenomenon  as  this  where  a  true  uou-sexual  gen- 
eration alternates  with  a  sexual  one  presenting  a  different 
structure  is  usually  distinguished  as  metagenesis  from  another 
form  of  alternation  of  generations  known  as  heterogony,  in 
which  the  first  generation  reproduces  parthenogenetieally, 
giving  rise  to  a  second  generation  differing  in  form  from  the 
first  and  reproducing  by  the  sexual  method.  Typical  exam- 
ples of  this  process  are  to  be  found  among  the  Trematode 
worms  (q.v.),  where  the  sexual  worm  gives  rise  to  a  sporocyst 
in  the  interior  of  which  ova,  developing  parthenogenetieally, 
give  rise  to  a  larva  which  later  on  transforms  to  the  adult 
worm.  In  a  less  perfect  form  heterogony  occurs  in  many 
lower  Crustacea  {Daphnin),  which  throughout  the  warmer 
portion  of  the  year  produce  "  summer  eggs  "  which  develop 
parthenogenetieally,  male  animals  appearing  only  for  a  short 
period  in  the  autumn,  as  a  rule,  when  the  females  produce 


•■  i 


M;] 


, 


62 


INVERTEBRATE  MORPHOLOGY. 


"  winter  eggs  "  which  develop  after  fertilization.  Here  no 
difference  of  form  exists  between  the  two  generations,  but 
such  cases,  as  well  as  those  in  which  two  sexual  generations 
unlike  in  form  and  habitat  alternate  with  each  other,  are 
usually  associated  with  the  more  typical  examples  as  in- 
stances of  heterogony. 


LITERATURE. 

0.  Hertwig.     Beitrage  zur  Kenntiiiss  der  Bildung,  Befriichtung  und  Theilung 

den  thiermhen  Eies.    Morpholog.  Jabrbucb,  i.,  iii.  and  iv.  1875-78. 
£.  van  Beneden  and  A.  Keyt.     Nouvdles  rccherehes  sur  la  fi'coadation  et  la 

divmoii  mitodque  ehcz  VAscaride  inegaloeephalc.      Bulletin  de  la  Societe 

royale  Belgiquc,  XIV.  1887. 
Th.  Boveri.     Zellenstudien.     Jenaisclie  Zeitsclirift,  xxii.  1888  and  xxiv,  1890. 
H.  lol.     Le  quadrillf  des  centres.     Arclii\es  de.s  Sciences  pbys.  et  naturelles 

(Jenive,  .xxv.  1891. 
0.  Hertwig.      Vergleich   der  Ei-  rtml  SamenHldung  bei  Nematoden.    Arcbiv 

fiir  inikrosk.  Anatomic,  xxxvi.  1890. 
F.  M.  Balfour.     A  Treatise  on  Comparative  Embryology.     London,  1880. 
E.  Metschnikoff      Emhryologische  Studien  an  Mediiseit.     Vienna,  1886. 
E.  Haeckel.     Die   Gastrula  und  die  Eifurchung  der  Thiere.     Jenaiscbe  Zeit- 

.scbr.,  IX.  1875. 


TRWUOPLAX,  THE  DIVYEMW^  AND  OHTUONECTID^.       63 


CHAPTER  IV. 


I'RIOHOPLAX,  THE  DlCYEMID.Ii:  AND  ORTHONECTID^. 


"\    ! 


Before  passing  on  to  a  description  of  the  first  type  of 
Metazoa,  it  will  be  necessary  to  consider  a  few  forms  which 
can  hardly  be  assigned  to  it  and  yet  present  too  great  a  dif- 
ferentiation of  their  component  cells  to  warrant  their  reference 
to  the  Protozoa.  A  third  siibkiugdom,  the  MeHozod,  has  been 
proposed  for  them,  but  until  more  is  known  of  the  relations 
of  some  of  them  at  least  to  other  forms  the  establishment  of 
such  a  subkingdom  seems  inadvisable. 

Trichoplax  adhwrem. 


Fig.  80.— .4,  Surface  View  and  R  TnANsvKRSE  Section  through  Tricho- 

plar  (nftpr  Schulzk). 
b  =  botryoidal  sinicture.  r  =  refractive  bodies. 

In  the  marine  aquaria  at  Gratz,  Vienna,  and  Berlin  there 
has  been  found  u  small  organism  (^Fig  30,  A)  measuring  from 


64 


INVEliTEBRATE  MORPHOLOGY. 


>: 


m 


m 


5    ■' 


1.5  to  4  ram.,  but  capable  of  great  alteration  of  form.  It  is 
flattened,  and  creeps  about  upon  the  walls  of  the  aquaria  in 
an  amoeboid  manner.  It  consists,  however,  of  numerous  cells 
(Fig.  30,  B),  the  upper  surface  being  covered  by  a  flattened 
ciliated  epithelium,  and  the  lower  formed  by  a  layer  of 
columnar  cells  also  ciliated,  while  the  space  between  the  two 
surfaces  is  occupied  by  a  network  of  branching  cells,  the 
branches  appeariug  to  unite  with  those  of  adjacent  cells  and 
with  prolongations  from  both  the  upper  and  the  lower  epithe- 
lium. The  arrangement  suggests  the  three  germ-layers  ecto- 
derm, endoderm,  and  mesoderm,  but  until  more  is  known  con- 
cerning the  reproductive  processes  such  an  homology  is 
unwarranted.  At  present  the  organism  is  only  known  to  re- 
produce by  division,  and  no  structures  have  been  discovered 
which  may  be  identilied  as  ova  or  spermatozoa.  Beneath  the 
u})per  epithelium,  imbedded  in  the  cells  of  the  middle  tissue, 
huge  refractive  spheres  (Fig.  30,  B,  r)  and  yellowish-green 
botryoidal  masses  (/>)  occur,  but  they  have  apparently  no  con- 
nection with  reproduction. 

The  Dicyemid^. 

The  DicyemidfB  are  elongated  vermiform  organisms  which 
are  parasitic  in  the  renal  organs  of  the  Cephalopods.  The 
various  species  of  Dicyema  (Fig.  31)  vary  in  length  from 
0.5-7  mm.  and  are  all  very  simple  in  structure,  consisting  of 
a  single  elongated  central  cell  (Fig.  31,  C)  extending  from  one 
end  of  the  body  to  the  other  and  covered  by  a  number  of 
ciliated  cells  arranged  in  a  single  layer.  Some  of  these,  situ- 
ated at  one  end  of  the  body,  are  smaller  than  the  others  and 
mark  oft'  the  anterior  extremity ;  there  is  no  mouth  or  diges- 
tive tract  aud  no  sense-organs. 

Reproduction  is  carried  on  by  the  development  of  germ- 
cells  {g)  produced  by  the  division  of  the  nucleus  of  the  central 
cell  and  the  concentration  around  the  nuclei  so  produced  of  a 
portion  of  its  protoplasm.  The  development  of  these  germ- 
cells  is  apparently  parthenogenetic  and  no  male  Dicyemn  is  as 
yet  knoAvn.  In  young  individuals  the  germ-cells  segment  in 
the  interior  of  the  central  cell  and  give  rise  to  "  vermiform  " 


TRICHOPLAX,  THE  DICYEMID^  AND  ORTHONECTID^. 


65 


a 


X- 


g 


embryos  (Fig.  31,  V)  similar  to  and  developing  directly  into  the 
adult  form.  Another  form  of  embryo  is,  however,  produced 
by  older  individuals,  its  formation  being 
accompanied  by  a  peculiar  behavior  of  the 
germ-cells.  The  nucleus  of  each  one  first 
divides  into  two  unecpial  parts,  the  smaller 
part  separating  as  a  paranucleus  and  under-  /  f^ifeiV  S 

going  no  further  development.  The  germ- 
cell  now  segments,  and  an  embryo  (Fig. 
31,  e)  consisting  of  a  single  large  cell 
partially  surrounded  by  smaller  cells  re- 
sults. The  smaller  cells  are  now  thrown 
off  and  separate  somewhat  from  each  other,  A^  0i 
and  the  larger  cell  repeats  the  segmentation- 
process,  the  smaller  cells  being  again  throAVu  JQ)\ 
off;  and  this  may  happen  three  or  four 
times,  the  result  being  the  production  of 
three  or  four  concentric  layers  of  small 
cells  surrounding  a  single  larger  one,  all 
lying  in  the  central  cell  of  the  parent.  The 
large  cell  undergoes  no  further  develop- 
ment, but  the  smaller  ones,  except  those 
of  the  last  generation,  develop  into  "  infu- 
soriform  "  embryos  of  a  peculiar  and  com- 
plicated structure.  The  cells  of  the  last 
generation  develop  into  "vermiform"  ^t^- Yiq.  ^\.—Dicyema  ty- 
bryos  similar  to  those  found  in  young  pu8  (combined  from 
Dicyemids.  ««^«'*'     ««"'*"^    ^'^ 

''  ,  ,  Whitman). 

The  fate  of  the  "  infusoriform  "  embryos  c  =  ceiitml  cell. 

has  not  been  determined.     Since  they  are  e  =  tmbiyos. 

ciliated  it  seems  not  improbable  that  they  ^  "^  genn-colls 

,        ,,        T  .,•  .ii  .       A  =  nucleus  of  ceut  rill 

serve  for  the  dissemination  of  the  species  ^.^.i, 

audits  transference  from  one  Ce})hal()})od  j' =  vcrniifonn     em- 
host  to   another.      It   has,    however,  been  bryo. 
suggested  that  they  may  develoj)  into  males. 

The  Outhonectida. 

The    Orthonectids    are    parasitic   on    Echinoderms    and 
Nemerteau  worms  and  resemble  in  structure  the  Dicyemids, 


U 


66 


IN VEHTEBHA  TK  MOliPlIOLOG  T. 


\ 


I 
( 


ill 


the  ectoderm  cousistiiif^  of  a  number  of  ciliutecl  cells  arranged 
iu  a  siu^le  layer  aud  eiiclosiuj^  a  mass  of  j^erm-cells  wliich 
corresj)()iid  to  the  central  cell  of  JJicyenia.  Between  the  f^erm- 
cells  aud  the  ectoderm  tine  nucleated  fibres  occur  which  are 
presumably  muscular. 

Three  forms  of  individual  are  known  to  occur  in  the  <^enus 
Rhopalura,  one  being  a   male,  aud  the  other   two   females. 


Pig.  S2.—Rhopnlura  Guirdii  (after  Jvlis). 
A,  mule  ;  B,  round  ft'iniilc  ;  C,  flat  female. 

The  male  (Fig.  32,  ^4)  is  about  half  the  size  of  the  females, 
which  measure  about  0.25  mm.  in  length,  aud  ])resents  a  met- 
americ  arrangement  of  the  ectoderm  which  does  not  extend  to 
the  internal  cells.  The  cells  of  the  anterior  segment  have 
their  cilia  directed  auteriorl}-,  and  are  succeeded  by  a  segment 
consisting  of  several  rows  of  small  uon-ciliated  cells  each 
containing  a  refractive  body,  and  behind  this  there  follow  three 
or  four  segments  formed  of  cells  provided  with  cilia  directed 
backwards.  One  of  the  female  forms  (Fig.  32,  B)  is  elon- 
gated, and  is  segmented  like  the  male  except  that  the  segments 
are  more  numerous  and  the  second  non-ciliated  segment  con- 
sists of  a  single  row  of  cells  destitute  of  refractive  bodies. 


. 


id 

r 

re 


TRICIIOPLAX,  THE  DlCYEMlD.li  AND  OHTIlONKCnD.E.       67 

The  other  ieiiiide  (Fi^.  3  ,  6*)  is,  on  the  coutraiv,  ovoid,  lliit- 
teued,  and  uiisoj^inented,  beiuj^  ciliated  all  over ;  it  dillera 
furthermore  from  the  elouj^.ited  female  in  possessiii}^  on  one 
side  uear  the  aiiterit)r  extremity  a  grauular  mass  eoiitainiuj^  a 
lar^e  uucleus  whose  sij^uitieaiice  is  entirely  problematical. 

Associated  with  the  difference  of  form  of  the  two  females 
there  is  a  difference  of  function.  In  the  elonj^ated  form  when 
the  ova  are  mature  the  anterior  two  sej^ments  split  off  as  a 
cap  and  allow  the  ova  to  escape,  and,  on  fertilization,  these 
j];ive  rise  to  males.  In  the  ovoid  form,  however,  the  ova  are 
imbedded  in  a  gelatinous  mass,  and  are  liberated  by  the 
breaking  up  of  the  parent  into  a  number  of  fragments;  from 
the  ova  females  of  both  forms  develop. 

The  systomutie  position  and  afiinitics  of  the  Dicycmidie  and  Orthonec- 
tiihi'  is  a  matter  of  uncertainty.  They  liave  been  held  by  some  autiiors  to 
possess  artinities  with  tlie  (Jfej^arinida  and  by  others  to  bo  degenerate  thit 
worms,  while  otiiers  havt;  sought  to  traee  resemblances  to  the  Rotifers. 
The  granuhir  mass  with  the  hu'ge  nucU'iis  which  occurs  in  the  ovoid 
Rhojhtlnia  lias  lieeii  supi)osed  to  represent  a  rudiment  of  a  digestive  tract, 
while  the  superlicial  metamerism  of  the  male  and  elongate  female  of  the 
Orthonectida  may  possibly  point  tea  derivation  from  more  highly  organ- 
ized ancestral  forms.  There  can  be  but  little  doubt  that  the  Dieyemida'  and 
Orthonectida  are  closely  related,  but  at  present  suflicient  evidence  is  want- 
ing to  warrant  any  definite  conclusions  as  to  their  relationships  to  other 
forms. 


i 


LITERATURE. 

TRiniOl'LAX   ADH.KUENS. 

F.  E.  Schulze      IHht    Trichoplax  aiVuvrens.      Abbandl.  Akad.  Wiss.  Berlin, 
1891  (see  also  Zoolog.  Anzciger,  vi.  1883). 

DICVKMIDiE. 

C.  0.  Whitman.    A  Contribution  to  the  Emhnjology,  Life-Jiiatory,  and   Cliisiti- 
ficatioti  of  the  Dicycmiih.    Mitth.  a.  d.  Zool.  Station  zu  Neapel,  iv.  1883. 

OUTIIONECTIDiK. 

C.  Julin.    Contributions  d  Vhistoire  des  Meaozoaires.    Archives  de  Biologie,  iii. 
1883. 


-W 


IN  VEUTEBHA  TE  MOlil'BOLOO  Y. 


CHAPTER  V. 


TYPE  C'(ELENTEKA. 


I 
i 


The  Cojlentera  inehule  the  diplobhistic  Metazoa,  only 
two  geriu-hiyers,  the  ectoderm  and  eudoderm,  being  re  pre- 
sented in  their  organization  (Fig.  33).     Between  these  two 

hiyerw,  however,  a  third  (Fig.  33,  nnj) 
is  invariably  present,  which  in  its 
primitive  condition  is  not  cellular,  but 
consists  of  a  gelatinous  or  fibrous  sub- 
stance secreted  by  one  t)f  the  two 
cellular  layers.  Usually,  however, 
cells  from  the  endoderm  or  ectoderm 
wander  into  it,  and  sometimes  are  so 
numerous  as  to  give  it  the  appearance 
of  a  cellular  layer.  Even  in  such 
cases,  hf)wever,  the  gelatinous  matrix 
is  the  fundamental  substance  of  the 
Fig.  33.— Dia<ji{am  ok  ////-  layer,  Avhich  it  seems  preferable  to 
dm  TO  8IIOW  TiiK  Gen-  term  the  me«or/?(m,  rather  than  to  implv 

KKAL     StkUCTUUE     OF     a  ,  ,  I'll  L  •    i  1  " 

,,  an  homology  winch  does  not  exist  by 

COiliENTKUATE.  ... 

ec  =  ectoderm.  designating  it  the  mesoderm. 

en  =  eiidockrm.  In    consequence    of     the    absence 

wir  =:  mcsogldii.  ^^f    the    mesoderm     the    CVelenterates 

present  in  the  interior  only  a  single  cavity.  Consequently 
it  may  be  said  that  the  c<»']om  is  not  represented  in  the 
C(elentera,  though  their  central  cavity  is  usually  regarded 
as  equivalent  to  both  ccelom  and  euteron  of  the  higher 
forms.  The  so-called  endoderm,  however,  seems  to  be  homol- 
ogous with  their  mesoderm  plus  endoderm,  and  may  be  more 
accurately  termed  the  mcs-emlodermf  and  it  seems  preferable 
to  regard  the  c<elom  as  not  yet  differentiated. 

Another  feature  which  obtains  throughout  the  group  is 
the  radiate  ground-form.     In  many  but  one  axis  can  be  de- 


TYPE  CCKLENTKIU. 


69 


termiued,  and  in  the  Spoiij^es  the  form  may  become  so  irre^u- 
hir  that  thay  ma}'  be  considered  to  be  destitute  of  axes.  In 
such  forms  as  the  Mcdusic,  however,  a  typical  radiate  form 
occurs,  there  being  two  or  more  simihir  axes  at  right  angles 
to  the  vertical  one,  and  thronghout  the  higher  members  of 
the  group  this  radiate  symmetry  is  more  or  less  apparent, 
though  it  becom«>s  (hu-idedly  obscured  in  certain  Authozoa  by 
a  pronounced  tendency  towards  bilaterality,  which  iu  a  few 
forms  {(\'rviiiflii(f<t')  actually  replaces  it. 

In  correspt)ndence  with  their  low  grade  of  general  struc- 
ture there  is  no  very  exteusivt;  difl'erentiation  of  tissues.  A 
considerable  degree  of  division  of  labor  of  course  occurs 
among  the  cells,  and  <m'11s  having  the  same  function  may  be 
aggregated  together  so  as  to  form  a  somewhat  definite  tissue, 
as  in  the  case  of  the  nerve,  n)uscle,and  reproductive  cells,  but 
even  iu  these  tissues  there  seems  to  be  a  considerable  amount 
of  individuality  retained  b}-  the  constituent  cells,  and  the  tis- 
sues can  only  be  reganled  as  exceedingly  diti'use.  Of  organs, 
except  in  some  cokudal  forms  with  division  of  labor  among 
the  constituent  individuals,  it  is  hardly  correct  to  speak,  the 
Co'ltMiterates  not  having  i>rogress  mI  beyond  the  organ  stage 
of  individuality. 

The  ty})e  Co'lentera  may  be  divided  into  two  subtypes, 
the  Pori/era,  or  Spongt  s,  aud  the  Cnidiiria. 


I.  SuuTYi'i:  Porifera. 

The  Sponges,  on  account  of  their  fixed  life  aud  irregular 
form,  were  long  regarded  as  i)lants,  and  it  is  only  within  com- 
paratively recent  times  that  their  true  relationsiiii)s  have  beeu 
ascertained.  They  are  almost  exclusively  marine  in  habitat, 
occurring  in  large  uundxas  in  the  warmer  seas,  aud  inhabit 
the  ocean  depths  as  well  as  the  shallower  waters.  A  few 
genera,  e.g.  SpomjiUa,  Ephyatia,  represented  by  numerous 
species,  are  inhabitauts  of  fresh  water. 

The  simplest  Sjjouges  (Fig.  Si)  have  the  form  of  a  hollow 
cylinder  fixed  at  oue  end,  while  at  the  other  is  an  opening, 
the  osculum,  and  scattered  over  the  surface  of  the  cylinder 
are  a  number  of  smaller  opeuiugs,  the  pores.     Through  these 


1 


70 


INVKUTKBRA TK  MORPUOLOQ  Y. 


11 


I 


tl| 


water  passes  into  the  ceutrul  cavity,  the  eculeuteron,  and 
escapes  by  the  osculuiu.  The  exterior  of  the  body  is  covered 
by  a  layer  of  flat  cells,  the  ectoderm,  aud  the  co'leuterou  is 
liued  by  collared  cells  provided  with  a  siiij^le  Haj^ellum  aud 
reseinbliii|4  j^reatly  Autotlagellata  beloiij^'iuj^  to  the  geuus 
Cixiotiiga.     These  cells  constitute  the  eudoderiu,  aud  betweeu 


fWkS^ 


09 


Fio.   34.— An  Ascon       Fig.   35.  —  Diaoram   to   snow   the  General 
Si'ON(ifc:.J««,e«a /)/•*-  STiUJcnruE  ok  a  Sycon  Sponge. 

morUialin  infifi-  Hab-  Tlie  upper  portion  rei)iescnts  the  simplest  con- 

CKKL  from  SoLLAs).  ditiou,  the  complexity  increasing  downwurds. 

c/;  =  ciliuted  cliiimber.  ic  —  iubulent  cuiinl. 

Os  =  osculum.  p  =  inbuleut  pore. 

pr  =  prosopylc. 

it  aud  the  ectoderm  is  the  mesoglcea,  iu  which  are  imbedded 
larj^e  uumbers  of  cells,  giving  it  almost  the  appearance  of  a 
cellular  layer. 

In  such  simple  Sponges  the  mesoglcea  is  comparatively 
thiu  and  the  pores  open  almost  directly  into  the  coeleuteron 
liued  by  the  collared  cells.  This  arrangemeut  constitutes  the 
first  or  Ascon  type  of  structure.  In  the  majority  of  forms  a 
much  greater  complexity  arises  from  the  walls  of  the  simple 
cylinder  being,  as  it  were,  drawn  out  into  a  number  of  fiuger- 
like  processes,  each  of  which  communicates  by  a  wide  open- 


TYVK  CiKLKNTKHA. 


71 


iuR  witli  tbo  ciivity  of  tho  orij^iuul  cyliudor  (Fij^.  35).  The 
cells  liuiuj^  tbo  I'lMitni!  cavity  boooiiK'  riiittened,  tbo  collari'd 
cells  beini'  fmiud  oulv  in  tlu^  iuterioi"  «>f  tbe  seooiubirv  cvliu- 
ders  wbicb  rudiato  t'lom  tb«>  t'cutnil  cbainber  {ir).  Porrs, 
icniied  pnmopiifi's  ipr),  (»('(Mir  in  tb(?  walls  of  tbe  secondaiy 
cylinders,  wbicb  arc  closed  at  tbeir  free  en«ls.  Tbroiij^b  these 
|»rosu|>yles  water  passes  into  tbe  interior  of  tlu*  radiatinj^  cyl- 
inders, tbeu(Hf  into  tlu;  central  cavity  and  so  to  tbe  exterior 
by  tbe  osculuni.  rurtber  complication  occurs  by  tbe  walls 
of  tbe  radiatin''  cvliiulers  connny;  in  contact  with  each  other 
and  fusing  in  a  more  or  less  irrej^ular  manner,  tln^  s[)ace  be- 
tween tbe  various  cylinders  beinj^  thus  divided  into  a  serie.-; 
of  more  or  less  well-detined  iubalent  canals  (/c)  into  wbicb  tbe 
water  j)asses  through  pores  {p)  which  lie,  morpholo«;ically,  be- 
tween tbe  extremities  of  the  radiating'  cvliuders.  The  cavities 
of  these  cylimlers  now  form  the  ciliated  chambers,  and  Sponges 
in  wbicb  they  possess  tbe  cylindrical  form  are  said  to  belong 
to  the  Sycon  type.  The  annexed  diagram  (Fig.  35)  illustrates 
the  dilfereut  stages  of  complexity  met  with  in  S3'cou  Sponges. 

Tbe  next  complication  consists  of  tbe  branching  of  tbe 
ciliated  cbamlxns,  though  they  still  retain  a  cylindrical  shape, 
and  tbeir  separation  from  the  central  cavity  by  a  tract  lined 
with  flattened  cells  (Fig.  3(5,  A)  ;  and  finally  tbe  collared 
cells  become  limited  to  a  portion  of  tbe  radial  chambers, 
tbe  ciliated  canals  thus  becoming  circular  in  shape  and  united 
with  tbe  central  chamber  by  long  and  rather  slender  canals 
lined  with  flattened  cells  (Fig.  30,  B).  This  constitutes  what 
is  termed  tbe  Lexicon  type  of  structure.  In  this  type  tbe 
pores  upon  the  surface  of  the  Sponge  frequently  do  not  open 
directly  into  tbe  canals  leading  to  tbe  ciliated  chambers, 
but  into  a  wide  lacunar  cavity,  tbe  suhifcrmal  spuce,  lying  be- 
low tbe  cortical  layers  of  tbe  sponge,  and  with  this  tbe  canals 
communicate. 

To  these  complications  of  arrangement  further  complexity 
is  added  by  the  occurrence  in  many  Sponges  of  what  may  be 
considered  budding,  in  some  cases  leading  to  tbe  formation 
of  definite  branches,  or  in  others  ])roducing  only  a  number 
of  oscula,  each,  however,  with  its  own  canal  system. 

Tbe  general  characteristics  of  tbe  ectoderm  and  endoderm 


* 


!l. 


ri 


IN  VEliTEUHA  TE  MOHPUOL  0  G  Y 


have  beeu  iiiilicateJ  iu  the  pieceiliuj^  clesciiptiou  of  the  cauul 
systems  ;  the  iiieso^kjuu  ie(|uiies,  however,  further  uotice.  It 
cousists  of  u  gelatinous  luatrix  which,  luiwever,  coutaius  hirj^e 
numbers  of  cells  ])reseutiuj^  a  cousiderable  amount  of  dif- 
ferentiation. Some  are  amoeboid  in  form,  others  contain  pij^- 
ment,  others  ajj;ain  are  elon«;ated  and  spindle-shaped,  forming 
tiie  contractile  cells,  others  form  the  repr«jductive  elements, 
ova  and  spernuitozoa,  while  others  aj^ain  are  skeletoj^enous 
in  function,  well-developed  skeletal  structures  being  present 


Fl(i.  30— Two    FlUtniES    SIIOWINU    DiPFliUKNCES    in    THK    (.'nMIM.KXlTY   OF 

SruucTiMUi  OF  A  Li!;i;c(»N  Si'onok. 

A.   /<«'«ct7/a  rtter  (after  Dkndyi;  B.   ihcardla  lohuLavi»  ieXwv'&K\\K\.7x.\. 

ec  —  ciliated  cliumbcr.  p  =  iulmloiit  pore,  ap  —  spicule. 

in  almost  all  Sponges.  In  some  forms  the  skeleton  presents 
the  form  of  siliceous  spicules  either  of  a  simple  needle-like 
form,  or  presenting  modifications  of  a  four-  or  six-rayed 
grouud-ft)rm,  or  tinally  assuming  the  form  of  hooks,  anchors, 
(»r  sj)iny  spheres.  In  another  group  there  is,  associated  usu- 
ally with  needle-like  siliceous  sj)icules,  a  network  of  a  horny 
material  termed  siMuijiolin  which  forms  a  su[)portivt!  scatlold- 
ing  for  the  soft  parts  of  the  Sponge,  and  lastly,  in  another 
group  the  spicules  are  composed  of  carbonate  of  lime  and 
present  a  variety  of  forms  (Fig.  3(5,  .1). 


TYPE  CCELEHTEUA. 


?:} 


ul 
It 

if. 


«- 


IS 

It 


Nerve-cells  have  also  beeu  tlescribed,  thouj^h  a  dethiito 
nervous  systoin  cauuot  be  said  to  exist.  Elongated  retractih* 
processes  have  been  observed  projecting  from  the  surface  of 
certain  sponges,  and  at  the  base  of  each  is  a  group  of  stellate 
cells  each  of  which  stands  a  long  slender  ])rolongLtion  into 
the  process.  To  these  cells  a  nnrvous  function  has  been  at- 
tributed and  the  processes  have  beeu  considered  sensory  ; 
A.ith  the  exc('j)tion  of  thest?  structures,  however,  no  sense- 
organs  or  ncrvc-elenients  have  been  yet  observed. 

The  Sponges  may  be  arranged  according  to  the  nature  of 
their  skeleton,  in  four  orders. 

1.  Order  Calcarea. 

In  the  Calcarea  the  skeleton  is  always  present  and  is 
formetl  of  s])icuh!S  consisting  of  carbonate  of  lime.  'J'he 
grou])  contains  forms  of  various  complexity  of  structure,  from 
the  simj)le  cylindrical  Leucofiohma  constructed  upon  the 
Ascon  t3'po  through  Sycon  forms  such  as  O'nniti '.  to  repre- 
sentatives of  the  third  and  fourth  ty})es.  Indeed  it  is  only  in 
this  group  that  the  Ascon  and  Sycon  tyj)es  of  structure  an^ 
found.  All  the  kn«)\vn  species  are  nuiriue  and  live  at  only 
slight  depths. 

2.  Order  CornacuspongiflB. 

The  sk«'l(^ton  of  the  Cornacuspongije  consists  either  of 
siliceous,  needle  like  spicules,  fre<puMitly  more  or  less  united 
b\-  sjjongiolin,  or  else  entirely  of  a  network  of  fibres  com- 
])osed  of  the  lattei'  substance,  'jike  the  Calcarea  they  are 
inhabitants  of  shaihtw  water  and  ar»»  foi-  the  most  part  marine, 
tlioiigh  some  forms  [SfH)ii<filIa,  Fig.  157,  h)>li//(li(fi(i)  live  in  fresh 
water.  These  fresh-water  forms  ai'e  of  a  greiMi  color  due  to 
chlorophyll,  the  presence  of  this  pigment  being  supposed  by 
some  observers  to  depend  on  numerous  unicellular  algje  liv- 
ing in  the  substance  of  the  Sponges.  To  this  group  belongs 
also  the  S))o.  4e  of  commerce  {J'JusfxiiK/ia),  whose  value  de- 
])ends  uj)i)n  tht;  i'utire  absence  of  siliceous  spicules,  and 
which  is  found  in  the  shallow  waters  of  the  eastern  portion 
of  the  Metliterranean,  in  the  lied  Sea,  and  in  the  Western 
Hemisphere  in  the  waters  surrounding  the  Bahama  Islands. 


1  li 


<lf 


i'l 


74  INVKliTEliRATM  MOIiPHOLOU Y. 

W.  OnliT  Spiculispongiae. 

TliG  skeletou  in  the  S[»iculis[)()nj^i!i'  is  occjisionjillj'  entirely 
wuntin^,  Jis  in  tlie  {j;enus  //iilLsarcd,  hut  usujilly  consists  of 
siliceous  s})icul('s  usuully  tt'truxiiil  or  rod-  or  club-shapod, 
sometimes  interlocking  with  one  another  so  jis  to  form  a  firm 


Fiu.  y?.— A  S.MAI, I.  tipongilla  with  only  a  siNdi.K  ()s(  ti.im  (from  Hcxley). 

a  =  iiilialfiil  \m\a.  c  —  cilialt'd  cliamlic  r  sucii  lliroiii;!! 

d  —  osculuin.  the  tissues. 

skeleton.  One  of  the  members  of  the  ;j;i()U])  is  tlu;  "boriuf^ 
s[)ou<j;e,"  C'ltona,  which  excavates  clumnels  in  and  assists  in 
the  disintejjjration  of  oyster-shells,  frecjuently  attackinjjj  the 
shells  of  living  animals  ant  I  contributing  to  their  destruction. 

4.  Older  Hyalospongiae. 

The  Hyalospongiio  are  essentially  deepsiNi  forms,  and  are 
characterized  by  the  possession  of  six-rayed  siliceous  spicules 
as  skeletal  (demiMits.  Tin^  .«picuh's  may  become  fused  to- 
gether to  form  a  firm  siliceous  m^twork  having  tin;  api)»'ar- 
ance  of  spun  glass,  as  in  the  genus  I'Jii/tlccfrlld,  commonly 
known  as  Venus'  Flower-baskt't. 

JieprodKclian  ()/'  f/ic  Pori/cni. — Sexual  reproduction  occurs 
probabl}'  througlnnit  the  entire  group  of  the  S[)ougeH,  the  re- 


TYPK  CiKLKNTEliA. 


76 


productive  oloinouts,  ova  ami  spermatozoa,  dift'oreutiatiiig 
from  mes()<j;l(r;il  cells.  Mauy  Spouj^es  are  liermaphroilite,  the 
Hj)ermat()/,();i  dcvclopiu^  usually  somewhat  iu  advauee  of  the 
ova,  but  SOUK!  I'oniis  seem  to  have  separate  sexes.  The  ova 
are  fertilized  \vhil(>  still  withiu  the  tissues  of  the  pareut  ami 
undergo  a  portion  of  their  deveh)i)ment  there,  later  breakiufj; 
throuj^h  into  a  eanal  and  so  passinj^  to  the  exterior  as  a  cili- 
ated free-swinnniu^  structure.  The  sej^mcMitatiou  of  the 
ovum  in  typicul  cases  results  in  the  formation  of  a  blastula, 
which  becomes  converted  into  a  soliil  ciliated  sterrula  by 
immi<j;rati()ii.  After  swimmin;j;  about  for  a  time  the  sterrula 
los«>s  its  cilia  and  settles  down,  a  cavity  aj)pearin},'  iu  its  in- 
terior, which  later,  in  forms  which  possess  ciliated  chambers, 
gives  ris(i  to  these  striu'tnres  as  a  series  of  (xjuclies  in  connec- 
tion with  which  I'anals  arise.  The  [)ores  and  the  osciUum 
finally  break  throuj^h,  in vaj^i nations  from  the  exterior  givinj; 
rise  to  the  former. 

In  som(>  forms,  however,  the  blastula  staj^e  undergoes 
invagimition,  usually  of  a  rather  piiculiar  form,  iu  which  case 
a  gastrula  results  instead  of  a  sterrula.  The  gastrula  settles 
<lown  ami  beconn^s  fixed  by  the  pole  at  which  the  blastopore 
occurs,  tlu^  further  «levelo[)ment  being  similar  to  that  found 
in  the  immigration  ty})es. 

Iu  ad<lition  to  tin;  sexual  nu'thod  most  Si)onges  also  ]>os- 
sess  the  |)owerof  non-sexual  reproduction,  dependent  on  thfir 
ca])al)ilities  for  regeneration.  A  detached  portion  of  a  Sponge 
will,  und»!r  favoiabh^  coiulitions,  regenerate  into  a  new  indi- 
viilual,  anil  this  power  has  been  applied  to  the  artificial  re- 
production of  the  commercial  Spongers.  In  some  forms  in 
addition  to  this  a  process  of  intermil  budding  occurs,  a  num- 
b(>r  of  the  nn'sogheal  cells  aggregating  together  and  develop- 
ing into  an  oval,  ciliatiul,  sterrula-like  structure  which,  leaving 
the  ])arent,  develops  into  an  adult  Sponge  (Es/htcIIh).  Iu  the 
fresh-watiM-  SpoiHjUld  this  process  is  carried  to  the  greatest 
extent,  and  towards  the  approach  of  winter  in  temperate 
latitudes  compli»tely  replaces  the  sexual  method.  The  in- 
ternal buds  of  S/KiiKjilla  luv.  known  as  [jvininnh's  ami  are 
especially  adaj)ted  for  tiding  the  spiu'ies  over  unfavorable 
couditions,  sucli  as  cold  or  drvn<!ss,  which  tins  vegetative  in- 


ii 


I  ■> 


i>i' 


1. 


(6 


INVERTEBltATE  MOliPlIOLOU  Y. 


ilividiiJil  cannot  witlistand.  Tliev  are  spherical  bodies  con- 
sistiu}^  of  a  mass  of  cells  richly  laden  with  food-matt<^r,  and 
enclosed  in  a  double  chitinous  wall,  with  an  oi)euing  at  one 
])oint,  a  number  of  siliceous  spicules,  in  the  allied  j^enus 
J'.'phydnfm  of  a  very  characteristic  form  and  known  as  (itnpJn'- 
lUacs,  beiuj^  arranfjjed  between  the  two  layers  of  the  wall.  On 
the  approach  of  cold  weather  the  Sponge  dies  down  and  the 
gemmules  thus  fall  to  the  bottom  of  the  ponds  or  streams, 
where  they  remain  unchanged  until  the  approach  of  warmer 
weather,  when  the  internal  cellular  mass  flows  out  through 
the  pore  (which  is  closed  only  by  a  thin  membrane)  and  de- 
velops into  a  new  Spongilla. 

The  relationships  of  tlie  Sponges  have  long  been  a  matter  of  discussion. 
For  a  long  time  they  were  regarded  as  plants  and  later  as  colonies  of  Pro- 
tozoa, but  the  discovery  of  sexual  reproduction  in  them  and  of  their  mode 
of  development  demonstrated  that  they  were  to  be  considered  Metazoa. 
At  i)resent  the(|uesti<)u  as  to  whether  they  are  to  be  associated  with  the 
CtdUaria  among  tiie  Co'lenterates  or  regarded  as  a  distinct  type  is  still 
open,  thougii  the  weiglit  of  evidence  and  authority  is  in  favor  of  their 
Cu'lenteratf  character.  Such  simple  forms  as  Lencosolotia  ceriainly  point 
ill  that  direction,  and,  if  the  occurrence  of  a  sterrula  formed  by  immigra- 
tion prove  the  typical  mode  of  development,  the  embryology  of  the  Sponges 
presents  stages  up  to  thy  formation  of  the  ciliat<'d  chambers  which  are 
step  by  step  comparable  to  what  occurs  in  the  Cnidarla. 

II.  Subtype  Ciiidaria. 

The  CniiJarhi,  like  the  Sponges,  have  in  their  simplest 
forms  the  general  form  of  a  hollow  cylinder  open  at  one  end 
and  consisting  of  but  two  cellular  layers,  the  ectoderm  and 
endt)derm,  between  whidi  is  interposed  a  tibrous  or  gelatinous 
mesoghi'a  which  may  ()r  may  not  contain  cells.  Differences 
from  the  Sponges  are  found  in  the  occurrence,  exce})t  in  one 
or  two  forms,  of  a  number  of  elongated,  contractile  processes 
or  tentacles  around  the  mouth  of  the  cylinder  (see  Fig.  HJ3), 
and  in  the  absence  of  inhalent  jion^s  upon  its  surface.  Such 
simple  forms  are  known  as  poh/ps,  and  they  are  usually 
attached  organisms  with  little  or  no  power  of  locomotion.  A. 
large  number  of  Cuidafiu  })resent  a  v(!ry  different  form,  how- 
ev»'r,  b«'iug  disk-  or  bell-sha])ed,  a  process  conii)arable  to  the 
clapper  of  a  bell    hanging  down  from  the    centre  and  hav- 


TYPE  CiELENTmiA. 


77 


iug  at  its  extremity  the  inouth-opeuiuj;.  This  leads  into 
a  central  cavity,  the  ciL'leuterou,  lying  in  tlie  substance  of 
the  bell,  and  from  this  pouches  or  line  canals  radiate  out 
towards  th(i  rim,  where,  in  some  cases,  they  are  united  by  a 
circular  canal  which  runs  completely  round  the  bell  at  this 
region.  To  the  margin  of  the  bell  tentacles  are  usually 
attached,  and  sense-organs,  presenting  frequently  consider- 
able complexity  of  structure,  are  found  in  the  intervals  be- 
tween the  tentacles  or  at  their  bases.  These  forms,  known 
as  medusa',  are,  as  a  rule,  free-swimming,  propelling  them- 
selves through  the  water  by  vigorous  contractions  of  the  bell. 
All  Cnidaria,  whether  of  the  polyp  or  medusa  form,  pos- 
sess in  their  tissues  i)eculiar  elements  altogether  unrepre- 
sented in  the  Sponges.  These  are  the  ^_  d 
ncmnfocysfs  or  so-called  thread-cells. 
Each  consists  of  an  oval  or  spherical 
cyst  (Fig.  38,  v)  with  a  membranous  wall 
and  fluid  contents,  the  wall  being  pro-  * 
longed  at  one  end  into  a  long,  exceed- 
ingly delicate,  hollow,  thread-like  fila- 
ment sometimes  furnished  with  spines  at 
its  base,  and,  in  an  undisturbed  cyst,  is 
invagiuated,  and  coiled  up  in  the  interior 
(Fig.  38,  t).  These  nematocysts  are  pro- 
duced  by  and  enclosed  within   special  Fig.     38.  —  Nematocyst 

cells     known     as   cnidohhiHtH   (cm)    lying        Cici-i-  of  Phi/salia. 

1,     •      .1  ,1  1  •      ii     •        "  =  iioniatoi'yst. 

i)rincipally  in  the  ectoderm,  and  m  tlieir     ,         . ,    ., 
*  r      J  ,.   .  ct  =  cnitldcil. 

most    highly   developed    form   ditferen-  <,«  =  cuidoblast. 
tiatiiig  below  into  a  su}>])()rting  stalk  (.v)    n  —  nerve  prolongation, 
which  rests  upon  the  outer  surface  of     «  =  supporting  i)roces3, 
the  mesoghx'a.     J'rom  the  outer  extremi- 

tv  there  jjrojects  beyond  the  surface  of  the  ectoderm  a  short 
hair-like  ]>rocess,  iha  cni(locil{cl),  and,  in  addition,  an  exceeding- 
ly flue  ])rocessof  son:e  'engtli  (/.)  is  given  oft'  at  the  junction  of 
the  stalk  with  the  cyst-containing  portion  of  the  cell.  These 
two  processes  are  supposed  to  be  sensitive,  the  hmger  one  per- 
haps bringing  the  cuidoblast  into  connection  with  nerve-cells 
lying  elsewhere  in  the  (ectoderm.  In  some  way  not  yet  prop- 
erly understood,  a  stimulation,  such  as  a  touch  by  some  foreign 


78 


INVEUTEBHATE  MOliPUOLOG  Y. 


boil}-,  produces  au  e  vngiuutiou  of  the  coiled  thread  from  the  cyst, 
in  the  interior  of  wliich  it  has  beeu  bathed  by  the  Huid  contents. 
Theeva^inatioii  is  of  siitiicient  force  apparently  to  puuctiire  the 
skin  of  many  animals  and  so  inoculate  the  contents  of  the 
cyst,  which  are  of  a  poisonous  nature,  and  produce  intlam- 
niiitory  disturbances,  and  in  minuter  orj^anisms  paralysis  or 
dt'ath.  To  the  presence  of  these  structures  jelly-tishes  owe 
their  stinging  j)owers,  and  they  form  efficient  weapons  both 
for  obtaining  food  and  for  warding  ofl'  enemies. 

The    Cnidaria   may    conveniently    be  divided    into    three 
classes : 

I.  Class    Jlifdromcdustv. 
II.        "      Scj/pliotiiedusai. 
III.        "      Anthozoa. 


(■• '' 


li 


1.  Class  Hydromedusje. 

The  Hydroraedusa^  present  both  polyp  and  medusa  forms, 
the  members  of  some  of  the  orders  contained  within  the 
class  being  only  of  the  medusa  form,  while  in  another  order 
only  the  polyp  form  occurs.  Usually,  however,  both  forms 
occur  in  more  or  less  perfect  developm<Mit,  representing  two 
stages  in  tli«^  life-history  of  an  individual  and  succeeding  one 
anotlun-  in  the  definite  manner  which  has  been  already 
described  as  an  alternation  of  generations.  The  i)()lyi)s  pos- 
s(»ss  th(^  powiM'  of  non-sexual  reproduction  by  budding  and 
give  rise  by  this  method  not  only  to  medusa^,  but  also  to 
other  ])oly])  individuals  which  may  remain  in  connection  with 
one  another  and  thus  give  rise  to  branching  colonies.  As  a 
rule,  the  medusa  individuals  se|)arate  from  the  polyj)  and 
lead  a  free  life,  but  in  the  order  Sip/nnioplionv  they  may 
remain  in  connection  with  each  other  ami  with  polyp  individ- 
uals, uudei'going  various  adaptations  of  form  in  accordance 
with  ditl'erent  functions  which  they  assunu',  the  whole  form- 
ing a  colony  presenting  in  a  high  degree  a  divisi(m  of  labor 
among  the  component  individuals. 

As  has  already  been  })ointed  out  the  structure  of  the  })olyj) 
form  differs  considerably  from  that  of  the  medusa.  The 
polyp  is  more  or  less  cylindrical  in  form,  tapering  off  above 


TYPE  CaCLKNTEKA. 


79 


I' 

e 
[e 
11- 
[r 

le 

n 


so  that  the  free  end  has  a  somewhat  couical  shape,  the  montli 
beiuf^  situated  at  the  eud  oi  the  cone.  At  the  base  of  the 
coue,  which  is  termed  the  hyi^o- 
stotne,  there  is  usually  a  circle  of 
teutacles,  though  occasionally 
they  are  scattered  irregularly  over 
the  surface  of  the  hypostome,  and 
a  second  circle  of  tentacles  may 
be  present  at  the  base  of  the 
polyp  (Peinuiriu).  In  colonial 
forms  each  individual  [hifdnintli) 
(Fig.  39,  ////)  is  situated  at  the  ex- 
tremity of  a  stalk  or  hijdrocitutus 
(he),  the  vari(jus  stalks  either 
uniting  together  to  form  a  branch- 
ing colony  or  else  arising  from  a 
network,  the  hi/tIrorlnz((,  which 
covers  the  surface  upon  which 
the  colony  grows ;  or  occasion- 
ally each  hydriuith  arises,  with- 
out the  intervention  of  a  hydro- 
caul  us,  from  u  tlat  plate-like 
ex])ansiou  ciiiiuuoii  to  all.  In 
either  case  the  t'(i>leuter(m  is 
continuous  tlirougliout  the  entire 
coKmy,  the  tlesliy  substance  of 
the  hydrocaulus  and  h3'droriiiza, 
voenoHdVC  {co),  being  tubular  ami  in 
\lirect  continuity  with  the  body- 
walls  of  the  liydranthh.  The 
ctenosarc  is  encdosed  within  a  cliitinous  substaiic*'  termed  the 
pi'ri.S(ir\  [p)  secreted  by  the  ectodermal  cells  ami  sometimes 
prolonged  at  the  end  of  each  Liydrocaula.s  into  a  cup-like 
structure,  the  hydrotheca  (ht),  into  which  the  hytlranth  may 
be  retracted  ;  occasional!'  tiiis  ectodernuil  secretion  taki!S 
the  form  «»f  carl)onate  of  ii-Lit;. 

The  ectodtiim  general] v  4iti))ws  a  considerable  amount  of 
ditl'erentiatioa  of  its  c  ^^r^txunit  esells.  In  addition  to  the 
cniJoblasts,  which  Ilxw  been  aoxeiuiv  described  (p.  77),  epithe- 


y 


FlO.  39.— POUTION  OF  A  Col,«)NY 
(»F    TIIK    C'AMl'.XNULAKtAN     llv- 

uuoiD  Vlytia 

CO  —  caMiosarc. 
/ic  —  liydrocaiilus. 
ht  —  hydrolliccii. 
hy  =  hydriiiitli. 
ffo  =  uoiiollu'cti. 
m  =  inoiilh. 

p  =  pcrisurc. 

t  =  teiilticle. 


I 


■  f^jmm 


80 


IN  VEUTKBliA  TE  MOliPUOLOO  Y. 


■A* 


"i 


Fio.  40.—^  =  Eplthelio- 
inusculur  cell  of  C'u'leiu 
turulo  ;  B  =  iniiscuiiir 
coll ;  C  =  seusory  cell. 


lio-muscular,  niiiuculur,  glandular,  and  nerve  cells  are  gener- 
ally to  be  found  in  it.  The  epitlielio-iuuscular  cells  (Fig.  40, 
A)  sire  the  most  numerous  uml  consist  of  columnar  colls,  one 

extremity  of  which  bears  a  cilium  and 
helps  to  form  the  outer  surface  of  the 
body,  while  the  other  is  prolonged  into 
a  somewhat  spindle-shaped  process  of 
highl}'  contractile  muscular  substance. 
The  muscle-cells  (Fig.  40,  Ji)  are  modi- 
fications of  these,  having  lost  their 
connection  with  the  surface  of  the  body, 
the  cell-protoplasm  and  nucleus  form- 
ing a  small  elevation  on  the  muscle- 
libre.  The  muscle-Hbres  rest  upon  the 
outer  surface  of*  the  mesoghea,  and  in  the  ectoderm  are,  as  ji 
rule,  arranged  longitudinally,  so  that  by  their  contraction 
they  cause  a  shortening  or  retraction  of  the  polyp.  The  nerve- 
cells  are  of  two  kinds :  (1)  sensory  cells  (Fig.  40,  ('),  which  are 
slender  cells  whose  free  end  bears  a  single  cilium,  while 
the  inner  end  is  produced  into  one  or  more  slender  nerve-proc- 
es.ses  which  are  supi)osed  to  place  these  cells  in  connection 
with  (2)  the  ganglion-cells.  These  are  stellate  cells  lying  in 
the  deeper  layers  of  the  ectoderm,  just  external  to  the  muscle- 
cells,  and  sending  off  delicate  processes  in  various  directions 
so  as  to  form  a  i)lexus  of  nerve-fibres  ramifying  through  the 
ectoderm. 

The  mesogkra  is  thin  and  more  or  less  fibrous  in  structure, 
and  rarely  contains  cells.  The  endoderm-cells  are  large  and 
are  of  the  epithelio-muscular  variety,  the  muscle-fibres  hav- 
ing a  circular  arrangement  producing  by  their  contraction  a 
diminution  of  the  diameter  of  the  polyp.  The  proto])lasmic 
portion  of  each  cell  is  furnished  with  a  single  flagellum  and 
is  digestive  in  function,  taking  food-particles  into  its  sub- 
stance and  there  digesting  them.  In  addition  to  these,  gland- 
ular cells  also  occur  in  liie  endoderm,  especially  in  the  region 
of  the  hypostome. 

The  medusa  forms  have  the  slia])o  of  a  bell  (Fig.  41) 
which  may  be  either  shallow  oi  dee]),  the  mouth  of  the 
bell  in  all  cases  being  partially   closed  by  a  fold  of  tissue 


TYPE  C(ELENTEl{A. 


81 


which  projects  from  the  rim,  ami  is  known  as  the  velum; 
from  tlio  pitsseuce  of  this  structure  these  meilusio  have  beeu 
termed  crasptulote  medusw.  The  cavity  between  this  ami  the 
bell  is  th(!  snhumhreRar  cavity,  into  which  projects  u  process 
corresponding  to  the  clapper  of  the  bell  and  termed  the 
manvJbrium.  At  the  free  end  of  this  is  the  mouth,  which 
o})ens  into  a  canal  traversing  the  manubrium  and  communi- 
cating at  its  base  with  the  gastric  cavity  lying  in  the  sub- 
stance of  the  bell.  From  this  four  (in  some  cases  more) 
canals  or  ])ouches  radiate  out  towards  the  rim  and  com- 
municate there  with   a  circular   canal    which   extends   com- 


Ft(}.  41. — Liriope  xcutigera  (after  Brooks). 

pletely  round  the  margin  of  the  bell,  just  where  the  velum 
joins  it.  The  canal  of  the  manubrium,  the  gastric  cavity, 
and  the  radiating  and  circular  canals  together  constitute  the 
coelenteron.  To  the  margin  of  the  bell  tentacles,  varying  in 
number,  are  generally  attached,  on  or  between  the  bases  of 
which  sense  organs  are  to  be  found. 

The  medusro  differ  not  only  in  form  but  also  in  habit  from 
the  polyps,  being  free-swimming  organisms,  propelling  them- 
selves through  the  water  by  expelling  the  water  from  the 
subumbrellar  cavity  through  the  velar  opening  by  sudden 
and  rhythmical  contractions  of  the  bell.  In  accordance  with 
this  free  mode  of  life  sense-organs  are  present,  and  a  higher 


82 


jy  VEHTEDHA  TE  MOltP/lOLOU  Y. 


developme?'.t  of  the  nervous  system  than  is  fouucl  in  tlie 
polyps  obtains.  The  seuse-or^'uns  vurv  both  in  strueture  und 
function  in  tliireiont  luedusii',  in  smne  beiu«^  spots  uf  ])i<;ni(Mit 
sometimes  provitleil  witli  a  refractive  lens  and  functioning'  as 
li^ht-percii)ient  structurt's.  Such  medusa-  (Fij<.  45)  are 
li'iiiied  uciUate,  or,  since  the  eyes  are  on  the  surface  of  the 
bo'll  and  are  not  enclosed  within  a  chamber  formed  by  the 
j^'rowth  around  them  of  the  adjoining  tissues,  they  are  some- 
times termed  ijn uinophthitlmutoua.  lu  other  forms  agaiu  the 
sense-or^aus  consist  of  a  ^roup  of  cells  coutaiuiuf);  in  their 
interior  crystals  of  carbonatt;  of  lime  and  haviu<j;  a  somewhat 
vesicular  appearance,  and  in  the  neighborhood  of  these  are 
sensory  cells  with  lonj^  stiti*  cilia  to  which  are  imparted  any 
vibrations  which  the  crystals  may  manifest.  Such  crystal- 
coutaiuinj^  cells  are  known  as  otocifstti,  on  the  supposition  that 
they  form  auditory  organs,  but  it  seems  more  probable 
that  their  function  is  that  of  equilibrium  or<^ans,  informing 
the  medusa'  of  their  position.  Medusiu  posst;ssiuj^  such 
or{^aus  (Fig.  43)  are  termed,  to  distinguish  them  from  ocellate 
forms,  veaiculutc  niodusjc. 

The  ectoilerm  which  covers  the  outer  surface  of  the  bell  is 
much  flattened,  but  near  the  rim  it  becomes  columnar  and 
is  ciliatetl,  some  of  the  cells  assuming  the  form  of  sensory 
cells  similar  in  appearance  to  those  of  the  polyps,  their 
slender  nerve-tilanients  forming  a  delicate  plexus  in  the 
deeper  layers  of  the  ectoderm,  in  which  are  here  and  there 
iuibedded  stellate  ganglion-cells.  A  special  uerve-riug  sur- 
rt>unds  the  bell  a^  its  margin.  The  ectoderm-cells  which 
cover  both  surfac(?s  of  the  velum  and  the  subuuibrellar  surface 
are  of  the  epithelio-muscular  and  muscular  ty})es,  the  mus- 
cle-tibres  being  for  the  most  ])art  arninged  circularly.  lu 
the  deeper  layers  of  the  subumbrellar  ectoderm  numerous 
ganglion-cells  with  long  prolongations  ocuur  forming  a  net- 
work immediately  external  to  the  muscle-pr  jcesses,  and  at 
certain  definite  regions  collections  of  reproductive  cells  occur. 

The  mesogh^a  of  the  subumbrellar  surface  and  of  the 
velum  is  thin,  resembling  that  of  the  polyps,  but  in  the  convex 
portion  of  the  bell  it  is  very  thick  and  gelatinous  in  texture,  and 
may  in  some  cases  contain  cells.    The  endoderm  is  throughout 


TYrK  (  (KLKNTKHA. 


83 


le 
|l 

t 

l-s 


the  ec'Iciitcron  eiliatod  ;  in  tlie  teiitaclo.s,  wliicli  may  1)(>  oitluM* 
hollow  or  solid,  it  luav  I'otaiu  its  ciliatoil  character,  or  eiso  in 
tiio  solid  tciiti'tdes  form  a  solid  axis  similar  to  that  of  the 
tentacles  of  the  [>oly|>  forms. 

Such  .lie  the  ^'eneral  structural  features  common  to  all 
the  Hvdroiiiedusie.  It  remains  to  add  here  c(»rtain  points  l»y 
which  esjiecially  the}'  are  distinj^'uished  from  otiusr  ^'rouiis  of 
Cnidaria.  These  are  :  (<tjthe  ectoderm  and  endoderm  meet  at 
the  m<)uth-o|»enin;^  ;  {It)  the  sensci-orj^ans  when  present  ar(! 
never  modilh'd  tentacles:  (c)  a  velum  is  always  present  in  the 
medusa  foinis  ;  and  {<!)  the  reproductive  elements  have  thiiir 
origin  in  tlie  ectoderm. 

1.  Order  HydrarieB. 

To  this  order  })elon^s  tlm  IIi/<lnt,  a  common  inhabitant  of 
fresh-water  ponds  and  streams  in  all  parts  of  the  world.  It 
is  a  simple  cylindrical  orj^auisiu,  adherent  by  one  extremity 
to  foreign  hoditfs,  though  not  fixed,  beinj^  able  slowly  to 
change  its  position,  litdow  the  short  liypostome  is  a  single 
row  of  exceed! nj^ly  extensible  tentacles,  which  are  hollow,  a 
l)eculiarity  found  in  no  other  Hydromedusan  polyps.  So  lonj^ 
as  conditions  are  favorable  and  uutrition  abundant  Hydt'd 
reproduces  by  budding,  the  buds  se])aratin<^  from  the  parent 
form  after  a  certain  period  of  {growth.  Sexual  rcsproduc- 
tion  also  occurs,  the  spermatozoa  dev(dopin«^  in  the  ectoderm 
a  little  below  the  I'nv^  of  tentacles,  while  the  ova  form  in  the 
same  laj'er  sf)mewhat  lower  .lowu  on  the  body,  the  animals 
beiuf^  hernia] )hr()tlites.  The  ova  after  fertilization  remain 
imbedded  in  the  ectoderm  of  the  parent  for  some  time,  but 
later,  developing  around  them  a  cyst,  they  sink  to  the  bottom 
of  the  water,  and  there  remain  usually  for  some  time  without 
developing  further.  In  this  condition  they  are  able  to  with- 
stand a  considerable  amount  of  cold  and  dryness,  and  so 
may  tide  the  species  over  unfavorable  conditions.  No  medusa 
stage  occurs  in  Hydra. 

Hydra  grisea  ],.  is  a  brown  form,  relatively  large  in  size,  while  H. 
rfriilt's  L.  is  smaller  and  of  a  dark-green  color  due  to  chlorophyll-contain- 
ing corpuscles  imbedded  in  the  ectoderm-cells  and  supposed  by  some 
authors  to  be  uaioellular  Algui  of  a  symbiotic  habit.     (See  p.  20.) 


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84 


mVEKTEBRA TE  MORPHOLOG  7. 


2.  Order  NarcomedussB. 

In  the  Hydrariae  the  polyp  form  occurs  without  a  medusa, 
in  the  order  Narcomedusce  the  reverse  is  the  case,  since  in  it 
the  medusa  form  is  the  only  one  present  in  a  typical  state  of 
development.  The  medusae  are  usually  somewhat  lens- 
shaped  structures  (Fig.  42),  with  a  lobed  margin,  the  velum 
{v),  instead  of  being  horizontal,  being  pendent  from  the  mar- 


FiG.  42. — Cunoctantha  octonaria  Hakck. 

m  =  uiargiu  of  bell.  ot  =  otocyst. 


(After  Brooes). 
V  —  velum. 


gin  {in)  and  extending  up  in  the  intervals  betveen  the  lobes. 
At  the  apex  of  each  one  of  these  intervals  is  situated  a  short, 
stiff,  solid  tentacle  which  is  usually  bent  backward  over  the 
exumbrellar  surface  and  is  tipped  by  a  knob  of  nematocysts. 
The  cavit}'  of  the  short  manubrium  leads  into  a  gastric 
chamber  which  is  prolonged  out  towards  the  margin  into  broad 
pouches  which  lie  opposite  the  tentacles  and  the  intervals  be- 
tween the  lobes,  and  around  the  margin,  following  the  edge  of 
the  lobes  and  therefore  having  a  festooned  arrangement,  runs 
a  narrow  circular  canal  which  communicates  with  each  radial 
pouch  at  the  apex  of  each  interlobular  interval.  This  struc- 
ture is,  however,  absent  in  the  American  species  Cunoctantha 
octonaria.  The  reproductive  organs  develop  in  the  subum- 
brellar  ectoderm  covering  the  pouches  and  sometimes  extend 
on  to  the  manubrium.  Around  the  margin  of  the  lobes  are 
seated  club-shaped  projecting  otocysts  {ot)  composed  of  an 
external  layer  of  ectoderm  surrounding  a  number  of  eudo- 
dermal  cells,  one  or  more  of  which  contain  a  crystal  of 
carbonate  of  lime.  The  ectoderm  cells  in  the  neighborhood 
are  largel}'  sensory  and  provided  with  long  cilia,  their  inner 
ends  contributing   to   the  formation  of  the  marginal  nerve- 


ring. 


TYPE  C(ELENTERA. 


86 


Ja. 


3.  Order  Trachymedusffi. 

In  this  order  ouly  the  medusa  form  is  present  and  it  re- 
sembles in  many  respects  that  of  the  Narcomedustie.  The 
bell  is  somewhat  flattened,  but  is  not  lobed  at  the  margin ; 
while  the  velum  has  the  usual  horizontal  position  and  the 
long  slender  manubrium  projects  some  distance  outside  of  the 
subumbrellar  cavity.  From  the  gastric  cavity  four,  six,  or 
eight  radiating  canals  arise,  ,/hich  join  at  the  margin  the  cir- 
cular canal,  and  in  the  ectoderm  of  the  subumbrellar  surface 
over  a  small  area  along  the  line  of  these  canals  the  reproduc- 
tive cells  develop.  At  the  margin  a  number  of  sense-organs, 
otoc3'sts,  are  present,  agreeing  essentially  in  structure  with 
those  of  the  Narcomedusie  and  are  sometimes  projecting  or 
in  other  cases  more  or  less  enclosed  in  a  cavity  formed  by 
the  growing  up  around  them  of  the  adjacent  substance  of  the 
bell.  The  tentacles  vary  somewhat  in  shape  and  structure  in 
different  genera.  In  lihopalonema  they  are  all  solid,  eight 
being  somewhat  longer  than  the  other  eight  at  whose  bases 
are  the  otocysts  ;  in  IJriope  (Fig.  41)  four,  situated  opposite 
the  radiating  canals,  are  hollow  and  extensible,  while  the 
other  four  are  solid  and  bent  back  over  the  exumbrellar  sur- 
face as  in  the  Narcomedusae  ;  while  in  Geryonia  all  the  six 
tentacles  are  hollow. 


m 


N' 


4.  Order  Gampanularise  or  Leptomedusee. 

In  this  order  both  the  polyp  and  the  medusa  form  occur 
in  the  life-history  of  the  individual,  alternation  of  generations 
being  the  rule.  The  polyp  generation  takes  the  form  of  a 
branching  colony,  which  generally  shows  a  certain  amount  of 
division  of  labor  among  the  component  individuals.  The 
nutritive  hydrauths  or  trophopolyps  present  the  typical  form 
already  described  (p.  78),  and  each  is  seated  in  a  cup  of  per- 
isarc,  the  hydrotheca,  into  which  the  polyp  may  retract.  Such 
polyps  reproduce  only  polyp  forms,  but  in  addition  others 
destitute  of  mouth  and  tentacles,  and  known  as  gonopolyps, 
occur,  which  produce  by  budding  numerous  medusoe  or  medu- 
soid  buds,  the  polyp  and  its  progeny  being  together  enclosed 
in  a  cup  of  perisarc,  differing  in  shape  from  the  hydrotheca 


ii 


86 


INVERT EBHATE  MORPHOLOGY. 


\ 


iuul  knowu  as  the  gonotheca  (Fig.  39,  go).  Ou  account  of  the 
presence  of  these  cups  the  polyps  of  this  order  are  sometimes 
termed  culyptohlastic.  In  the  typical  Campanularian  polyp 
colonies,  such  as  those  of  A'ucope  or  Obelia,  no  further  differ- 
entiation of  the  polyps  is  found,  but  in  the  family  Plumulari- 
dfe  in  the  neighborhood  of  each  of  the  small  hydrothecro  there 
are  one  or  more  slender  extensible  polyps  lacking  mouth  and 
tentacles  whose  endoderm  is  a  solid  axial  cord,  while  the 
ectodermal  cells  send  off  long,  streaming,  pseudopodia-like 
processes,  these  polyps  apparently  playing  the  part  of  food- 
providers  for  the  colony. 

The  medusiB  (Fig.  43)  are  usually  very  shallow  bells,  wdth 

depending  from  the  margin,  and 
resemble  the  Trachymedusfe  in 
that  the  reproductive  organs 
develop  on  the  line  of  the  ra- 
diating canals.  Of  these  there 
are  in  the  majority  of  cases 
four,  though  occasionally,  as  in 
^quorea,    they    may    be    very 


numerous  hollow  tentacles 


numerous.       Sense-organs     are 


always  present  at  the  margin  of 
the  bell  and,  as  in  the  Trachy- 
FiG.  4d.—Rhegmatodes  tennis,  Ag.    medusae  and  Narcomedusse,   are 

alwa^'s  otocysts,  the  medusfe 
belonging  to  the  vesiculate  category.  A  marked  difference 
obtains  between  the  otocysts  of  the  LeptomedussB  and  those 
of  the  two  preceding  orders  in  that  the  calcareous  crystals 
are  in  the  former  developed  in  ectoderm  cells.  The  otocysts 
furthermore  primitively  occur  on  the  inner  surface  of  the 
velum,  where  they  are  lodged  in  a  slight  depression,  which 
may,  however,  deepen  so  much  that  the  otocysts  appear  to  be 
imbedded  in  the  substance  of  the  bell. 

In  the  typical  Campanularians  free-swimming  medusoe  are 
developed,  and  according  as  the  polyps  or  the  medusae  attract 
especial  attention  the  order  may  be  termed  that  of  the  Cam- 
panulariaB  or  that  of  the  Leptomedusse.  In  a  few  forms,  such 
as  lihegmatodes,  up  to  the  present  no  polyp  generation  is 
known  to  occur,  and  conversely  in  certain  genera,  such  as  Ser- 


r 


TYPE  CCELENTERA. 


87 


iM?«na  aud  IlaJecium,  aucl  in  the  Plumularklee,  e.g.  Aglaophenia, 
it  would  appear  at  first  sij^lit  that  no  medusa  geueratiou  oc- 
curred. This,  however,  is  not  strictly  accurate,  the  appear- 
ance depending  on  the  medusa-buds  in  these  forms  never 
becoming  free-swimming  but  remaining  in  a  more  or  less 
undeveloped  condition,  the  ova  and  spermatozoa  becoming 
mature  notwithstanding  the  imperfect  development  of  the 
medusae.  Alternation  of  generations,  however,  exists  in  these 
forms  just  as  much  as  in  the  typical  Campanulariaus,  for,  no 
matter  how  degenerate  the  buds  of  the  gonopolyps  may  be, 
they  must  still  be  regarded  as  the  medusa  generation. 


lit 


5.  Order  Tabulariae  or  AnthomedussB. 

In  this  order,  as  in  tha  preceding  one,  a  well-marked 
alternation  of  polyp  and  medusa  generations  occurs.  The 
polyps  are  united  to  form  colonies,  the  individual  hydrauths 
being  constructed  on  the  Tubularian  type,  i.e.  the  perisarc 
ceases  at  the  base  of  each  hydranth  so  that  there  is  no 
hydrotheca.  The  tentacles  show  a  greater  variety  of  form 
and  structure  than  in  the  Campanularian  forms,  and,  though 
sometimes  filiform  and  arranged  in  a  single  cycle  at  the  base 
of  the  hypostome  (Margelis),  are  yet  in  other  forms  scattered 
irregularly  over  the  surface  of  the  hypostome  {Olava),  or  may 
be  club-shaped  {Coryne),  or  in  addition  to  scattered  club- 
shaped  tentacles  a  circle  of  filiform  ones  may  occur  at  the 
base  of  the  hydranth  as  in  Pennaria.  A  division  of  labor  is 
not  the  rule  as  in  the  Campanularians,  but  nevertheless  this 
phenomenon  is  in  some  cases  carried  to  a  much  greater  ex- 
tent than  in  that  group.  In  some  forms  special  gonopolyps 
are  present  from  which  all  the  medusae  arise,  but  more  fre- 
quently any  of  the  hydranths  may  produce  these  structures. 
The  gonopolyps  when  they  occur  are  never  enclosed  within  a 
gonotheca,  and  hence  the  term  gyninoblastic,  frequently  applied 
to  the  polyps  of  this  order.  In  the  genus  Hydrtzctinia  (Fig. 
44)  a  complicated  division  of  labor  occurs.  The  various 
hydranths  composing  the  colony  arise  from  a  flat  expansion 
common  to  all  and  formed  by  tlie  fusion  of  an  original  net- 
work of  coenosarcal  tubes,  and  on  the  surface  of  which  numer. 


88 


INVERTEBRATE  MORPHOLOOT. 


I 


ous  stout  spines  occur.  Some  of  the  hydranths  are  typical 
tropliopolyps  (Fig.  44,  tp)  with  filiform  teutacles  and  a  mouth, 
but  among  them  are  gonopolyps  {gp)  with  short-knobbed 
tentacles  below  which  the  medusa-buds  arise.  In  addition 
there  are  also  towards  the  periphery  of  the  colony  much 
longer  slender  hydranths  without  a  mouth  and  like  the  gono- 


FiG.  44. — Portion  of  Colony  op  Hi/draettnia  eehinata  (adapted  from  figure  by 

HiNCKS). 

gp  =  gouopolyp.         mp  —  offensive  polyp.  tp  =  trophopolyp. 

polyps  in  having  short-knobbed  tentacles,  but  differing  from 
them  in  their  greater  length  and  in  never  producing  medusa- 
buds.  Finally,  in  a  European  species  of  the  genus  a  fourth 
exceedingly  long  polyp  {mp),  destitute  of  both  mouth  and 
tentacles,  but  furnished  at  its  free  end  with  numerous  nemato- 
cysts,  occurs.  These  third  and  fourth  varieties  of  polyp 
probably  are  offensive  and  defensive  in  function,  procuring 
food  for  the  colony  and  warding  off  some  predatory  enemies. 
The  Anthomedusse,  as  the  medusa  forms  are  termed,  have 
much  deeper  bells  than  the  Leptomedusse,  and  differ  from 
the  latter  in  that  the  sense-organs  are  light-percipient  in  their 


TYPE  C(ELENTERA. 


89 


function,  whence  the  medusso  are  frequently  termed  ocellatc, 
and,  secondly,  in  that  the  re- 
productive organs  develop  in 
the  wall  of  the  manubrium 
instead  of  on  the  line  of  the 
four  radiating  canals.  Ten- 
tacles as  a  rule  occur  at  the 
margin  of  the  bell,  and  it  is  at 
their  bases  that  the  eyes  are 
formed.  Occasionally,  as  in 
Margelis  (Fig.  45),  tentacles 
also  arise  from  the  end  of  the 
manubrium.  As  a  rule,  the 
sexual  generation  is  composed 
of  free-swimming  medusae,  as 
in  Coryne,  3IargeliSj  and  Pen- 
naria,  but  not  infrequently  the 
medusa-buds  become  retarded 
in    their    development,   as    in  Fia.  ^S.-Meuvsa  of  Margelis  caroli- 

Hydractinia,  Clava,   Tuhularia^ 

and  to  an  extreme  extent  in  Evdendrium,  and  all  intermediate 

stages  between  the  two  extremes  are  to  be  found. 


6.  Order  Hydrocorallinae. 

The  Hydrocorallinae  are  colonial  marine  forms  represented 
by  the  Stag's-horn  Coral,  Millepora,  and  characterized  by  the 
densely  ramified  coeuosarcal  tr.bes  being  enclosed  in  a  mass 
of  carbonate  of  lime  secreted  by  the  ccenosarcal  ectoderm 
and  taking  the  place  of  the  perisarc.  From  minute  pores  on 
the  surface  of  this  calcareous  mass,  the  corallum,  the  hydranths 
protrude  and  present  a  well-marked  polymorphism.  The 
pores  are  arranged  in  groups  consisting  of  a  central  one  sur- 
rounded by  a  varying  number  of  smaller  ones.  From  the 
central  pore  protrudes  a  hydranth  with  a  circle  of  short  ten- 
tacles tipped  with  knobs  containing  numerous  nematocysts ; 
this  is  the  trophopolyp  or  gasterozoid  (Fig.  46,  g).  From  the 
smaller  surrounding  pores  more  elongated  hydranths  protrude, 
destitute  of  a  mouth,  and  with  short  scattered  tentacles  also 


90 


INVERTEBRATE  MORPUOLOOT. 


knobbed ;  these  are  probably  oflfeusive  polyps,  of  use  to  the 
colony  in  obtaining  food,  and  are  known  as  dactylozoids  (Fig. 
46,  d).  The  cavities  in  the  corallura  in  which  the  gasterozoids 
live  are  divided  by  transverse  partitions  into  chambers  into 
the  outermost  of  which  the  hydranth  may  be  retracted,  the 
arrangement  recalling  what  occurs  in  the  corallum  of  the  fossil 
Tabulate  Corals.  In  the  genus  Stylaster,  however,  which 
forms  a   rose-red   branching   corallum,  these   partitions  are 


~c» 


Fig.  46.— Portion  op  Colony  of  Millepora  (after  Mosblet). 
CO  =  corallum.  d  =  ductylozuid.  g  =  gasterozoid. 

wanting,  a  calcareous  cone,  the  columella,  projecting  upwarda 
from  the  floor  of  the  cavity. 

In  one  species  at  least  of  Millepora  a  well-marked  and  typi- 
cal alternation  of  generations  occurs,  medusae  being  formed 
which  are  set  free  and  develop  the  reproductive  elements  in 
the  ectoderm  of  the  manubrium.  In  the  majority  of  the 
members  of  the  order,  however,  the  medusa-buds  are  never 
set  free,  and  are  usually  much  degenerated,  and  indeed  in 
Millepora  alcicornis  they  may  be  said  to  have  completely  dis- 
appeared, and  with  them  the  alternation  of  generations.  The 
medusa-buds  develop  on  the  walls  of  the  ccenosarcal  tubes, 
and  lie  in  cavities  in  the  corallum  which  open  to  the  exterior 
by  a  pore  through  which  the  egg-embryos  escape. 


TTVE  CaSLENTERA. 


91 


I  If. 


7.  Order  Siphonophorae.  ' 

The    Sipbonophores    are   free-swimminf?    Hydromedusan 
colonies,  the  coustitueut  iudividuals  of  which  show  a  high 
degree   of   division    of    labor.      The 
various  forms   of   individuals  which 
are  to  be  found  in  different  members 
of  the  order  are  usually  not  present 
in  any  one  colony,  some  genera  })OS- 
sessiug    some    forms    which    others 
lack  ;  it  will  be  convenient,  therefore, 
to  consider  an  ideal  form  in  which  the 
various    modifications    are     present. 
Each  colony  (Fig.  47)  consists  of  an 
axis  or  stolon  on  which  the  various 
individuals   are    seated    and    which 
places  them  in  connection  with  each 
other  ;  it  is  usually  long  and  slender, 
but  in  some  cases,  as  in  Porpiia,  may 
be  reduced   to   a   disk.     At  the   ex- 
tremity of  the  stolon  may  be  found 
a  float  or  pneumatophore  (Fig.  47,  p), 
a  double-walled  sac  containing  air  in 
the  interior,  and  which  is  to  be  re- 
garded as  a   modified  medusa  form. 
Next  to  it  come  usually  several  medusa 
forms  lacking  manubrium,  tentacles, 
and  sense-organs,  and  possessing   a 
locomotor   function  ;    these    are   the 
swim-bells  or  nedocalyces  (n).     At  in- 
tervals along  the  rest  of  the  stolon 
are   situated   groups    of  individuals,  Fi^.  47. 
each  group  covered  over  by  one  or 
more  scale-like  structures  (cs),  which 
are  again  highly  modified  medusae,  and 
in  each  group  is  to  be  found  a  tropho- 
polyp  {tr),  a  vase-like  polyp  form  with 
a  wide  trumpet-shaped  mouth,    and 
having  near  its  base  a  single  tentacle 
(t)  which  bears  along  one  side  a  row  of  numerous  secondary 


-DiAGUAM   OF   A   Sl- 

PHOKOPHORE  Colony. 
cs  =  covering  scale. 
n  =  uectocalyx. 
p  =  pneumatocyst. 
r  =  reproductive  polyp. 
8   =  seusoiy  polyp. 
tr  =  nutritive  polyp. 
t    =  tentacle. 


92 


INVERT EBli ATE  MOliPUOLOQY. 


branches,  each  richly  provided  with  nematocysts.  Associated 
with  this  nutritive  individual  is  usually  a  reproductive  form, 
which  in  some  cases  may  take  the  form  of  an  Anthomedusa, 
separating  from  the  colony  and  leading  a  free  life,  as  in 
Velella,  or  may  be  medusoid,  presenting  a  medusa  form,  but 
lacking  a  mouth  and  tentacles  and  never  separating  from  the 
colony,  or  finally  a  gonopolyp  (r)  may  occur  which  bears 
numerous  much-degenerated  medusoid  buds.  In  some  forms 
there  is  still  another  form  of  individual  (s),  resembling  a  tro- 
phopolyp,  but  being  destitute  of  a  mouth  and  having  a  sim- 
ple tentacle  without  the  secondary  branches.  From  its  great 
sensibility  to  stimuli  this  is  supposed  to  be  a  sensory  polyp. 

In  some  forms,  such  as  Diphyes,  no  pneumatophore  occurs, 
but  nectocalyces  are  present ;  in  others,  as  Agalma,  both  occur 
and  the  colonies  resembie  somewhat  the  diagrammatic  form 
described ;  while  in  a  third  group,  including  the  Portuguese 
man-of-war  Caravella,  the  pneumatophore  becomes  largely 
developed  and  nectocalyces  are  wanting,  the  stolon  at  the 
same  time  being  contracted  to  a  disk  lying  on  the  lower  sur- 
face of  the  pneumatophore.  In  VeleUa  and  Porpita  the  stolon 
is  reduced  to  a  disk,  but  the  pneumatophore  is  wanting. 

Alternation  of  generations  of  a  typical  form,  complicated, 
however,  by  the  polymorphism,  occurs  in  such  forms  as 
Velella,  which  possess  a  free-swimming  medusa ;  in  the  ma- 
jority, however,  it  is  obscured,  as  in  many  Tubularian  hy- 
droids,  by  the  greater  or  less  degeneration  of  the  medusa 
An  alternation  of  another  kind,  however,  occurs  in  some 
forms,  the  bunches  of  individuals  separating  from  the  stolon 
and  leading  for  a  time  an  independent  existence,  during  which 
their  medusoid  reproductive  individuals  become  mature. 

The  complicated  polymorphism  of  the  Siphonophore  colonies  leads  to  a 
merging  of  the  individualities  of  the  component  individuals  in  that  of  the 
entire  colony,  a  process  which  reaches  its  highest  pitch  in  such  forms  as 
VeleUa.  The  various  polyp  and  medusa  forms  of  the  colony  may  be  con- 
sidered as  organ-individuals,  and  by  their  integration  an  individuality  of  a 
higher  grade — a  metamere-individual — \s  produced. 


Devdopment  of  the  Hydromedusce. — It  has  been  mentioned 
as  one  of  the  characteristic  features  of  the  Hydromedusae  that 
the  reproductive  elements  arise  in  the  ectoderm.     They  reach 


TYPE  CCELENTERA. 


93 


their  maturity  iii  the  meclusno  or  metlusoiJ  buds  except  in  the 
Hydrarite,  iu  which  this  stage  is  entirely  wautiuf,',  and  in  cer- 
tain HydrocoralliuoB,  iu  which  it  has  disappeared,  and  may 
likewise  first  become  differentiated  in  the  medusa.  In  many 
forms,  however,  iu  which  an  alternation  of  j^enerations  occurs 
they  arise  in  the  polyp,  sometimes  at  a  point  far  distant  from 
where  the  medusa-buds  will  arise,  aud  reach  these  structures 
only  after,  it  may  be,  a  rather  extensive  series  of  wanderings. 
Thus,  to  take  an  extreme  case,  in  Eiidendrium  the  ova  arise 
in  the  ectoderm  of  the  main  stem  of  the  piunately  branching 
colony  a  short  distance  below  the  terminal  hydranth ;  as  new 
branches  are  formed  iu  this  same  region  the  young  ova 
migrate  into  them,  passing  through  the  supporting  layer  and 
wandering  among  the  endoderm  cells.  Later  on  when  the 
gonopolyps  arise  on  the  lateral  branches  the  ova  wander  into 
them,  still  in  the  endoderm,  and  finally  when  the  medusa- 
buds  develop  on  the  gonopolyps  the  ova  continue  their  endo- 
dermic  course  into  them  and  eventually,  again  passing  through 
the  mesogloea,  take  up  their  final  position  in  the  medusoid 
ectoderm.  Gradations  between  such  an  extensive  migration 
and  cases  in  which  none  occurs  are  to  be  found,  and  it  may  be 
stated  as  a  general  rule  that  the  more  the  medusoid  buds 
depart  from  the  medusa  form  the  greater  is  the  migration 
undergone  by  the  reproductive  cells. 

As  a  rule  the  Hydromedusse  are  of  separate  sexes,  the  sepa- 
ration aflfecting  the  entire  colonies — or,  to  put  it  slightly  dif- 
ferently, the  medusae  are  always  unisexual,  and  a  polyp  colony 
Avlien  it  occurs  gives  rise  to  medusae  or  medusoid  buds  all  of 
the  same  sex.  The  Hydrariae,  however,  are  exceptions  to  the 
rule,  being  hermaphroditic. 

A  blastula  results  from  the  segmentation  of  the  ovum,  and 
this  is  converted  into  either  a  sterrula  by  immigration  or  a 
dibla^tula  (i.e.  a  hollow  two-layered  organism  without  mouth 
or  tentacles)  by  delamination  (see  p.  55).  If  a  sterrula  be 
formed,  it  assumes  the  diblastula  condition  by  a  hollowing  out 
of  the  central  mass,  and  after  swimming  about  for  some  time 
in  this  condition,  if  a  polyp  is  to  be  formed,  it  settles  down 
upon  some  foreign  body,  a  mouth  breaks  through  and  ten- 
tacles appear,  producing  the  first  hydranth. 


04 


IN  VERTEBRA  TE  MORPIIOLOG  Y. 


In  some  forms,  such  na  Hyilractinia,  the  frec-swimrain;?  embryo  when 
it  settles  down  becomes  converted  into  a  flat  plate-like  expansion  without 
mouth  or  tentacles,  from  which,  as  a  bud,  the  first  hydranth  arises. 

If,  liowover,  the  ovum  develops  directly  into  a  medusa,  as 
iu  the  Trachymedusa)  and  Narcoraedusie,  the  breakiug  throuf»h 
of  the  mouth  aud  the  formation  of  teutacles  takes  place 
■while  the  embryo  is  still  free-swimminj^,  aud  the  staj^e  so 
produced  may  resemble  closely  a  free-swimming  hydranth,  as 
iu  Ciinoctantha,  or  may,  by  the  great  development  of  mesoglcea 
at  the  extremity  opposite  the  mouth,  assume  a  rather  globular 
form,  as  iu  Liriope.  As  the  teutacles  develop  and  the  bell 
becomes  differentiated  b}'  the  extension  laterally,  as  it  were, 
of  the  embryo,  the  velum  arises  at  the  margin  of  the  bell. 
At  this  time  the  coileuteron  is  a  flattened  cavity  extending 
to  the  margins  of  the  bell,  but  later  it  becomes  obliterated 
along  four  lines,  and  the  obliteration  of  the  cavity  extending, 
the  radiat'Lig  and  circular  canals  aud  the  gastric  cavity  alone 
persist,  a  layer  of  eudoderm-cells  sometimes  joining  them 
and  representing  the  o])Iiterated  portion  of  the  coelenteron, 
though  often  this  also  disappears. 

In  the  AnthomedusiT}  aud  Leptomedusse,  in  which  the  medu- 
sse  arise  by  budding  from  the  polyps,  the  buds  are  at  first 
tubular  outgrowths  of  the  body-wall  (Fig.  48,  A).  The  ecto- 
derm at  the  tip  of  the  bud  thickens,  depressing  the  central 
portion  of  the  eudoderra  (Fig.  48,  B),  and  on  the  appearance 
of  a  cavity  in  the  thickened  ectoderm,  the  subumbrellar 
cavity,  the  central  eudoderm  pushes  out  into  the  cavity,  carry- 
ing with  it  the  ectoderm  covering  it  aud  forming  the  manu- 
brium (Fig.  48,  C).  In  this  stage  the  bud,  though  still  lacking 
mouth  and  tentacles,  is  comparable  to  the  polyp  stage  of  the 
medusa  of  direct  developmer^  at  least  so  far  as  the  coelen- 
teron  is  concerned,  and  by  processes  identical  with  those 
occurring  in  the  directly-developing  embryo  the  radiating  and 
circular  canals  are  formed  (Fig.  48,  E\  and  on  the  formation 
of  a  mouth  at  the  extremity  of  the  manubrium  and  the  de- 
velopment of  tentacles  the  medusa  is  perfectly  formed. 

In  many  cases,  however,  as  already  stated,  the  medusa- 
buds  never  reach  their  complete  development,  but  become 
sexually  mature  while  sfill  imperfect  in  form.     The  stage  at 


TYPE  t'CELKNTKHA. 


90 


which  deveh>piueut  ceases  varies  iu  different  forms ;  in  Tuhu- 
laria,  for  instance,  the  niedusoid  bud  resembles  a  Uiedusa  «;x- 
cept  that  it  hicks  tentacles,  sense-orguns  and  mouth,  and  is 
not  free-swinimin}^ ;  iu  Cla  hi  not  only  does  development 
cease  at  an  earlier  stage,  but  a  certain  amount  of  degeneration 


Fig.  48. — Diagrams  showing  the  Development  of  a  Medusa-bud, 

A,  outpushing  of  body-wall  of  polyp  ;  B,  thickoinng  of  t'ctoderm  ;  C,  forma- 
tion of  subumbrellar  cavUy  ;  D,  tiaiisverse  section  lliroiigh  G,  along  tlie 
line  indicated  by  ab;  E,  fonnation  of  radial  canals  ;  F,  transverse  section 
through  E,  along  the  line  indicated  by  ab. 

cc  =  circular  canal.  rs  =  radial  pouch. 

m  =  cavity  of  manubrium.  su  —  subumbrellar  cavity. 

re  =  radial  canal.  v  =  velum. 

occurs,  the  rudimentary  subumbrellar  cavity  never  commu- 
nicating with  the  exterior  and  the  radiating  and  circular 
canals  being  entirely  obliterated ;  and  finally  in  Kudeindrium 
the  bud  never  develops  beyond  the  earliest  stage  iu  which  it 
is  a  simple  tubular  outgrowth  of  the  body-wall. 

The  Relationships  of  the  rat-ions  Orders  to  one  Another. — Since  the 
Hydrariae  show  such  a  simple  type  of  organization  it  is  generally  supposed 
that  they  represent  more  or  loss  closely  a  primitive  ancestral  form,  though 
their  usual  habitat  in  fresh  water  suggests  the  possibility  of  their  having 
undergone  some  d.  gradation.    If  they  do  represent  the  primitive  Hvdro- 


96 


Ilf  VERTEBRATE  MORPHOLOOr. 


medusan,  then,  it  is  evident  that  the  medusa  form  is  a  secondary  modifica- 
tion—a speciaiiiced  rc|)i'oductive  organ,  wiiicli  in  the  Narcomedusiie  and 
Trachymedusa}  has  become  so  important  tiiat  the  ancestral  polyp  form  is 
practically  suppressed  in  the  life-history.  On  this  view  it  must  be  sup- 
posed that  organisms  so  similar  as  the  medusie  of  tiie  Tubulariuu  and 
Campanularian  polyps  have  been  developed  entirely  iudepeudeuily  of  one 
another,  a  vlow  which  carries  with  it  many  difficulties,  and  that  the  medu- 
soid  buds  represent  stages  in  their  evolution. 

It  seems  more  probable,  however,  that,  leaving  the  Hydrariye  out  of  the 
question,  in  all  the  otlior  groups  the  medusa  was  the  parent  form.  This 
is  borne  out  by  the  fact  that  in  tlie  Narcoiuedusae,  which,  with  their  broad 
pouch-like  extensions  of  the  gastric  cavity,  are  the  most  primitive  of  all 

the  craspedote  medusa?,  there  is  no  fixed 
polyp  form.  It  has  been  shown,  how- 
ever, that  the  Narcomedusa?  and  Trachy- 
medusiB  in  tlieir  development  pass 
through  a  stage  wliich  may  be  considered 
to  represent  the  polyp  form,  and  if,  while 
in  this  form,  non-sexual  reproduction 
shoidd  have  taken  place,  the  buds  re- 
sembling the  immature  form  which  gave 
rise  to  them,  a  polyp  colony  would  result, 
some  of  the  buds  of  which  might  con- 
tinue their  development  and  become 
medusjie.  By  this  view  the  difficulties 
presented  by  the  similarity  of  the  medusa^ 
throughout  all  the  groups  where  iliey 
occur  are  overcome  and  the  medusoid 
buds  are  regarded  as  imperfectly  devel- 
oped or  degenerate  medusae.  Further- 
more this  view  js  rendered  more  than 
probable  by  the  development  of  Cunoc- 
tantha  and  the  allied  Cuniita.  The 
former  while  in  the  embryonic  polyp 
form  actually  does  bud  (Fig.  49),  the 
buds  resembling  tho  original  embryo  which  gave  rise  to  them,  and  all  the 
buds,  the  parent  embryo  included,  later  develop  into  medusjt?.  In  Cnninn, 
however,  the  parent  embryo  which  gives  rise  to  buds  inidergoes  no  further 
development,  only  the  buds  continuing  on  their  course  of  growth  to  medu- 
sa). In  this  case  a  true  and  typical  altenuition  of  generations  occurs  and 
points  out  a  simple  explanation  of  the  alternation  which  is  found  in  the 
AnthomedusiB  and  Leptomedusa^  In  these  the  polyp  colonies  are  the  re- 
sults of  non-sexual  reproduction  of  a  larval  medusa,  and  some  only  of  the 
individuals  so  formed  continue  their  development  to  medusa?. 

The  relationships  of  the  Hydrocorallinje  to  the  other  groups  a;e  not  yet 
quite  demonstrated.     It  would  seem,  however,  from  the  medusa-bud  which 


Fig.    49. — Budding   Lauva   op 

Cunoctanthit    octonnria     (after 
McCrbadt,  from  Brooks). 
a  =  egg  larva. 
bb  =  budded  larva?. 


TYPE  CCELENTERA. 


07 


occurs  in  one  species  of  Millepora  having  its  reproductive  organs  in  the 
walls  of  tho  manubrium  that  the  affinities  of  the  group  are  vvitii  the  Antho- 
medusae,  and  that  an  exceptional  amount  of  degeneration  of  the  metlusa 
had  occurred  in  correspondence  with  the  development  of  tlie  calcareous 
corallum. 

The  Siphonophores  are  evidently  allied  to  the  Anthomedusje,  judging 
from  the  characters  of  their  medusie.  The  colony,  however,  contains  both 
medusa  and  polyp  individuals,  the  former  not  being  in  all  cases  reproduc- 
tive as  in  tho  Anthomedusie.  The  eml)ryology  of  those  members  of  the 
order  which  have  been  studied  in  this  particular  indicates  that  they  too  must 
be  regarded  as  i)roduced  by  budding  in  embryonic  stages,  some  of  the  buds 
developing  to  medusa*,  others  remaining  in  the  polyp  stage.  A  further 
differentiation  of  the  medustu  took  place  by  which  the  i>neumat()piiore, 
nectocalyces,  and  covering  scales  have  been  specialized  from  medusa'  origi- 
nally reproductive,  the  pneumatophore  probably  represiMiting  the  parent 
individual  of  tho  colony.  It  is  interesting  to  note  in  this  connection  that 
in  some  forms  tho  reproductive  mednsje  after  having  expelled  their  ova  or 
spermatozoa  become  converted  into  nectocalyces. 


II.  Class  Sctphohedusa. 

lu  the  ScyphomedusiB  the  nieilusa  form  is  preeminent,  the 
polyp  form  beiu*;  placed  in  the  buckgroumi  and  occurring 
only  as  a  larval  stage,  though  iu  some  forms  it  assumes 
somewhat  greater  importance  on  account  of  the  power  it  may 
possess  of  reproduction  by  transverse  division. 

The  medusjB  are  usually  free-swimming,  though  a  few  of 
the  more  lowly  organized  forms  are  attached  throughout  their 
lives  by  a  prolongation  of  the  exumbrellar  surface  (Fig.  52), 
forming  a  connecting  link  between  the  free-swimming  forms 
and  the  polyp.  As  a  rule  they  reach  a  much  greater  size 
than  do  the  Hydromedussr,  from  which  they  are  further  dis- 
tinguished by  («)  the  absence  of  a  velum,  (ft)  by  the  sense- 
organs  when  present  being  modified  tentacles,  and  (c)  by  the 
reproductive  cells  always  arising  in  the  endoderm.  On  ac- 
count of  the  first  of  these  characters  the  Scyphomedusio  are 
sometimes  termed  the  Acrmpeda. 

They  are  all  more  or  less  bell-shaped,  a  number  of  ten- 
tacles  usually  hanging  from  the  margin  of  the  bell  (Fig.  53). 
These,  however,  are  frequently  secondarily  developed,  there 
being  in  the  simpler  forms  eight  primary  tentacles  which  may 
persist  as  tentacles,  or  four  or  all  of  them  may  be  converted 


li  i 


98 


INVERTEBRATE  MORPHOLOGY. 


into  sense-organs  (Fig.  50,  s)  situated  near  the  margin  of  the 
bell  and  more  or  less  enclosed  in  special  chambers  by  the 
growth  around  them  of  folds  of  the  bell  substance  (?).  From 
the  centre  of  the  subumbrella  there  hangs  the  manubrium 
(wi),  the  extremit}'  of  which  is  frequently  prolonged  into  four 
elongated  mouth-lobes,  and  above  it  communicates  with  the 
gastric  cavity,  gr,  which  in  simple  forms  extends  to  the 
margin  of  the  bell,  being  obliterated  only  at  four  interradial 
points  oi"  lines.  The  four  broad  radial  pouches  thus  produced 
correspond  with  the  radiating  canals  of  the  craspedote  medu- 
sae, and  the  line  of  fusion  being  imperfect  at  the  margin  of  the 


Fig.  50.— Diagram  op  Scyphomedusa. 
g  =  gastric  cavity.  r  =  reproductive  organs. 

I  =  lobe  covering  the  sense-organ.  s  =  sense-organ. 

m  =  mouth.  9g  =  subgenital  cavity. 

mf  =  mesenterial  filaments.  t  =  tentacle. 

bell  a  communication  between  adjacent  pouches  is  present 
comparable  to  the  craspedote  circular  canal.  This  condition 
is,  however,  only  retained  in  the  simpler  forms  ;  in  the  higher 
Scyphomedusae  the  lines  or  points  of  obliteration  may  be 
omitted,  and  by  secondary  obliterations  taking  place  over 
various  areas  of  the  ccelenteron,  and  by  its  irregular  extension 
towards  the  rim  of  the  bell  as  this  grows  in  diameter,  a  com- 
plicated branching  arrangement  of  the  peripheral  portion  of 
the  coelenteron  is  produced,  in  which,  however,  the  original 
4-radial  arrangement  is  as  a  rule  distinctly  indicated.  Along 
the  interradial  axes  there  are  four  deep  depressions  of  the 
subumbrellar  surface,  the  funnels  or  subgenital  chambers  (Fig. 
60,  sg),  above  which  lie  the  horseshoe-shaped  reproductive 
organs,  r,  developed  in  the  coelenteric  endoderm,  one  limb  of 
each  horseshoe  lying  in  each  of  the  adjacent  radial  pouches. 


TYPE  QCELENTEHA. 


99 


Finallj',  in  the  Hue  of  each  iuterradius  there  project  into  the 
cobleuteric  cavity  a  number  of  coarse  thread-like  tilameuts, 
the  laesenterial  filaments  {mf),  which  are  unrepresented  in  the 
Hydromedusfe. 

Such  is  the  general  structure  of  the  Scyphomedusse ;  the 
modifications  will  be  better  described  in  connection  with  the 
various  orders  into  which  the  class  may  be  divided.  In  histo- 
logical structure  the  resemblance  to  the  Hydromedusae  is  so 
great  as  to  do  away  with  the  necessity  of  a  detailed  account, 
except  as  regards  the  sense-organs.  As  already  stated,  these 
when  present  are  modified  tentacles  and  partake  of  the  char- 
acters of  both  eyes  and  otocysts.  They  are  usually  short 
finger-like  stalks,  lying  in  a  notch  of  the  rim  of  the  bell 
and  covered  over  by  folds  (covering  plates,  Fig.  51,  cp)  arising 
from  the  adjacent  substance  of  the  bell  on  either  side  of  tho 


Fig.  51. — Mauginal  Sense-okgan  op  Rhopnlonema  (after  Hertwig). 
ce  =  ccelenteric  cavity.  en  =  endoderm. 

cp  =  covering  plate.  o  =  eye. 

ec  =  ectoderm.  ot  =  otocyst. 

niche  and  frequently  uniting  so  that  the  stalks  lie  in  pouch- 
like cavities.  The  ectoderm  of  the  finger-like  stalks  contains 
numerous  sensory  and  ganglion  cells,  and  at  one  or  more 
regions  pigment-cells  are  associated  with  these  to  form  the 
eye  (o),  which  may  be  further  perfected  by  the  addition  of  a 
cuticular  lens.  The  stalks  are  hollow,  containing  a  prolonga- 
tion of  the  ccelenteric  cavity  {cc)  lined  by  endoderm,  and  at  the 
tip  of  the  stalk  the  endoderm-cells  are  filled  with  crystals  of 
carbonate  of  lime,  the  whole  mass  of  crystals  forming  a  rather 
large  otocyst  {ot).  The  covering  plates,  furthermore,  above  the 
sensory  stalks  are  usually  grooved,  the  bottom  of  the  groove 


^   iJi 


L 


100 


INVERTEBRATE  MORPHOLOGY. 


being  liued  by  sensory  cells  to  which  an  olfactory  function  is 
attributed.  The  marginal  sense-organs  of  the  ScyphomedusaB 
are  thus  much  more  complicated  in  structure  and  in  addition 
have  a  different  mode  of  origin  than  those  of  the  Hydrome- 
dusse. 


1.  Order  Stauromedusee. 

In  this  order  some  of  the  forms  are  fixed  throughout  their 
adult  life,  e.g.  Ziwernaria  (Fig.  52) ;  while  others  are  free- 
swimming  when  mature,  as  Tessera.  The  deep  bell  has  at 
the  margin  eight  tentacles— the  primary  tentacles,  none  of 
which  become  modifi.ed  into  sense-organs,  these  structures 
being  wanting  in  the  group.     In  some  species  of  Zvcernaria 


Fig.  52,— Halidyattu  auricula  divided  lonoitudinally  (after  H.  James-Clark). 

/  =  funnel.  mf  =  mesenterial  filaments. 

go  =  reproductive  organs.  rp  =  radial  pouch. 

ia  =  iuterradial  adhesion.  t  =  modified  primary  tentacle. 

these  primary  tentacles  {t)  are  somewhat  altered,  and  in  all 
the  margin  of  the  bell  is  produced  into  eight  knob-tipped 
lobes,  to  each  of  which  a  bunch  of  secondary  tentacles 
may  be  attached.  The  coelenteron  extends  out  to  the  margin 
of  the  bell,  and  is  interrupted  along  the  interradii  by  a  point 


TYPE  C(ELENTERA. 


101 


{Tessera)  or  Hue  {Zucernaria)  of  adhesion.  The  depressions 
of  the  subumbrellar  surface,  the  funnels  (/),  are  very  deep  in 
Lvcernaria,  extending  almost  to  the  summit  of  the  bell. 


>  so 


2.  Order  Peromedusae. 

In  this  order  the  adult  medusae  are  always  free-swimming, 
and  are  characterized  by  the  bell  being  pointed  in  shape  and 
about  its  middle  marked  with  a  distinct  constriction.  The 
ccelenterou  is  obliterated  at  only  four  points,  as  in  Tessera, 
and  they  differ  from  the  Stauromedusse 
by  possessing  four  sense-organs,  the 
four  interradial  tentacles  of  the  primary 
series  of  eight  becoming  modified  to 
form  these  structures,  while  the  radial 
ones  retain  their  original  character. 

3.  Order  GubomedussB. 

This  order,  of  which  Charybdea  (Fig. 
53)  is  a  typical  example,  is  characterized 
by  the  bell  being  of  a  cubical  shape. 
The  interradial  obliterations  of  the 
coelenteron  are  linear,  and,  as  in  the  pre- 
ceding order,  four  of  the  primary  ten- 
tacles, these  being  the  only  ones  which 
develop,  are  modified  to  form  sense- 
organs.  In  this  order,  however,  it  isFiQ.5d.— Charybdea  mar- 
the  four  radial  tentacles  which  form  the 
sense-organs  (so),  the  four  interradial  per- 
sisting as  tentacles  (t). 

4.  Order  Discomedusee. 

In  this  order,  which  includes  the  majority  of  the  known 
ScyphomedussB,  all  the  eight  primary  tentacles  are  converted 
into  sense-organs,  a  number  of  secondary  tentacles  usually 
developing  at  the  margin  (Fig.  54).  The  primary  interradial 
obliterations  of  the  coelenteron  do  not  develop,  but  on  the 
other  hand  secondary  obliterations  frequently  make  their 


aupialis  (after  Claus). 
80  =  sense-organ. 
t  =  tentacle. 


102 


INVERTEBRArE  MORPHOLOGY. 


appearance,  wliicli,  combined  with  irregularities  of  growth  of 
tbe  coeleuterou,  give  its  peripheral  portions  an  irregular  out- 
line (Cyanea),  or  convert  them  into  a  series  of  anastomosing 
canals,  e.g.  Aurdia.  The  margin  of  the  bell  is  usually  more 
or  less  lobed,  eight  of  these  lobes  being  especially  distinct 
and  carrying  the  sense-organs,  the  intervals  between  them 
being  usually  occupied  by  the  secondary  tentacles  when  these 
are  present.  The  depressions  of  the  subumbrellar  surface  are 
no  longer  deep  funnels,  but  form  rather  shallow  subgenital 


Pig.  54. — Pelagia  cyanella  (after  AaAssiz). 

chambers  with  thin  roofs,  into  the  cavity  of  which  the  repro- 
ductive organs  bulge  out. 

In  many  forms  the  margins  of  the  mouth  are  prolonged 
into  long  fringed  lobes,  and  in  one  family,  the  jRhizostomidce 
(e.g.  Stomolophiis),  the  margins  of  these  lobes  may  fuse  to- 
gether, leaving,  however,  a  large  number  of  minute  openings 
along  the  line  of  fusion.     These  lead  into  canals  traversing 


TYPE  CCELENTF.RA. 


103 


the  substance  of  the  lobes,  and  uniting  finally  in  a  larger 
central  canal  at  the  upper  extremity  of  which  lies  the  original 
mouth. 

Development  of  the  Scyphomedusce. — The  segmentation  of 
the  ovum  leads  as  usual  to  a  blastula  form  which  may  be- 
come solid  by  immigration  and  subsequently  hollow  out,  or 
may  abbreviate  these  processes  by  a  typical  invagination. 
The  gastrula  resulting  passes  in  some  forms  gradually  into  the 
adult  condition  (Pelagia),  without  ever  relinquishing  its  free- 
swimmiug  habits,  though  in  the  majority  of  cases  in  which  the 
life-history  is  known,  cases  confined  entirely  to  the  Discome- 
dusae,  the  embryo  settles  down  and  leads  for  a  time  a  fixed 
sessile  existence,  resembling  very  much  a  polyp.  This  polyp 
form  (Fig.  55,  A),  known  as  the  Scyphostoma,  difi'ers,  mate- 
rially from  the  hydroid  polyp ;  in  the  first  place,  from  the 
body-wall  there  project  into  the  coelenteron  four  linear 
ridges  (mesenteries)  which  extend  from  the  neighborhood  of 
the  mouth  to  the  posterior  end  of  the  coelenteron,  and  later 
give  rise  to  the  mesenterial  filaments  of  the  medusae  ;  and  in 
the  second  place,  a  deep  funnel-like  depression  extends  into 
each  one  of  these  mesenteries  from  the  oral  surface  of  the 
Scyphostoma,  and  produce  later  the  subgenital  chambers  of 
the  medusae. 

The  Scyphostoma  may  develop  directly  into  the  free- 
swimming  medusae,  but  in  a  number  of  forms  it  undergoes  a 
series  of  transverse  divisions  (Fig.  55,  B)  into  a  number  of 
saucer-like  structures  produced  at  the  edges  into  eight  blunt 
notched  prolongations.  These  separate  from  the  parent  Scy- 
phostoma and  are  known  as  Ephyrce  (Fig.  55,  C),  developing 
into  the  adult  Discomedusan  by  the  intervals  between  the 
eight  lobes,  which  carry  the  sense-organs,  filling  out,  b}'  the 
development  of  tentacles,  and  by  the  growth  of  the  coelenteron. 
A  typical  alternation  of  generations  is  thus  brought  about. 

There  can  be  little  doubt  but  that  the  ancestral  Scyphomedusan  was  a 
sessile  organism  with  much  resemblance  to  Lucertiaria.  From  this  two 
lines  of  descent  arose,  both  marked  by  a  development  of  sense-organs  from 
tentacles  and  terminating  in  the  Peromedusae  and  CubomedusaB  respec 
tively,  the  Discomedusae  resulting  in  the  culmination  of  this  sense-organ 
development.    So  far  as  the  Discomedusae  tliemselves  are  concerned  the 


104 


IN  VEltrEBHA  TE  MORPHOLOQ  Y. 


Ephyra  seems  to  represent  an  ancestral  stage,  since  some  mature  medusae 
resemble  this  stage  very  closely  and  it  occurs  in  the  life-history  of  all,  and 
earlier  than  this  is  the  Scyphostoraa  representing  tlie  Lncernaria  stage  of 
evolution.      The  Scyphostoma  has  superficial  resemblance  to  a  bydroid 


Fig.  55.—^,  Scyphostom  of  Aurelia;  B,  Strobila  of  Aurelia ;  G,  Ephyra  of 

Pelagia  (all  after  Agassiz). 

polyp,  which  resemblance  is  almost  an  identity  in  the  earlier  stages  of  the 
Scyphostoraa  before  the  development  of  the  mesenteries  and  funnels.  This 
suggests  a  relationship  of  the  Scyphomedusae  to  the  Hydromedusee  only 
through  the  polyp,  the  separation  of  the  two  classes  having  occurred  be- 
fore the  appearance  of  the  medusae  on  the  scene. 


III.  Glass  Anthozoa. 

The  Anthozoa  never  assume  the  medusa  form,  but  are  ses- 
sile, usually  colony-producing  polyps  of  the  Scyphostoma 
type.  Typically  they  are  cylindrical  structures  (Fig.  56)  at- 
tached at  one  extremity,  the  fease,  and  bearing  at  the  other 
extremity  the  mouth  in  the  centre  of  a  flat  surface,  the  disk, 
around  the  margins  of  which  are  a  number  of  hollow  tenta- 
cles. The  coelenteron  is  imperfectly  divided  into  a  number 
of  chambers  by  longitudinal  partitions  arising  from  the  body- 
wall,  the  mesenteries  (Fig.  57,  me),  the  various  intermesenterial 


TYPE  VaSLENTEUA. 


105 


cliaiiibers  communicating  freely  with  a  central  space.  The 
mouth  does  not  open  directly  into  the  ccBleuteron,  as  in  the 
hydioid  polyps,  but  into  a  tube,  the  fstomatodoeum  (at),  lined 
by  ectoderm  and  communicating  freely  below  with  the  central 
cceleuteric  space.  Certain  of  the  mesenteries  in  their  upper 
portions  are  attached  to  the  endodermic  surface  of  the  stoma- 
todauiu,  but  below  its  lower  end  all  have  free  edges.  Aloug 
this  free  edge  there  runs,  in  most  of  the  mesenteries,  a  cyliudri- 


Fig.  66.  —  Metridium   margina- 
tum, Les. 


Pig.  57.  —  Diagrammatic  Traks- 
VERSE  Section  through  Edward- 

Sia  IN  liEOION  OF  STOMATODiEUM. 

me  =  lueseuiery. 
rm  =  retractor  muscle. 
81    =  sipboDoglyphe. 
st    =  stotuuiudteum. 
I-IV  =  mesenteries  in  the  order  of 
their  development. 


cal  thickening,  the  mesenterial  filament,  composed  in  its  lower 
portion  of  cnidoblasts  and  gland  cells,  and  usually  longer  than 
the  mesentery,  on  the  edge  of  which  it  forms  a  complicated 
system  of  coils  and  twists.  In  its  lower  part  the  filament  in 
some  forms  separates  from  the  mesentery  as  a  bunch  of  fine 
filaments  richly  provided  with  nematocysts  and  capable  of 
being  protruded  from  the  mouth  or  through  pores  in  the 
body- wall.  The  upper  part  of  the  filament  is  usually  of  dif- 
ferent structure,  bearing  on  each  side  a  band  of  elongated 
ciliated  cells  whose  function  it  is  to  produce  a  circulation  of 
the  fluids  in  the  cceleuteron. 

The  reproductive  cells  develop  in  the  endoderm  of  the 


106 


IN  VERTEBRA  TE  MORPHOLOG  Y. 


meseuteries,  wheuce  tbey  are  shed  into  the  intermeseuterial 
chamber  aud  make  their  exit  by  the  mouth.  The  nervous 
system  is  well  developed  especially  in  the  ectoderm  of  the 
disk  and  tentacles,  though  it  also  occurs  in  the  endoderni. 
It  possesses  the  general  character  of  the  Coelenterate  nervous 
tissue  consisting  of  sensory  cells,  nerve-tibres,  aud  ganglion- 
cells.  The  muscular  system  is  very  well  developed  both  in 
the  ectoderm  and  endoderm,  the  muscle-fibres  being  generally 
longitudinal  in  the  former  layer,  aud  in  the  latter  circular  in 
their  direction.  At  certain  regions  of  the  body  the  muscle- 
fibres  are  especially  abundantly  developed,  the  mesogloea 
being  thrown  into  complicated  folds  for  their  support,  so  that 
it  is  possible  to  distinguish  certain  definite  muscles.  One  of 
these  is  developed  upon  one  face  of  each  mesentery,  and,  its 
fibres  being  directed  longitudinally,  it  forms  a  strong  retractor 
(Fig.  57,  rm)  for  the  disk  aud  tentacles ;  a  second  is  developed 
in  the  endoderm  of  the  body-wall  a  short  distance  below  its 
junction  with  the  disk,  and  its  fibres  may,  by  the  growth  of 
the  mesogloea  around  them,  become  imbedded  in  that  layer ; 
it  forms  a  more  or  less  powerful  sphincter,  serving  to  cover  in 
the  disk  and  tentacles  when  these  have  been  retracted  by  the 
mesenterial  retractors. 

The  Anthozoa  are  constructed  upon  a  radial  symmetry, 
as  are  the  other  Ccelentera,  this  symmetry  appearing  in  the 
arraugeiiient  of  the  mesenteries  and  tentacles  and  in  the  cylin- 
drical form  of  the  body.  Nevertheless  it  is  always  possible 
to  divide  the  Anthozoan  by  a  single  plane  into  two  similar 
halves,  that  is,  a  bilateral  symmetry  is  also  present  which  is 
produced  by  the  arrangement  of  the  retractor  muscles  on  only 
one  face  of  each  mesentery  and  by  the  flattening  of  the  sto- 
matodsBum.  This  latter  feature  is  furthermore  usually  made 
more  pronounced  by  the  occurrence,  at  one  or  both  ends  of 
the  longer  transverse  axis  of  the  stomatodaeum,  of  a  distinct 
groove  lined  by  high  columnar  cells  with  long  cilia,  these 
grooves  forming  the  siphonoglyphes  (Fig.  57,  si),  and  by  the 
mesenteries  which  are  attached  to  the  stomatodaeum  in  the 
neighborhood  of  the  siphonoglyphes  usually  having  their 
retractor  muscles  on  different  faces  from  those  on  which  they 
occur  in  the  other  mesenteries. 


TYPE  C(ELENTEliA. 


107 


Frequently  the  ectoderm  of  the  authozoan  polyps  secretes 
»■  skeletal  substauce  which  may  either  be  carbonate  of  lime,  or 
else  an  organic  substance  of  a  horny  consistency.  In  the 
corals  the  secretion  takes  the  form  of  carbonate  of  lime  and 
forms  a  cup  (Fig.  58,  ap)  in  which  the  polyp  is  seated,  ridges 


fwijo  \Us 


Fig.  58. — Diaguam  of  the  Stuuctdue  op  a  Coual  (after  von  Koch,  from  Lano), 
(ip  =  exotheca.  h»  =  mesentery. 

fp  =  basal  plate.  ss  =  septa. 

Calcareous  skeleton,  wliite;  ectoderm,  shaded;  mesoglcea,  black;  endoderni, 

dotted. 


(septa,  ss),  over  which  the  soft  tissues  of  the  animal  are 
moulded,  projecting  up  from  the  bottom  of  the  cup.  The 
septa  may  be  united  by  delicate  tangential  bars,  synapticulfe, 
and  from  the  bottom  of  the  cup  a  somewhat  cylindrical  colu- 
mella may  project,  other  upright  rods,  the  pali,  intervening  be- 
tween the  free  edges  of  the  septa  and  the  columella.  The 
upper  portion  of  the  body-wall  of  the  polj'p  is  reflected  over 
the  rim  of  the  calcareous  cup  and  may  produce  ridges,  costce, 
on  its  outer  surface  corresponding  in  position  with  the  septa, 
and  inasmuch  as  the  cup  is  continually  increasing  in  depth  so 
long  as  the  polyp  lives,  and  the  polyp  only  occupies  the  upper 
portion,  the  lower  part  may  from  time  to  time  be  separated 
oft'  by  a  transverse  partition  or  dissepiment. 

In  other  forms,  such   as  the  Alcyonarians,  however,  the 
skeleton  is  secreted  only  by  the  basal  ectoderm  and  the  colony 


1()8 


INVEUrEDHATE  MOHPUOLOUY. 


becomes  moulileil  over  it  (Fig.  69),  so  that  it  forms  a  central 
Iioruy  or  more  or  less  calcified  axial  support,  and  in  addition 

the  mesogloeai  cells  secrete  scattered 
particles  of  carbouate  of  lime,  haviug  a 
more  or  less  detiuite  form  for  each  spe- 
cies, but  not  uniting  together  to  form  a 
firm  skeleton. 

The  development  of  the  Anthozoa  is 
always    direct,     and     the     diploblastic 
Fig.  69.— Diagram  of  condition    is     produced     by    delamiua- 
YouNG  GonGoNiAN  (af-  tion  of  the  cells  of  the  blastula.     Many 
ter  VON  Koch).  adult    forms     possess    the     power     of 

division,  either  transverse  or  longitudinal,  the  latter  giving 
rise  to  complex  colonies  in  many  cases.  In  other  forms  the 
primary  polyp  may  develop  a  stolon  from  which  other  indi- 
viduals may  bud,  producing  a  diffuse  colony,  or  the  intervals 
between  the  individuals  may  be  filled  up  by  a  growth  of 
mesoglcea  traversed  by  a  network  of  canals,  forming  a  tissue, 
the  coenenchyme  (Fig.  60),  in  which  the  various  individuals  are 
imbedded. 

The  class  Anthozoa  may  be  divided  into  a  number  of 
orders,  whose  existence  depends  mainly  on  the  arrangement 
of  the  mesenteries. 


1.  Order  AloyonarisB. 

The  majority  of  the  Alcyonarians  produce  colonies  by  bud- 
ding. In  some  the  individuals  are  scattered  on  stolons,  in 
others  imbedded  in  a  ccenenchyme  {Alcyonium,  Fig.  60),  or  in 
others  united  to  form  fleshy  colonies  of  a  feather  or  reniform 
shape  {ReniUa),  the  whole  being  imbedded  in  the  sand  by  a 
fleshy  stalk.  In  some  of  the  groups  a  horny  or  calcareous 
skeleton  is  present  in  addition  to  the  calcareous  spicules  im- 
bedded in  the  mesogloea  and  may  form  a  central  axis  enclosed 
by  the  coenenchyme  (Fig.  59)  and  of  a  horny  consistency,  as  in 
Gorgonia  and  Zeptogorgia,  or  more  or  less  calcareous,  as  in 
Isis  and  Oorallium,  the  skeleton  of  the  latter  constituting  the 
red  coral  of  commerce.  In  the  Organ-pipe  Coral,  Tttbipora, 
each  individual  lives  in  a  calcareous  tube,  the  various  tubes 


TYPE  CaCLENTERA. 


109 


Ix'iiig  uuiteil  by  tranaverse  plates,  aud  in  TTeliopora  the  skeleton 
becomes  very  luassive,  resembling  that  of  the  ordinary  corals 
eveu  to  the  occurrence  of  septa  projecting  into  the  interior  of 
tlie  cups  which  contain  the  individual  polyps. 

Notwithstanding  these  manifold  variations  of  the  skeleton 
ami  of  the  colony  form,  the  individual  polyps  present  through- 
out  u  great  similarity  of  structure.  They  possess  only  eight 
pinnate  tentacles  and  eight  mesenteries  whose  retractor  mus- 
cles are  arranged  in  the  manner  shown  in  the  annexed  dia- 
grammatic cross-section  of  a  polyp  (Fig.  61).     In  lienilla  aud 


rm- 


Pig.  60.— Diagram  op  Youkg 
Colony  of  Alcyonium  (after 
VON  Koch. 


Fig.  61.— Diagrammatic  Transvbrsb 
Section  of  an  Alcyonarian. 
rm  =  retractor  muscle. 
81  =  siphoDoglyphe. 
I-IV  =  mesenteries. 


allied  forms,  such  as  Pennatida,  a  slight  polymorphism  occurs, 
certain  polyps  possessing  no  tentacles  and  functioning  as 
inhalent  zooids  througli  which  currents  of  water  pass  into  the 
coelenteric  cavities  of  the  colony  through  which  they  circulate. 

2.  Order  EdwardsiflB. 

The  Edwardsise  never  produce  colonies  nor  do  they  possess 
a  skeleton,  though  frequently  the  exte  ior  of  the  body  is  en- 
crusted by  foreign  particles.  They  live  usually  imbedded  H 
sand,  the  base  being  rounded  and  not  adhesive,  and  possess 
eight  (sometimes  sixteen  or  thirty-two)  simple  tentacles  and 
eight  mesenteries,  dififering  from  those  of  the  Alcyonarians  in 


110 


INVERTEBRATE  MORPHOLOGY. 


the  arraugemeut  of  the  retractor  muscles  as  shown  in  Figure 
67. 


3.  Order  Cerianthese. 

The  Cerianthero  are,  like  the  Edwardsise,  solitary  forms 
destitute  of  a  skeleton,  and  live  imbedded  in  sand  or  mud. 
The  basal  region  is  rounded  and  not  adhesive,  having  at  the 
centre  a  pore  which  communicates  with  the  coelenteron.  In 
Ceriajithus  a  fibrous  investment  surrounds  the  body  as  a  tube, 
secreted  by  the  ectoderm,  this  layer  of  the  body  being  further 
characterized  by  an  enormous  development  of  muscle-fibres 
arranged  longitudinally  and  supported  upon  slender  processes 
of  the  mesogloea  of  the  body-wall.  The  tentacles  are  simple 
and  very  numerous,  being  arranged  in  two  sets,  one  surround- 
ing the  margin  of  the  funnel- 
shaped  disk  and  the  other  im- 
mediately surrounding  the 
mouth.  The  mesenteries  are 
also  very  numerous  and  are 
distiuguished  by  the  absence  in 
the  adult  of  retractor  muscles, 
the  ectodermal  muscles  playing 
the  part  of  the  retractors,  and 
the  characteristic  Anthozoan 
sphincter  is  also  absent.  The 
arrangement  of  the  mesenteries 
(Fig.  62)  is  peculiar  to  the 
group,  new  ones  continuing  to 
form  during  the  entire  life  of 
the  animal  and  making  their 
appearance  one  on  each  side 
of  the  sagittal  plane  between 


Fig.    63,  —  Dtaohammatic   Trans- 
verse Section  op  a  Young  Ceri- 
anthus  (according  to  Carlgren). 
si  =  siphonotrlyphe. 

I-IV  =  the  Edwardsinn  mesenteries. 
1-4  =  secotidary  mesenteries. 


the  two  which  immediately  preceded  them.  The  older  mes- 
enteries are  thus  crowded  to  one  surface  of  the  body,  the 
dorsal  surface,  at  which  the  single  siphonoglyphe  (si)  occurs 
in  the  stomatodsBum,  and  the  four  on  either  side  of  the 
mid-dorsal  line  (T-TV)  are  the  homologues  of  the  eight 
mesenteries  of  the  EdwardsisB,  the  rest  being  secondary 
structures  not  represented  in  that  group. 


TYPE  CCELENTERA. 


Ill 


4.  Order  Antipathariee. 

The  members  of  this  order  are  all  colouial  and  secrete  a 
brauehiug  axial  skeleton  of  a  black  horuy  material.  The 
polyps  possess  usually  but  six  simple  teutacles,  and  as  a  rule 
ouly  six  mesenteries  are  present,  of  which  only  the  two  lying 
in  tlie  transverse  axis  bear  reproductive  organs  and  mesen- 
terial  lllaments  ;  in  some  forms  four  or  six  additional  imper- 
fectly-developed mesenteries  are  present,  but  six  seems  to  be 
the  number  typical  for  the  group. 

5.  Order  Erotactiniae. 

This  order  includes  a  group  of  forms,  all  simple  and  with 
simple  tentacles,  but  showiug  considerable  variation  in  the 
number  of  the  tentacles.  They  all 
agree  in  this  particular,  however, 
that  there  are  twelve  mesenteries 
arranged  in  pairs  (Fig.  63,  I-V1\ 
the  two  pairs  attached  to  the  si- 
phonoglyphe  region  of  the  stoma- 
todicum  having  their  retractor 
muscles  on  the  faces  turned  away 
from  each  other,  while  in  the  other 
four  pairs  they  are  on  adjacent 
faces.     The  two   former  pairs    are 

termed     the    divecave    me^^n^me.  F.^ea-DiAonAMMATrcTuANs- 
li^  ig.    bd,  J>  and   I)  ),  their  constit-     verse  Section  op  Gonaciinia. 
uent  mesenteries  lying  (me  on  each  ^<  ^  =  directive  mesenteries, 
side   of  the  sagittal  plane,  and  to-  ^'^^   ^  ^^"^  Edwardsiun  meseu- 

gether  with  one  mesentery  (//and  i^  FA     J*"""!'  •     * 
,.    ..  1        ,    ,,  y  ^  '^^  J^/ =  mesen  let  les  forming 

/)  from   each   of   the  other  pairs  pairs  with //und /. 

represent  the  eight  Edvvardsian  "^  =  secondjiiy  pair  of  mes- 
mesenteries.     To  these  six  primary  enteries. 

pairs  ii  varying  number  is  added  in  the  different  forms ;  it 
may  be,  on  each  side,  one  between  one  of  the  pairs  of  direc- 
tives and  the  adjacent  lateral  pair  (Scytophorm),  or  a  pair  in 
the  same  locality  {Gonactinia,  Fig.  63,  7),  or  two  pairs  (me  of 
which  corresponds  to  the  pair  of  Gonactinia,  the  second  pair 
lying  between  the  two  lateral  primary  pairs  {Oractid). 


D'-'  ^ 


112 


INVERTEBRATE  MORPHOLOOY. 


In  all  these  forms  there  is  a  strictly  bilateral  arrangement 
of  the  mesenteries,  and  a  tendency  for  them  to  arrange  them- 
selves in  pairs. 


6.  Order  Zoantheee. 

The  ZoantheaB  form  very  frequently  colonial  aggregates 
either  of  a  diffuse  stoloniferous  character  (Zoanthus)  or  of  a 
more  compact  form,  the  individuals  being  imbedded  in  a  coe- 
nenchyme  {Palythoa).  No  skeleton  is  present,  though  many 
forms  have  a  dense  crust  on  the  outside  of  the  body  formed 
of  particles  of  sand,  sponge-spicules,  radiolarian  and  fora- 

miniferan  shells,  etc.,  imbedded 
in  the  outer  portion  of  the  meso- 
gloea.  They  possess  a  varying 
number  of  simple  tentacles,  and 
there  is  only  a  single  siphono- 
glyphe  which  marks  the  ventral 
surface  of  the  body.  The  mes- 
enteries are  arranged  in  pairs, 
six  of  which  (Fig.  64,  /-  F,  II- VI, 
III  and  IV)  correspond  with  the 
six  primary  pairs  of  the  Protac- 
tinisB ;  of  these  the  dorsal  direc- 
FiG.  64.— DiAonAMMATrc  Trans-  tives  (D)  are  never  united  to  the 
VERSE  Section  op  Zoanthus.      stomatodsBum     and     the     dorsal 

f'rfr  =,d''-««^*7."^7"t«'-'*^«-        lateral  pair  (//,  VI)  consists  of 

/-F/as  in  preceding  ngure.  t     .  ^  •  ,     , 

1-4  =  pairs  of  secondary  mesen-  o^^    perfect    and   one   imperfect 

teries.  mesentery,  the  latter  being  ventral 

to  the  former.  The  ventral  lat- 
eral primary  pair  may  consist  -of  two  perfect  mesenteries  or 
may  have  the  same  arrangement  as  the  dorsal  lateral  pair. 
To  these  six  pairs  a  varying  number  of  secondary  pairs  (1-4) 
may  be  added,  the  new  pair  always  arising  immediately  on 
either  side  of  the  ventral  diractives.  Each  of  the  new  pairs 
consists  of  a  perfect  and  an  imperfect  mesentery,  the  latter 
being  the  dorsal  one  of  the  two,  these  secondary  pairs  thus 
differing  from  the  lateral  primary  mesenteries. 


TYfE  CiELENTERA. 


113 


7.  Order  Hexactiuise. 

In  the  HexactinisB  the  six  primary  pairs  of  mesenteries 
described  as  occurring  in  the  two  preceding  orders  are  again 
found  (Fig.  65,  /,  D,  and  D),  and  in  a  few  forms  (Halcanipa) 
may  be  the  only  ones  present.     As  a  rule,  however,  a  varying 

nr       i> 


Fig.  65.— Diagrammatic  Transvekse  Section  of  an  Hexactinian,  Aip- 

ttma,  with  only  the  mesenteries  of  the  first  cycle  perfect. 
D  =  directive  mesenteries.  /  =  mesenteries  of  the  first  cycle. 

5' =  reproductive  region  of  mesentery.       // =  "  "    "second" 

♦n/=  mesenterial  filament.  /// =  "  "    "third    " 

number  of  secondary  pairs  develop,  each  of  these  appearing 
in  the  interval  between  two  primary  pairs,  so  that  two  cycles 
of  mesenteries  (/  and  //)  may  be  distinguished.  Usually, 
however,  the  process  of  mesentery  formation  does  not  stop 
here,  tertiary  (///),  quaternary,  etc.,  cycles  being  developed, 
the  pairs  of  each  new  cycle  appearing  in  the  intervals  between 
the  pairs  of  the  cycles  already  present.  Consequently,  since 
there  are  six  primary  pairs,  the  second  cycle  will  consist  also 
of  six  pairs,  the  third  of  twelve,  the  fourth  of  twenty-four,  and 
so  on.  In  a  few  forms,  owing  to  the  precocious  development 
of  one  or  two  of  the  secondary  pairs  on  each  side,  the 
symmetry  becomes  converted  from  an  hexamerous  one  to  an 
octamerous  {Aiptasia  annidata)  or  a  decamerous  one  {Tealia). 
Since  the  tentacles  develop  in  connection  with  the  spaces 
between  the  mesenteries,  they  are  arranged  in  cycles  corre- 


114 


INVEliTEBRATE  MORPHOLOGY. 


spending  to  the  mesenteries.  Usually  but  a  single  tentacle 
communicates  with  each  space,  but  in  some  forms  a  series 
may  arise  on  the  roof  of  each  space  so  that  the  tentacles  have 
a  radiating  arraugemeut  (Discosoma)  or  may  appear  to  be 
irregularly  scattered,  as  in  some  corals  {Fungia).  They  are 
usually  simple  in  form,  though  they  may  be  in  some  cases 
pinnate  {Fht/manthits,  Thalassianthus)  or  even  branched. 
The  order  is  usually  divided  into  two  suborders : 

1.  Suborder  Malacodermata. 

This  includes  the  Sea-anemones  or  Actinians,  all  simple 
forms,  not  producing  colonies,  and  usually  attached  by  an 
adhesive  base.  They  never  form  a  skeleton  of  any  kind, 
though  they  may  develop  an  enveloping  cuticle,  usually  very 
thin  and  in  some  cases  encrusted  with  foreign  matter ;  this 
is  more  especially  the  case  with  deep-water  forms,  the  shallow- 
water  forms,  such  as  Metridium^  Burwdes,  etc.,  lacking  a 
cuticle.  Many  forms  possess  the  power  of  division,  the  in- 
dividuals so  produced  separating  completely  and  not  forming 
colonies ;  furthermore  some  forms  reproduce  non-sexually  by 
separating  off  portions  of  the  tissue  at  the  margin  of  the  base, 
each  portion  eventually  developing  into  an  adult  Actinian. 


2.  Suborder  Sderodermata. 

This  suborder  includes  the  ordinary  corals,  which  secrete 
a  calcareous  skeleton  of  the  character  already  described 
(p.  107).  A  few  forms  are  simple,  but  the  majority  produce 
complex  colonies  by  longitudinal  division  and  by  budding, 
while  in  others  the  division  is  only  carried  to  the  extent  of 
the  formation  of  an  individual  with  a  number  of  mouths,  as  in 
Fungia  and  Manicina.  In  most  of  the  forms  the  corallum  is 
tolerably  dense  and  may  be  either  branching,  as  in  Ocvlinay  or 
form  massive  blocks,  as  the  Brain-stone  Coral  {Mceandrina), 
but  in  Madrepora  it  is  more  or  less  porous. 

The  Corals  are  most  abundant  in  tropical  seas  and  in  shallower  water, 
the  Madrepores  forming  under  such  conditions  large  reefs,  in  the  lagoons 
of  which  the  Fungias,  Manicinas,  and  Maeandrinas  are  found.  In  colder  seas 
but  few  forms  {Astrangia)  are  found  in  shallow  water,  but  in  the  greater 


TYPE  C(ELENTERA. 


116 


•It^'pths  of  the  ocean  the  simple  forms  which  do  not  produce  colonies  are 
fiiMjueiitly  found. 

Jie/utionstn'ps  of  the  Anthozoa. — As  has  been  pointed  out,  it  seems  prob- 
at)lo  that  the  Anthozoa  are  to  be  traced  back  to  a  Scyphoston)a-liive  polyp 
latking  interradial  funnels.  No  four-mesenteried  form,  however,  is  known, 
a  large  gap  existing  between  the  Scyphostoma  and  the  Aleyonaria,  which 
are  probably  tiie  simplest  Anthozoa  known  to  us.  The  primitive  Aleyonaria 
wore  undoubtedly  simple  forms,  and  from  them  to  the  Edwardsia^  was  not 
a  very  great  step.  By  the  formation  of  four  additional  mesenteries  tiie 
Edwardsian  condition  became  converted  into  the  twelve-mcsenteried  con- 
dition which  forms  the  ground-form  of  the  Protactinia',  Zoantheae,  and 
Ilexactiniae,  the  various  stages  seen  in  the  ProtactinitB  indicating  the 
manner  in  which  the  Hexactinian  condition  has  been  brought  about. 
Th(!  Ceriantheae  seem  to  be  offsets  from  the  Edwardsian  condition,  but  it  is 
difTicnlt  in  the  present  state  of  our  knowledge  to  conjecture  the  affinities  of 
the  Antipatharia. 

It  is  noticeable  that  the  members  of  all  the  orders  except  the  Hexac- 
tiniie  have  a  strictly  bilateral  arrangement  and  development  of  the  mesen- 
teries ;  this  arrangement  becomes  gradually  modified,  first,  by  the  ten- 
dency of  the  mesenteries  to  arrange  themselves  in  pairs ;  second,  by  the 
formation  of  secondary  mesenteries ;  third,  by  a  tendency  for  these  to 
appear  in  pairs  ;  fourth,  by  a  tendency  for  such  pairs  to  appear  in  all  the 
intervals  between  the  primary  pairs.  Thus  the  Anthozoa  are  forms  whicii 
are  gradually  specializing  away  from  the  radial  symmetry  characteristic 
of  all  Coelenterates  towards  a  bilateral  symmetry,  and  the  more  pronounced 
radiality  of  the  Hexactiniae  is  a  secondary  condition. 


SUBKINGDOM  METAZOA. 

TYPE  V(ELENTEliA. 

Subtype  PORTFERA.— With  pores  in  tlie  walls  and  without  nemato- 
cysts. 

1.  Order  Ca^mrea. —Skeleton  calcareous. 

{a)  Ascon  type.     Leticosolenia. 
(?*)  Sycon  type.     Grantin. 

2.  Order   Cornacuspongia;.— Skeleton  of  spongiolin,  usually  with 

simple  siliceous  spicules. 

(a)  With  spicules  ;  fresh  water.     Spongilla,  Ephydatia. 

(b)  Without  spicules  ;  marine.     Euspongia. 

8.  Order  Spicnli'fiiwngifr,— Skeleton  of  uniaxial  or  tetraxial  sili- 
ceous sjncules.    Sometimes  entirely  wanting. 
(a)  Skeleton  wanting.     Halisarca. 
(6)  Skeleton  present.     Cliona,  Esperella. 
4.  Order  Hyalospongiw. — Skeleton    of   6-rayed  siliceous   spicules. 
Euplectella. 


116 


IN  VERTEBRA  TE  MORPUOL  00  Y. 


II.  Subtype  CNIDAETA.  Without  pores  in  walls  and  with  nematocysts. 
I.  Class  Hydromedus^. — Ectoderm  and  endoderra  meet  at  mouth.     Re- 
productive organs  develop  in  ectoderm.    Medusa  with  velum  ;  sense- 
organs  not  modified  tentacles. 

1.  Order  Hydrariw. — No  medusa  form  ;  tentacles  hollow.    Hydra. 

2.  Order  Narcomedusw.  -  No  hydroid  form ;  sense-organs  otocysts 

of  endodermal  origin ;   radiating  canals  represented  by  broad 
pouches.     Cunoctaidha,  Cunina. 
8.  Order  Tiachymedusce. — No  hydroid  form ;  sense-organs  otocysts 
of  endodermal  origin  ;  radiating  canals  narrow.     Liriope,  Qer- 
yonia,  Rf  jpalontma. 

4.  Order  Leptomedusw  or  Campanularice. — With  both  hydroid  and 

medusoid  forms,  the  latter  frequently  degenerate.     Hydranths 
with  hydrotliecae  ;  gonangia  present.     Medusa  with  otocysts  of 
ectodermal  origin  ;  reproductive  organs  on  radial  canals. 
(a)  Hydroid  and  medusoid  forms  both  well  developed.     Eu- 

cope,  Obelia. 
(6)  Hydroid  form  not  well  developed.    MqkioTea,  Rhegma- 

todes. 
(c)  Medusoid  form  degenerate.    Sertularia,  Halecium,  Aglao- 
phenia. 

5.  Order  AnthomedtiscB  or  Tubularice.    With  both  hydroid  and  me- 

dusoid forms,  the  latter  frequently  degenerate.    Hydroid  forms 
without  hydrotheesB  or  gonangia.    Medusoid  forms  with  eye- 
spots  ;  reproductive  organs  developed  in  wall  of  manubrium, 
(a)  Medusoid  form  well  developed.    Margelis,  Coryne,  Fen- 

naria. 
(6)  Medusoid  form  degenerate.    Clava,  Hydractinia,  Tubu- 
laria,  Endendrinm. 

6.  Order  Hydrocorallince. — Hydroid  forms  polymorphic ;  secreting 

calcareous  skeleton  by  ectoderm.    Medusoid  forms  usually  de- 
generate.   Millepora,  Stylaster. 

7.  Order  Siphouophora. — Free-swimming,  pelagic,  polymorphic  col- 

onies. 
(a)  Nectocalyces  present,  without  pneumatophore.    Diphyes. 
(6)  With  both  nectocalyces  and  pneumatophore.     Agalma. 

(c)  With  pneumatophore  only.     Caravella. 

(d)  Discoidal  forms  without  nectocalyces.     Velella,  Porpita. 
11.  Class  ScYPUOMEDUs^. — With  medusoid  form    only  in  adult   stage. 

Velum  not  present ;  sense-organs  are  modified  tentacles.     Repro- 
ductive organs  develop  in  endoderm. 

1.  Order  Stauromedusce. — With  the  eight  primary  tentacles  not  at 

all  or  but  slightly  modified.     Tessera,  Lucernaria. 

2.  Order  Peromedusce. — With  the  four  interradial  primary  tentacles 

transformed  into  sense-organs. 


TYPE  CaSLENTERA. 


117 


tentiiclcs 


four  radial   primary 
Charybdea. 
eight  primary  tentacles  trans- 


3.  Order  Cubomediisw. — Witli  the 

transformed  into  sense-organs. 

4.  Order   Dincoiiieihmt.  — With  all 

formed  into  sense-organs. 
(a)  Mouth-lobes  not  fused.     Cyanea,  Anrelia,  Pelagia. 
(6)  With  mouth-lobes  fused.     Utomolophns. 
III.  Class  A NTHOZOA.— Without  medusoid  forms.     Witli  ectodermal  stoma- 
todoeum  ;  ccelenteron  divided  into  chambers  by  vertical  mesenteries  ; 
reproductive  organs  developed  in  the  endoderm. 

1.  Order  ^%o»,aria.— Colonial  forms  with  eight  mesenteries  not 

arranged  in  pairs  ;  tentacles  pinnate. 

(a)  Without  axial  skeleton.     Renilla,  Alcyonium,  Pennatula. 
(6)  With  axial  skeleton.    Qorgonia,  Leptogorgia,  Isis,  Coral- 
Hum. 
(c)  With  tubular  calcareous  skeleton.     Tubipora. 

2.  Order  Edwardsm.—h\m}^\Q  forms  with  eight  mesenteries  not  ar- 

ranged in  pairs  ;  tentacles  simple.     Edwardsia. 

3.  Order  Ce/mwi/ica;.— Simple  forms  with  numerous  mesenteries 

not  arranged  in  pairs  ;  new  mesenteries  formed  on  each  side  of 
dorsal  mid-line.     Ceriauthus. 

4.  Order  Antipatharicc.—GoXomaX  forms  with  axial  horny  support ; 

with  six  simple  tentacles ;  mesenteries  not  arranged  in  pairs. 
Antipatharia. 

5.  Order  ProtaetinioE.—^\m\Ae  forms  with  twelve  primary  mesenter- 

ies arranged  in  pairs,  and  in  addition  one  unpaired  mesentery 
on  each  side,  or  one  or  two  pairs.  Scytophorus,  Gonactinia, 
Oractis. 

6.  Order  Zoanthece. — Simple  or  colonial  forms  with  twelve  primary 

mesenteries  arranged  in  pairs,  and  in  addition  a  varying  num- 
ber of  secondary  pairs  developed  on  each  side  of  the  primary 
pair  occupying  the  mid-ventral  line.     Zoanthus,  Palythoa. 

7.  Order  Hexactini(B.—^\m\)\G  or  colonial  forms  with  twelve  pri- 

mary mesenteries  arranged  in  pairs,  and  in  addition  a  variaVjle 
number  of  secondary  pairs  arranged  in  cycles,  the  newer  pairs 
developing  in  the  intervals  between  the  pairs  already  present. 
An  external  calcareous  skeleton  present  in  many  forms. 

(a)   Without  calcareous    skeleton    (Malacodennata).      Hal- 

campa,  Aiptasia,  Tealia,  Metridium,  Bunndes. 
(6)  With  H  calcareous  skeleton  (Sclerodermata).    Madrepora, 
Fungia,  Manicina^  Maandrin-a,  Astrangia. 


118 


INVEHTEBHATE  MORPHOLOGY. 


LITERATURE. 


A.   PoniKEKA. 

Voamaer.  Porifera.  Bronn's  Elassen  und  Ordnuugen  des  Thierreichs.  Leip- 
zig 11.  Heidelberg,  1887. 

£.  Haeokel.     Die  Kalkschwdmme.     Berlin,  1873. 

F.  E.  Schalze.  Untersuchungen  i'tber  Ban  und  Eiitwicklung  der  Spongien. 
Zeitschr.  fl\r  wissensch.  Zoolofjie,  xxv-xxxv.  1876-81. 

A.  Dendy.     Observations  on  the  /Structure  and  Glasinjicati'm  of  the  Calcarea 

heterucuila.     Quarterly  Journal  of  Microsc.  Science,  xxxv.  lo93. 

B.  Ton  Lendenfeld.     A  Monograph  of  the  Horny  Sponges.     London,  1889. 

F.  E.  Schulze.  Report  on  the  Hexactinellidm.  Reports  on  the  Scientific  Results 
of  the  Voyage  of  H.M.S.  Challenger.     Zoology,  xxi.  1887. 


B.  Cnidaria. 


£. 
0. 


6. 


GENEUAIi. 

Agassiz.     Contributions  to  the  Natural  History  of  the  United  States.    Vols. 

HI  and  IV.     Boston,  1860-63. 
Agassiz.     North  American  Acalephm.    Illustr.  Catalogue  of  the  Museum  of 

Coiup.  Zoology,  II.  Cambridge,  Mass.,  1865. 
Hetsclinikoff.     Embryologische  Studien  an  Medusen.     Wien,  1886. 
ft  R.  Hertwig.    Has  Nervensystem  und  die  Sinnesorgane  der  Medusen. 

Leipzig,  1878. 


HYDnOMEDUS^. 


Ray  Society. 


W 


J.  Allman.     A   Monograph  of  the  Gymnoblastic  Hydroids, 

London,  1871-72. 
Haeokel.     System  der  Medusen.    1.  Craspedoten.    Jena,  1879. 
N.  Moseley.     Report  on  certain  Hydroid,  Alcyonanan,  and  Madreporarian 
Corals.     Reports  of  the  Scientific  Results  of  the  Voyage  of  H.M.S.  Chal- 
lenger.    Zoology,  II.  1881. 

K.  Brooks.     The  Life-history  of  the  Hydromedusce.     Memoirs  Boston  Soc. 
III.  1886. 
Die  Entstehung  der  Sexuahellen  hei  den  Hydromedusen.    Jena, 


The  Structure  of  Cunoetantha  oetonaria  in  the  Adult  and  Lar- 
Studies  from  the  Biol.  Laboratory  Johns  Hopkins  University, 


Nat.  Hist., 

A.  Weismann. 
1883. 

H.  V.  Wilson. 
tal  Stages. 
IV.  1886. 

E.  Haeokel.     Report  on  the  Siphonopliortp.    Reports  of  the  Scientific  Results  of 
the  Voyage  of  H.M.S.  Challenger.     Zoology,  xxviii.  1889. 

A.  Agassiz.     The  Porpitidai  and  Velellidm  of  the  Ouif  Stream.    Memoirs  Mu- 
seum of  Comp.  Zoology,  viii.  1883. 


SCYPIIOMEDUBiB. 

£.  Haeokel.    System  der  Medusen.    IL  Acrnspeda.    Jena,  1880. 


TYPE  CCELENTERA. 


119 


ANTHOZOA. 

0.   von  Koch.     Die  Oorgoniden.    Fauua    u.   Flora  des  Qolfes  von  Neapel 
Monogr.  xv.  1887.  ^ 

A.  von  KflUiker.    Die  Pennatulidm.     Abbandl.  Senckenburg.  Nat  Oesellscli 
vii  and  VIII.  1872.  »i.  ueseu&cu., 

G.  Brook      Jteporton  the  Antipathnria.     Reports  of  the  Scientific  Results  of 
the  Voyage  of  II.M.S.  Challenger.     Zoology,  xxxii.  1889. 

A.  Andres.     Le  Attinie.     Fauna  und  Flora  des  Golfes  von  Neapel.     Monogr 
IX.  188o. 

0.  ft  E.  Hertwig.     Die  Actimcn  anatomisch  und  hiatologiscJi  untersncht     Jena 

ischeZeitschr.,  XIV.  1879. 
J.  P.  McMurrioh.     The  Phylogeuy  of  tlie  Anthozoa.    Journal  of  Morphology,  v, 


120 


INVERTEBRATE  MORPHOLOGY, 


CHAPTER  VI. 


THE  CTENOPHORA. 


The  group  of  forms  known  as  the  Gtenophora,  to  -which 
the  systematic  value  of  a  class  may  be  given,  present  no  little 
general  resemblance  to  the  Ccelentera,  but  at  the  same  time 
depart  so  widely  in  structural  and  histological  characters 
from  the  Cuidaria  and  Porifera  that  it  seems  advisable, 
until  further  evidence  is  forthcoming,  to  consider  them  as  a 
group  apart. 

All  the  Ctenophores  are  pelagic  and  are  of  great  transpa- 
rency and  delicacy,  due  to  the  nature  of  the  mesogloeal  tissue. 


mo 


Pig.  66. — Bolina  hydatina  (after  Chun). 

cp  =  ciliated  plates.  te  =  funnel-canal. 

g  =  stomach  or  funnel. 

I  =  lobe. 


mc  =  stomodseal  canal. 


In  form  they  vary  greatly,  some  being  almost  spherical  or 
pyriform  {PleurobracMa),  sometimes  with  broad  lobes  project- 
ing from  near  the  oral  extremity  (Fig.  66,  J)  {Bolina,  Mnemi' 


THE  CTENOPUOHA. 


121 


opsis),  others  being  ribbon-like,  as  Cestum,  the  Venus'  girdle, 
others  sac-like,  as  Bero'e  and  Idyia.  Indications  of  a  radial 
symmetry  are  seen  in  the  eight  longitudinal  bands  of  •cilia- 
plates  (cp)  which  serve  as  locomotor  organs,  but  this  is  thrown 
into  the  background  by  the  more  prououueetl  bilaterality. 
The  stomodaBum  is  flattened  in  one  plane  and  the  gastric 
cavity  ((/)  in  a  plane  at  right  angles  to  this,  two  tentacles,  one 
on  each  side  of  the  body,  lying  also  in  this  plane.  It  is  possi- 
ble accordingly  to  recognize  a  sagittal  plane,  that  of  the 
stomodaium,  and  a  transverse  plane,  that  of  the  gastric  cavity, 
and  corresponding  axes. 

The  mouth  lies  at  the  extremity  of  the  vertical  axis  which 
is  directed  backwards  in  locomotion,  and  opens  into  the  ecto- 
dermal stomodtBum,  which  is  flattened  parallel  to  the  sagittal 
plane.  At  its  upper  end  the  stomodteum  opens  into  the  en- 
dodermal  gastric  cavity  {g)  or  so-called  "  funnel,"  from  which 
live  canals  arise  ;  one  of  these  {tc)  passes  directly  upwards  to 
the  aboral  surface,  where  it  branches  and  opens  to  the  exterior 
by  usually  two  openings  ;  two  others  pass  downwards  parallel 
with  the  broad  surface  of  the  stomodaeum  {mc)  and  end 
blindly  ;  while  the  other  two  (Fig.  67,  re)  pass  directly  out- 
wards in  the  transverse  axis  of  the  body  and  end  at  the  base 
of  the  tentacles,  a  short  distance  before  their  termination 
giving  off  two  branches,  one  on  each  side.  These  branches 
soon  divide  and  give  rise  each  to  two  canals  which  commu- 
nicate peripherally  with  canals  running  longitudinally  below 
the  rows  of  cilia-plates. 

These  plates,  which  constitute  the  locomotor  organs,  are 
arranged  one  above  the  other  and  are  composed  of  fused  cilia 
arising  from  ectodermal  thickenings.  There  are  eight  merid- 
ional rows  of  these  ciliated  plates,  and  from  the  upper  end  of 
each  row  a  delicate  groove  lined  with  ciliated  cells  extends 
towards  the  aboral  pole,  each  groove  uniting  with  an  adjacent 
one  as  they  approach  the  pole,  so  that  their  number  is 
reduced  to  four  (Fig.  67,  eg).  These  pass  in  upon  the  floor  of 
a  dome- shaped  cavity  enclosed  by  fused  cilia  which  arch 
together  at  the  centre  but  do  not  quite  meet.  The  cavity  thus 
enclosed  is  somewhat  broader  in  the  sagittal  than  in  the 
transverse  axis  and  contains  the  aboral  sense-organ.     The  floor 


122 


INVERTEBHATE  MOHPIIOLOOY. 


of  the  cavity  is  formed  of  high  ciliated  cells  probabl}'  nervous 
in  function,  and  above  them  is  a  mass  of  otoliths  supported 
on  four  incurved  rods  of  fused  cilia,  one  of  which  forms  the 
termination  of  each  of  the  four  meridional  grooves. 

In  addition  to  this  sense-organ,  which  is  to  be  regarded 
as  of  the  same  function  as  the  otocysts  of  the  meduseo,  there 


Fig.  &7.—Pleurobrachia  seen  fok  the  Abokal  Pole  (after  Aoabsiz). 
eg  =  ciliated  groove.  p  =  polar  urea. 

ot  =  otocyst.  re  =  radial  caual. 

t  =  tentacle. 

lies  at  each  end  of  the  sagittal  axis  of  the  sensory  dome  a  so- 
called  pole-area  (Fig.  67,  p),  the  cells  of  which  are  furnished 
with  small  plates  of  fused  cilia,  each  area  being  surrounded 
by  a  thickened  ciliated  rim.  These  structures  are  from  their 
form  and  situation  supposed  to  be  sensory,  and  an  olfactory 
function  has  been  attriluited  to  them. 

The  tentacles  (Fig.  67,  i),  of  which  there  are  two,  situated 
at  the  extremities  of  Ihj  transverse  axis,  are  present  in  all 
forms  except  the  Beroids.  Each  tentacle  lies  in  a  deep 
depression  termed  the  tentacle-sheath  and  consists  of  a  princi- 
pal axis  which  gives  rise  to  a  large  number  of  secondary 
tentacles  arranged  upon  one  side  only.  Both  the  primary 
axis  and  the  secondary  tentacles  are  solid,  being  composed 
mainly  of  muscle-cells  and  containing  no  prolongation  of  the 
tentacular  vessel.     In  Mnemiopsis  and  its  allies  and  in  Cestum 


( 


TUE  CTENOPIIOHA. 


123 


Fia.  OH.— Adiiksivk  Ckm-s 

FUOM     TkNTACLK.    iiV    A 
CtenoI'IIOuk  (after  Chun). 
c  =  mllu'sive  cup. 
m  =  coutructilc  stalk 


the  primary  axis  is  practically  wanting,  the  secondary  tentacles 
arising  directly  from  the  bottom  of 
the  tentacle-sheath.  The  ectoderm  of 
the  secondary  tentacles  contains  nu- 
merous cells  supposed  to  be  sensory, 
and  also  so-called  adhesive  cells,  which 
in  this  group  replace  the  nematocysts. 
They  consist  of  a  slender  spirally- 
coiled  muscular  fibre  (Fig.  <)8,  m)  at- 
tached at  one  extremity  to  the  subja- 
cent tissue  and  terminating  at  the 
other  on  the  under  surface  of  a  hemi- 
spherical cap  (c),  whose  surface  is 
covered  by  small  spherical  masses  of 
a  sticky  secretion.  A  small  animal 
coming  into  contact  with   these   caps 

is  held  by  the  adhesive  secretion,  the  muscle-fibre  being  suf- 
ficiently elastic  to  yield  to  the  struggles  of  the  victim  and  t( » 
bring  it  in  contact  with  the  general  ectoderm  by  contracting 
when  its  struggles  cease. 

The  reproductive  organs  lie  in  the  outer  walls  of  the 
canals  which  lie  beneath  the  meridional  rows  of  plates,  but 
apparently  are  originally  derived  from  the  ectoderm.  All  the 
Oteuophores  are  hermaphrodites,  the  ova  being  arranged  oi. 
one  side  of  each  canal  and  the  spermatozoa  on  the  other,  in 
such  a  manner  that  the  adjacent  sides  of  any  two  canuis  bear 
the  same  kind  of  sexual  cells.  A  peculiar  phenomenon 
termed  Dissogony  has  been  observed  in  certain  forms,  consist- 
ing of  the  occurrence  of  two  periods  of  sexual  maturity  in  the 
life-history  of  the  individual,  the  reproductive  organs  ripening 
first  while  it  is  still  in  a  larval  stage  and  again  when  it  has 
reached  its  adult  form. 

The  main  bulk  of  the  body  of  a  Ctenophore  is  made  up  by 
a  gelatinous  tissue  intervening  between  the  endoderm  and 
ectoderm  and  which  may  be  termed  the  mesogloea,  though  it 
is  not  improbable  that  its  cellular  elements  are  in  great  part 
derived  from  embryonic  cells  corresponding  to  the  mesoderm- 
cells  of  higher  forms.  It  consists  of  a  gelatinous  matrix 
through  which  are  scattered  branched  cells  and  fibres.     Some 


124 


IN  VEliTEBHA  TE  MOIWUOL  OU  Y. 


of  the  latter  extend  throughout  the  entire  thickness  of  the 
mesogloea  and  are  inserted  by  their  branched  extremities 
into  tlie  ectoderm  on  the  one  side  and  the  endoderm  or 
stoniodjcal  ectoderm  on  the  other.  The^  are  contractile  in 
f  auction,  consisting  of  a  central  protoplasmic  axis  containing 
a  nucleus  and  of  a  peripheral  contractile  substance.  In  addi- 
tion to  these  there  are  other  much  finer  fibres  which  have 
been  supposed  to  be  nervous,  and  on  the  outer  surface  of  the 
mesoglcea,  between  it  and  the  bases  of  the  ectoderm-cells,  is 
a  network  of  stellate  ganglion-cells  whose  processes  overlap 
but  do  not  unite  with  each  other.  They  are  especially  abun- 
dant in  the  region  of  the  meridional  rows  of  plates ;  just  as 
the  slender  fibres  of  the  mesoglcea  are  especially  abundant 
below  the  aboral  sense-organ  and  the  meridional  grooves. 
Further  information  is,  however,  required  as  to  the  nervous 
system  of  the  Ctenophores. 

The  class  may  be  divided  into  two  orders : 


1.  Order  Tentaculata. 

The  members  of  this  order  possess  tentacles  either  with 
or  without  the  primary  axis.  The  simple  forms,  such  as 
Pleurohrachia,  belong  to  this  order,  as  well  as  the  lobate  and 
ribbon-shaped  forms.  In  the  lobate  forms,  such  as  Bolina 
(Fig.  66)  and  Mnemiopsis,  there  is  at  each  end  of  the  sagittal 
axis  a  large  lobe  developed  into  which  four  of  the  meridional 
canals  are  continued ;  two  of  the  canals,  those  nearest  the  ex- 
tremities of  the  transverse  axis,  pass  around  the  edge  of  the 
lobe  and  unite  with  each  other,  while  the  other  two,  which 
also  unite,  are  thrown  into  arabesque-like  twistings.  The 
Venus'-girdle,  Cestum,  is  ribbon-shaped,  being  flattened  in  the 
transverse  plane  and  much  drawn  out  in  the  sagittal  ])laue ; 
the  result  being  the  great  extension  of  four  of  the  meridional 
plate-rows  and  the  almost  complete  disappearance  of  the 
other  four.  In  its  young  stages,  however,  Cestum  is  a  spher- 
ical form  closely  resembling  the  simple  genus  Mertensia. 


THE  CTENOPIIOUA. 


125 


2.  Order  EurystomesB. 

This  order,  which  includes  the  Beroid  forms,  is  character- 
ized  by  the  entire  absence  of  tentacles  and  by  the  wide  bell- 
like stomodieuni.  The  meridional  canals  send  oil'  ahm^ 
their  course  numerous  branchiuj^  processes  into  the  mesogla'a 
and  are  united  around  the  mouth  by  a  circular  canal.  To 
this  order  belongs  the  Mediterranean  genus  JJero'e,  and  the 
genus  /dyia  of  the  northwest  Atlantic. 

Relationships  of  the  Ctenophora. — The  Ctenophores  have  been  by  luoisl 
jiiitliors  assigned  to  the  type  Coelentera,  ou  account  of  their  jelly-like  con- 
sistency and  the  presence  of  gastro-vascular  canals,  and  of  indications  of  a 
radiate  symmetry.  It  is  possible,  however,  that  these  characters  are  sin) 
ply  superficial,  and  that  the  group  possesses  but  very  remote  affinities  to 
the  Ccelenterates.  They  have  furthermore  been  regarded  by  some  as 
connecting  links  between  the  Coilenterates  and  the  Turbellarians,  and  in 
connection  with  this  idea  two  aberrant  forms  may  be  briefly  described. 
One,  Ctenoplana,  is  a  flattened  form,  on  the  middle  of  whose  dorsal  surface 
lies  the  otolith  sac,  and  at  a  short  distance  from  this  are  eight  short  rows  of 
cilia-plates,  each  in  a  slight  depression.  Twotertaeles  lie  in  the  transverse 
axis,  and  the  mouth  is  situated  at  the  centre  of  ihe  lower  surface  and  leads 
into  a  cavity  from  which  numerous  branching  gastric  pouches  arise  with- 
out any  definite  arrangement.  The  other  form,  Cirloplana,  is  also  flat- 
tened and  creeping  ;  the  mouth  lies  on  the  under  surface  and  opens  into  a 
wide  cavity,  which,  as  in  Ctenoplana,  gives  origin  to  a  number  of  pouches 
which  branch  and  give  rise  to  a  network  towards  the  periphery  of  the 
body  ;  a  canal  passes  from  the  gastric  cavity  towards  the  dorsal  surface  of 
the  body,  where  it  divides  into  two  branches  which  end  blindly,  and  lying 
between  them  is  a  vesicle  containing  otoliths  ;  two  tentacles  similar  to 
those  of  Pleurobracfria  lie  in  the  transverse  axis.  In  both  tiiese  forms  llio 
general  s.'rface  of  the  body  is  ciliated,  and  they  seem  to  represent  inter- 
mediate forms  between  the  Ctenophores  and  Turbellaria,  Ctenoplana  being 
more  closely  allied  to  the  former  and  Cveloplaua  to  the  latter. 

There  are  various  important  structural  diff'erences,  however,  between 
the  Coelentera  and  the  Ctenophores.  Among  these  may  be  mentioned 
the  structure  and  position  of  the  sense-organ,  the  structure  and  position  of 
the  mesogla'al  muscle-fibres,  the  structure  of  the  tentacles,  the  presence  of 
the  adhesive  cells  which  cannot  possibly  be  homologized  with  nematocyst- 
cells,  and  finally  the  early  differentiation  in  the  embryo  of  cells,  resembling 
the  mesoderm-cells  of  triploblasfic  animals,  which  give  rise  to  the  muscles  of 
the  tentacles  and  perhivps  to  some  of  the  mosoglceal  elements. 

It  seems  not  iiaprobablc  Uiat  the  affinities  of  the  Ctenophores  would  be 
more  accurately  indicated  in  the  classification  if  they  were  entirely  removed 


126 


il 


INVERTEBRATE  MORPHOLOGY. 


from  the  Ooelentera  and  associated  with  the  Turbellaria,  being  regarded 
as  highly  modified  forms,  adapted  for  pelagic  life,  descended  from  Tur- 
bellarian  ancestors.  The  evidence  which  has  been  brought  forward  in 
favor  of  a  relationship  of  the  Turbellaria  to  the  Ccelentera  through  the 
Ctenophores  would  support  this  view  as  well  as  that  it  was  intended  to 
support,  and  to  this  may  be  added  the  fact  that  while  the  peculiar  adhesive 
cells  of  the  Ctenophores  cannot  be  h..^ologized  with  any  of  the  histological 
elements  of  the  Cnidaria,  they  may  readily  have  been  evolved  from  the 
adhesive  cells  which  occur  in  the  ectoderm  of  many  Turbellarians. 

SUBKINGDOM  METAZOA. 

Class  CTENOPHORA.-Pelagic  organisms  provided  with  eight  meridional  rows 
of  plates  formed  by  the  fusion  of  cilia. 

1.  Order  2'e/*^ac..Z«te.-ctenophora  provided  with  tentacles 

(a)  WiUiout  lobes ;  more  or  less  oval  in  shape.     Pleurohraohia, 
Mertensia.  ' 

(6)  Lateral  lobes  occurring  at  oral  pole.     Bolina,  MnemicpJs. 
(c)  Ribbon-hke  form.     Cestum 

2.  Order  ^/.ry.v^o,„,^._without    tentacles;    stomod^um  w-dt 

bell-hke.    Beroi,  Idyia. 


and 


LITERATURE. 

B.Hertwig      Ueber  den  Bander  Ctenophorev.     Jenaiscbe  Zeitschr..  xiv   1880 

C.  Chun,     me  Ctenc^horen  drs  Oolfes  .on  Neapel.    Fauna  and  Flora  des  Uolfe; 

von  Neapel.     Monogr.  i.  1880.  uoues 


ft 


TYPE  PLATYUELMINTHES. 


127 


CHAPTER  VII. 

TYPE  PLATYHELMINTHES. 

The  Platyhelminths  constitute  a  gioup  which,  thoiigli 
presenting  a  much  higher  grade  of  organization  than  the 
Coelentera,  nevertheless  show  certain  general  structural 
similarities  to  the  representatives  of  that  tjpe.  Thus  upon 
the  exterior  of  the  body  tliere  is  a  thin  ectoderm  (Fig.  69, 


bm  vd  'D 

Fig.  69. -Diagrammatic  Transverse  Sections  through  variotts  Turbei. 
LAP.iA.     A,  a»   Actt'Ian  ;  B,  uu   Alloioca>Ian ;    C,  a  Rlmbdoca'lau  ;    IJ,  a 


Tiiclad. 


bm  =  busemei)t-membiane. 
d  =  iutesliue. 
ec  =  ectoderni. 
m  =  muscle- layer. 
n  =  uerve. 


vd  =  vas  deferens. 


0  =  ovary. 

od  =  oviduct. 
p  =  parciichyme. 

t  =  testis 

V  =  vilellariuin. 


A  ec),  below  which  is  a  hasement- membrane  (hm)  sometimes 
thin,  structureless,  and  destitute  of  cells,  sometimes  thicker 
and  enclosing  branched  cells,  and  strictlv  comparable  to  the 
mesogloea  of  the  Ccelenterates.  Within  the  basement-mem- 
brane  there  is  a  compact  mass  of  tissue  surrounding,  in  the 


128 


INVERTEBRATE  MORPHOLOGY. 


majority  of  forms,  a  cavity,  the  eiiteron  {d\  the  cells  lining 
the  walls  of  this  being  differentiated  into  a  digestive  epithe- 
lium or  endoderm.  The  space  between  the  euteron  and  the 
busement-membraue  is  occupied  by  the  mesoderm,  consisting 
peripherally  of  compact  layers  of  circular  and  longitudinal 
muscle-fibres  (m),  while  below  these  it  forms  a  mass  of  nu- 
cleated cells,  usually  vacuolated  so  as  to  resemble  a  network 
of  fibres  enclosing  spaces  and  constituting  the  parenchyma  (p). 
It  is  traversed  by  dorso-ventral  muscle-fibres  and  has  im- 
bedded in  it  various  organs  most  of  which  are  further  dif- 
ferentiations of  this  middle  germ-layer.  These  two  layers, 
the  endoderm  and  mesoderm,  are  together  comparable  with 
the  inner  layer  of  the  Coelenterates,  the  mes-endoderm,  and 
when  the  fnteron  exists  it  communicates  with  the  exterior,  as 
in  that  grc.  i  'a.  single  opening,  the  mouth,  the  Nemer- 
teans  only,  th  ost  highly  organized  class  of  the  Platy hel- 
minths, possessing  a  second  opening,  the  anus. 

These  homologies  are,  however,  associated  with  a  com- 
plexity of  organization  unrepresented  in  the  Coelenterates. 
The  Platyhelminths  all  present  a  typical  bilaterality  of  form, 
and  show  furthermore  a  well-marked  antero-posterior  as  well 
as,  in  most  cases,  a  dorso-ventral  differentiation.  The  body  is 
usually  flattened  and  more  or  less  vermiform,  whence  the  name 
of  the  group,  and  is  adapted  to  a  creeping  habit,  certain  para- 
sitic forms,  and  some  Nemerteans  which  live  buried  in  sand, 
being  the  only  forms  not  presenting  such  a  mode  of  life. 

The  greatest  contrast  to  what  occurs  in  the  Coelenterates 
however,  is  presented  by  the  development  of  compact  organs. 
The  nervous  system  is  no  longer  an  altogether  diffuse  tissue, 
scattered  in  a  thin  layer  throughout  the  body,  but  a  large 
number  of  ganglion-cells  are  aggregated  into  a  compact  mass, 
the  brain,  embedded  in  the  mesoderm  parenchyma  near  the 
anterior  end  of  the  body,  and  from  this  there  pass  backwards 
two  or  more  longitudinal  cords  of  nerve-fibres  which  give  otf 
branches  extending  to  all  parts  of  the  body  and  forming  a 
network  below  the  basement-membrane  from  which  the  pe- 
ripheral muscles  derive  their  nerve-supply.  In  some  cases 
nerves  have  been  observed  to  pass  from  this  network  through 
the  basement-membrane  to  come  into  connection  apparently 


TYPE  PLATYHELMINTHES. 


129 


with  nerve-cells  lying  between  the  inner  ends  of  the  ectoderm- 
cells  as  well  as  with  sensory  cells  resembling  in  general  form 
those  already  described  as  occurring  in  the  Cnidaria.  It  must 
not  be  understood,  however,  that  the  ganglion-cells  are  limited 
in  their  distribution  to  the  lower  layer  of  the  ectoderm  and 
the  brain ;  on  the  other  hand,  they  are  scattered  along  the 
nerve-cords  which  arise  from  the  brain,  the  Platyhelminths 
presenting  in  the  structure  of  their  nervous  system  a  condi- 
tion intermediate  between  the  diffuse  arrangement  of  the 
ganglion-cells  seen  in  the  Cnidaria  and  the  more  perfect  ag- 
gregation occurring  in  higher  types. 

An  excretory  system  of  branching  tubes  traversing  the 
mesoderm  parenchyma  and  opening  to  the  exterior  is  also 
present.  It  consists  usually  of  two  main  tubes,  nephridia, 
from  Avhich  numerous  branches  arise,  terminating  in  blind 
funnel-like  extremities  (Fig.  70,  f)  lying  in  the  meshes  of  the 
parenchyma.  Each  funnd  (Fig. 
78,  B)  is  closed  by  a  single  cell 
(<c),  from  which  there  projects 
into  the  tube  a  bundle  of  cilia 
{Jl\  and  which,  from  the  resem- 
blance of  the  motion  of  these  cilia 
to  a  flame  flickering  in  the  wind,  is 
known  as  a  flame-cell.  The  larger 
tubes  are  lined  by  a  layer  of 
cells  which  seem,  in  certain  cases 
at   least,  to   be    ciliated,   but  the 

smaller     branches    consist    of    a-,       „„     ^ 

„  Fig.  70— Excuktohv  System   of 

ot    cells    succeeding    one     the  Antekiou  Poktion  op  the 


series 

another  in  a  single  row,  the  canal 


Body  op  Planaria  montana  (after 


running  through   the  centres  of    chichkoff). 
the  cells  and   being   thus   intra-  ~  "  ~   y 

cellular.  The  tubes  throughout  the  entire  system  contain 
a  fluid  in  which  particles  resembling  guanin  in  their  behavior 
to  reagents  have  been  seen,  and  there  is  little  room  for  doubt 
but  that  the  tubes  have  an  excretory  function. 

Finally,  a  complicated  reproductive  apparatus  (see  Figs. 
68-70)  is  present,  the  Platyhelminths  being  for  the  most  part 
hermaphrodite.     The   testes  consist  of  from   two   to   many 


130 


INYERTEBRATE  MORPHOLOGY. 


globular  bodies  Avliose  ducts  unite  to  form  two  vasa  deferentia 
opening  to  the  exterior  through  a  muscular  introraittent  organ, 
and  sometimes  dilating  to  form  reservoirs,  the  seminal  vesicles, 
in  which  spermatozoa  may  be  stored  up  until  required  for 
fertilization.  The  female  apparatus  is  somewhat  more  com- 
plicated. The  ovaries  are  usually  two  in  number  and  their 
products  pass  to  the  exterior  through  special  tubes,  the  ovi- 
ducts, which  may  be  exceedingly  long  and  with  the  terminal 
portion  dilated  to  form  a  uterus  in  which  the  ova  may  pass 
through  certain  stages  of  their  development.  Connected  with 
the  oviducts  there  is  usually  a  pouch-like  structure,  the  semi- 
nal receptacle,  for  the  reception  of  spermatozoa,  and  further- 
more they  may  receive  the  products  of  two  other  glands 
which  supply  the  yolk  and  the  shell  for  the  ova.  The  yolk- 
glands  are  in  some  cases  very  voluminous,  forming  what  is 
termed  the  vitellarium,  and  have  been  apparently  developed 
by  the  separation  of  a  portion  of  the  original  ovary,  their 
cells,  wliich  manufacture  the  yolk  material,  being  accordingly 
equivalent  to  germ-cells.  The  evidence  for  this  supposition 
is  derived  from  the  arrangement  found  in  some  Turhellaria 
and  will  be  pointed  out,  together  with  the  variations  which 
the  complex  of  organs  presents,  in  the  descriptions  of  the 
various  groups. 


I.  Class  Tubbellabia. 

The  Turhellaria  derive  their  name  from  the  fact  that  the 
ectoderm  is  furnished  with  cilia,  which  form  the  locomotor 
organs  of  the  animals,  whose  gliding  motion  over  the  sur- 
face of  the  objects  among  which  they  live  is  very  charac- 
teristic. The  majority  of  the  members  of  the  class  lead  a 
free  life,  some  in  fresh  and  some  in  salt  water,  and  some  even 
on  land,  creeping  about  on  the  under  surfaces  of  stones  or 
weeds.  A  few,  however,  are  parasitic  either  upon  the  outside 
of  the  bodies  of  their  hosts  (BdeUura)  or  in  a  few  cases  living 
in  the  body-cavity  or  even  being  imbedded  in  the  tissues. 

In  addition  to  the  ordinary  ciliated  cells  the  ectoderm  con- 
tains numerous  sensory  as  well  as  gland  cells.  Special 
glands  secrete  in  most  of  the  groups  peculiar  rod-like  bodies 


TYPE  PLATriJELMINTHES. 


131 


which  lie  scattered  about  iu  the  ectoderm  betweeu  its  coiiipo- 
neut  cells  or  may  jtroject  more  or  less  beyond  its  surface. 
These  rhabdites,  as  they  are  termed,  are  produced  as  a  secre- 
tion by  cells  lying  usually  in  the  mesoderm  and  connected 
with  the  exterior  by  a  slender  neck  passing  through  the  base- 
ment-membrane, the  rhabdites  thus  making  their  way  to  the 
exterior.  The  rhabdite-cells  are  ectodermal,  their  position  in 
the  mesoderm  being  quite  secondary,  and  in  fact  iu  one  group 
they  are  confined  to  the  ectodermal  layer.  The  function  and 
nature  of  the  rhabdites  have  been  variously  interpreted,  some 
authors  considering  them  equivalent  to  the  Cnidariau  nemato- 
cysts,  but  it  seems  more  probable  that  they  are  the  condensed 
secretion  of  cells  which  originally  produced  a  mucous  sub- 
stance and  by  slowly  dissolving  in  water  produce  a  viscid 
slime  of  sufficient  tenacity  to  retain  organisms  coming  in  con- 
tact with  it. 

In  addition  to  these  structures  many  forms  possess  adhe- 
sive cells,  columnar  cells  which  produce  a  strongly  adhesive 
secretion  which  is  poured  out  in  drops  upon  the  free  ex- 
tremity of  the  cell,  recalling  in  this  respect  the  adhesive  cells 
of  the  Ctenophores.  These  cells  seem  to  be  of  use  mainly  in 
enabling  the  worms  to  adhere  to  the  surface  on  which  they 
are  creeping,  and  are  especially  developed  towards  the  hinder 
end  of  the  body.  Another  organ  of  adhesion  in  the  form  of  a 
muscular  sucker,  situated  usually  about  the  middle  of  the 
ventral  surface,  is  present  in  certain  marine  Turbellaria,  but 
the  majority  of  the  members  of  the  group  lack  such  struc- 
tures. 

The  nervous  system  consists  of  a  brain  from  which  a  num- 
ber of  nerve-cords  arise,  varying  somewhat  in  their  arrange- 
ment in  the  different  orders.  Sense-organs  of  one  kind  or 
another  are  usually  present  in  addition  to  the  widely-distrib- 
uted sensory  cells  of  the  ectoderm.  A  large  number  of  forms 
possess  eyes,  which  in  some  Polyclads  may  be  exceedingly 
numerous,  and  usually  consist  of  a  patch  of  pigment  lying  in 
the  mesoderm  and  upon  which  a  refractive  lens-like  structure 
lies.  In  a  few  cases,  as  in  Microstoma,  the  eye  is  simply  a  patch 
of  pigment  in  the  ectoderm  near  the  anterior  end  of  the  body. 
An  otocyst,  consisting  of  a  spherical  vesicle  filled  with  fluid 


132 


INVERTEBRATE  MORPHOLOGY. 


and  containing  an  otolith  of  carbonate  of  lime,  is  present  in 
some  of  the  lower  Turbellaria,  as  Monotus^  and  rests  directly 
upon  the  surface  of  the  brain ;  these  structures  probably,  as 
in  the  Cnidaria,  are  sense-organs  of  equilibrium  rather  than 
of  aiulition.  In  the  Polyclads  tentacles  are  frequently  pres- 
ent, sometimes  capable  of  being  retracted  and  serving  as 
organs  of  touch,  and  in  certain  Rhabdoccels  there  is  a  ciliated 
depression  on  each  side  of  the  head  richly  supplied  with 
nerves  forming  what  has  been  considered  an  olfactory  organ. 


1.  Order  Acoela. 

The  Accela  form  a  group  of  lowly-organized  Turbellaria 
exclusively  marine  in  habitat  and  leading  an  active  and  free 
existence.  They  all  possess  a  mouth  (Fig.  71,  m)  situated  on 
the  ventral  surface  and  leading  into  a  short  pharynx,  though 
in  some  forms  this  may  be  absent ;  but  beyond  this  there  is 
no  trace  of  a  digestive  tract,  the  food  passing  from  the 
pharynx  into  the  parenchyma  {p),  where  it  is  digested.  Ow- 
ing to  the  lack  of  a  digestive  tract  these  forms  are  strictly 
two-layered  (Fig.  69,  A\  only  the  ectoderm  and  mes-endoderm 
being  represented,  and  consequently  are  exceedingly  interest- 
ing as  indicating  the  manner  in  which  the  differentiation  of 
the  triploblastic  condition  has  been  derived  from  the  diplo- 
blastic. 

The  nervous  system  has  been  described  in  Convoluta  as 
consisting  of  a  bilobed  ganglion  surrounding  the  otocyst,  and 
in  front  of  this  and  united  to  it  by  commissures  is  a  second 
pair  of  ganglia.  From  the  anterior  ganglia  there  arise  by  a 
common  stem  two  nerves  on  each  side  which  pass  backwards, 
one  along  the  edge  of  the  body  and  the  other  a  little  internal 
to  it,  while  the  posterior  ganglionic  mass  gives  rise  to  two 
nerves  which  pass  backwards,  one  on  each  side  of  the  median 
line.  All  six  nerves  send  off  numerous  transverse  branches 
which  unite  to  form  with  the  nerve-cords  a  square-meshed 
network.  In  addition  to  the  single  otocyst  (Fig.  71,  ot)  two 
pigment-spots  lying  in  the  ectoderm  and  representing  light- 
percipient  organs  (e)  are  present,  as  well  as  a  peculiar  refrac- 
tive highly-movable  organ,  lying  in  the  median  line  on  the 


TYPE  PL  A  TYIIELMINTIIES. 


133 


anterior  margin  of  the  body,  which  is  supposed  to  be  tactile 
in  function. 

No  excretory  apparatus  has  as  jet  been  described  for  the 
Acoela,  but  a  reproductive  system 
with  some  interesting  peculiarities 
occurs.  The  male  apparatus  consists 
of  numerous  spherical  testes  (/)  whose 
ducts  unite  to  two  vasa  deferentia, 
dilating  below  to  form  the  seminal 
vesicles  {vs)  and  uniting  in  the  mus- 
cular intromittent  organ.  The  female 
organ  is,  however,  relatively  simple, 
consisting  of  two  club-shaped  ovaries 
(ov)  whose  short  oviducts  opeu  almost 
directly  to  the  exterior  near  the  pos- 
terior end  of  the  body  by  a  pore  (c^^  9 ) 
common  to  both  male  and  female  ap- 
paratus. There  is  no  vitellarium,  no 
shell-gland,  no  seminal  receptacle, 
and  no  special  uterus,  a  state  of 
affairs  indicating  great  simplicity  of 
structure  compared  with  what  is 
found  in  the  other  orders. 


2.  Order  AUoiocoela. 


The  members  of  this  order  are 
marine  with  the  single  exception  of 
Plagiostoma  lemani,  which  is  found  in 
the  deep  waters  of  the  Swiss  lakes. 
They  present  a  distinct  advance  upon 
the  Acoela  in  that  a  well-defined  diges- 
tive tract  is  present  (Fig.  69,  B),  the 
interval  between  it  and  the  peripheral 
musculature  being  completely  filled  up  by  the  usual  paren- 
chyma and  the  organs  imbedded  in  it.  These  forms  are  then 
triploblastic,  possessing  well-defined  ectoderm,  mesoderm,  and 
endoderm,  a  condition  found  in  all  the  higher  orders. 

The  mouth  varies  somewhat  in  position,  lying  either  near 


Fig.    71.— Diagham   op   an 

AcffiLOUS      TUKUELLAUIAN 

(after  von  Graff). 

e  =  eye. 
m  =  Dioutb. 
ot  =  otocyst. 
ov  =  ovuiy. 
p  =  parenchyma. 
t  =  testis. 

vs  =  vesicula  seminalis. 
5  $   =  reproductive  orifice. 


134 


IN  VEHTEBJtA  TE  MOHPJIOLOG  Y. 


the  anterior  or  the  posterior  eud  of  the  body,  and  opens  into 
a  pharyngecd  pouch,  whose  walls  are  thickened  by  uiuscle-iibres 
in  such  a  way  as  to  form  a  somewhat  bulbous  mass  sharply 
marked  off  from  the  parenchyma  which  surrounds  it.  In 
Monohis,  however,  the  pharynx  is  more  developed,  pr.  ejecting 
as  a  stronj^  (drcular  fold  into  the  pharyngeal  pouch  and  form- 
ing what  is  termed  a  plicated  pharynx.  This  at  its  inner 
extremity  communicates  with  the  sac-like  intestine,  usually 
quite  simple  but  occasionally  somewhat  pouched,  and  ter- 
minating, as  in  all  the  Turbellaria,  blindly. 

The  nervous  system  consists  of  a  bilobed  ganglionic  brain- 
mass  from  which  pass  backwards  two  nerve-cords  which  may 
{3/onotiis)  or  may  not  present  transverse  anastomosing 
branches,  and  in  addition  a  number  of  smaller  branches  pass 
forward  to  be  distributed  to  the  anterior  eud  of  the  body. 
Eyes,  consisting  of  pigment-spots  seated  upon  the  brain,  are 
frequently  present,  and  in  Monotus  an  otocyst  is  found,  while 
lateral  ciliated  depressions  on  each  side  of  the  head  occur  in 
Plagiostoma. 

The  excretory  system  is  present,  but  presents  no  notable 
departures  from  the  typical  arrangement.  As  regards  the 
reproductive  organs,  the  testes  resemble  those  of  the  Acoela, 
but  the  ovaries  are  comparatively  small  and  the  separate 
vitellaria  are  large  and  sometimes  branched,  opening  into  a 
cavity,  the  genital  atrium,  common  to  them,  the  oviducts 
and  the  intromittent  organ,  and  communicating  with  the  ex- 
terior by  a  single  median  pore  situated  near  the  posterior  end 
of  the  body.  In  a  few  forms  the  vitellaria  are  not  diflferen- 
tiated  from  the  ovaries,  presenting  a  condition  similar  to  that 
found  in  the  Acoela. 


3.  Order  Rhabdocoela. 

The  Rhabdocoela  are  found  both  in  fresh  and  salt  water  and 
are  usually  small.  They  possess  a  distinct  tubular  digestive 
tract  (Fig.  69,  G,  d)  without  lateral  pouches  or  branches,  but 
the  principal  characteristic  lies  in  the  presence  in  the  paren- 
chyma of  large  spaces  resembling  the  coelomic  cavities  of 
higher  types,  a  feature  not  repeated  in  any  other  Turbellaria. 


TYPE  PLATYUELMINTUES. 


135 


A 


It 


! 

t 


-vl 


ov 


The  mouth  is  situated  at  various  rej^ious  of  the  body  iu 
different  forms,  beiu<;  anterior  in  Microstoma,  while  iu  J/r.so- 
atoma  (Fij^.  72)  it  is  situated  at  the  middle  of  the  ventral  sur- 
face. The  walls  of  the  pharyn- 
geal pouch  iph)  may  be  quite 
simple,  as  in  the  Acoila  which 
possess  a  pharynx,  or  may  pre- 
sent u  muscular  thickening  form- 
ing a  bulbous  pharynx,  but  no 
further  complexity  occurs,  al- 
tliough  in  certain  forms,  such  as 
FrorhynchuSy  the  pharynx  is  capa- 
ble of  being  protruded  from  the 
mouth,  acting  probably  as  a 
delicate  tactile  organ. 

The  nervous  system  {n)  is 
essentially  similar  to  that  of  the 
AUoioccela  ;  two  or  four  eyes  {oc) 
frequently  occur,  though  otocysts 
are  wanting,  while  the  ciliated 
depressions  on  the  side  of  the 
head  supposed  to  be  olfactory  iu 
function  occur  in  Microstoma  and 
Prorhynclais  and  allied  forms. 

The  excretory  system  consists 
occasionally  of  a  single  nephri- 
dium  with  numerous  branches 
which  open  near  the  posterior 
end  of  the  body,  but  more  usually 
two  main  tubes  are  present  open- 
ing near  the  middle  of  the  body 
either  directly  to  the  exterior  cr 
into  the  pharyngeal  pouch  {Meno- 
stoma\  though  in  some  cases  they 
uuite  near  the  posterior  end  of  the 
body  into  a  single  tube  which 
opens  to  the  exterior  by  a  single  median  pore  ( Vortex). 

The  reproductive  system  presents  considerable  variation 
iu  the  structure  of  the  female  apparatus,  but  the  testes  {t)  are 


Fl«.72.— A  HllABDOCffiLOUS  TUK- 

HELLAUIAN,  Mesostoma  apleudi- 
dum  (after  von  Graff)- 


at  = 

atg  = 

d  = 

n  = 
oc  — 
ov  = 

P  = 

ph  = 

sr  = 

t  = 


atrium. 

atrial  gland. 

iiitcsliue. 

brain. 

eye. 

'Wary. 

Pv  •As. 

p."    'ynx. 

receptaculum  seminis. 

testis. 


vi  =  yolk-gland. 


136 


INVEHTEDHA TE  MOliPlIOL OG  Y. 


uniformly  two  simple  club-shaped  bodies  uniting  below  to 
form  a  common  seminul  vesicle.  The  female  apparatus  may 
consist  of  a  single  ovary  (oy)  combined  with  a  vitellarium  or 
of  two  such  structures,  but  usually  there  is  a  separation  of  the 
vitellarium  {vi).  In  the  more  complicated  cases  there  is  but 
a  single  small  ovary  opening  almost  directly  into  the  genital 
atrium,  which  receives  also  in  addition  to  the  intromittent 
organ  the  ducts  of  the  two  vitellaria.  Its  walls  are  further- 
more pouched  out  into  a  seminal  receptacle  an  !-,ac-like 
c:.vity  which  serves  as  a  uterus,  while  a  peculiar  muscular 
sac,  lined  by  a  strong  cuticle,  the  bursa  copuldtrix,  serves  for 
the  reception  of  the  intromittent  organ  during  copulation. 
As  stated,  however,  numerous  variations  from  such  a  condition 
occur,  and  it  is  not  possible  to  describe  any  one  arrange- 
ment characteristic  of  all  the  Bhabdocoels. 


4.  Order  Tridadea. 

The  Triclads  constitute  a  group  of  forms  with  very  definite 
structural  peculiarities,  occurring  principally  in  fr'^sh  water 
{Planuria,  DendroccpliDu,  Phagocata),  though  a  fev  nis  are 
terrestrial  {Bipalium),  and  a  still  smaller  uum^^.  marine 
{Gunda,  IMelloura).  As  a  rule  they  are  elongated  in  form, 
one  of  the  terrestrial  species  reaching  a  length  of  2  cm.,  and 
are  for  the  most  part  free-living,  though  Bdelloura  and  Syncoe- 
lidium  are  ectoparasites  of  the  King-crab  {Limulus).  The 
mouth  is  situated  in  all  cases  behind  the  middle  of  the  body 
and  leads  into  a  somewhat  capacious  pharyngeal  pouch  (Figs. 
69, 1),  and  73,  ph)  in  which  lies  a  muscular  cylindrical  pharynx 
capable  of  protrusion  from  the  mouth-opening.  The  diges- 
tive tract  at  the  base  of  the  pharynx  divides  into  three 
branches,  one  of  which  passes  forward  in  the  median  line, 
giving  off  simple  or  branched  diverticula  on  both  sides,  while 
the  other  two  pass  back'.vards  on  either  side  of  the  pharyngeal 
pouch,  giving  off  diverticula  only  from  the  outer  side.  The 
intestinal  branches,  whose  number  has  suggested  the  name  of 
the  order,  and  their  diverticula  are  imbedded  in  a  compact 
parenchyma,  no  well-marked  ccelomic  spaces  being  present 
(Fig.  69,  D). 


' 


TYPE  PL  A  Till/iLMlA  TIIK8. 


137 


,1 


TliG  uervous  system  cousists  of  u  bilob.'d  braiu  lyiug  iu 
the  juiteiior  part  of  tlie  boily  and 
from  which  two  nerve-cords  pjiss 
backwards,  united  at  intervals  by 
cross-commissures  and  |^iviii«^  ort' 
on  tiieir  outer  sides  brunches 
wliich  anastomose  with  one  uu- 
otlier,  forming  a  network. 

In  Gunda  seyinetitatii  the  transverse 
cununissurus  a^rue  in  number  and  ar- 
niiigemeiit  with  tlio  hitei'ul  brunclies  on 
tliu  uno  hand  and  with  tlie  diverticuhi  of 
the  intestine  on  tlie  other,  the  arrange- 
ment of  the  two  systems  producing  an 
appearance  of  metamerizution  which  is 
most  striking,  especially  as  it  atTects  as 
well  the  excretory  and  reproductive  sys- 
tems. In  this  form  an  indication  is 
afforded  of  the  manner  in  which  the 
more  pronounced  and  t  v  [)ical  metameri- 
zation  of  the  higher  types  has  been  pro- 
duced by  the  more  or  less  completed 
multiplication  of  the  organs  and  the 
integration  of  the  parts  so  formed  into 
a  metamere  (see  p.  43). 

Eyes  are  usually  present,  fre- 
quently provided  with  lenses,  and, 
thouj^h   usually  two   iu   uumber, 

may  be  very  numerous  and  situ-  Fig-  73.— A  Triclad  Turbei.lari- 

AN,  Synaxlidium  pellucidv.m  (after 
Whekleb). 

ex  =  excretv/ry  system. 
g  =  accessory  gland. 
ggl  =  vaginal  glands. 
n  =  uervous  system. 
od   =  oviduct. 
ov  =  ovary. 

p  =  reproductive  orifice. 
ph  =  piiarynx. 
t  =  testis. 
u  =  uterus. 
vd  =  vas  deferens. 
vi  =  yolk-gland. 


ated  along  the  margin  of  the  body. 
No  otocysts  occur,  and  the  sides 
of  the  anterior  end  of  the  body 
are  iu  some  forms  produced  into 
more  or  less  elongated  processes 
which  may  possibly  be  mainly 
sensory  iu  function,  while  behind 
them  are  areas  of  strongly  ciliated 
cells  richly  supplied  with  nerves 
and  presumably  corresponding 
with  the  ciliated  depressions  oc- 
curring iu  the  same  region  in  the  Alloiocoela  and  Khabdocoela. 


138 


mVEUTEBHATE  MOliPIIOLOG Y. 


The  excretory  system  (lifters  from  that  of  the  h>wer  orders 
in  that  the  two  h)iigitiuliiial  iiephritlia  opeu  on  the  dorsal  sur- 
face of  the  body  by  uumerous  pores,  which  in  Gnnda  corre- 
spond in  number  with  the  intestinal  diverticula  and  nerve- 
commissures.  The  reproductive  apparatus  consists  of  nu- 
merous testes  (Fig.  73,  f\  as  in  the  Ac(ela  (arranged  metameri- 
cally  in  Gunda),  whose  ducts  unite  to  vasa  deferentia  [vd) 
uniting  in  the  muscular  iutromittent  organ  which  projects  into 
the  genital  atrium.  Two  small  ovaries  {ov)  occur  in  tlie  ante- 
rior end  of  the  body,  their  large  oviducts  passing  backwards  to 
unite  in  a  muscular  bursa  copuhdrix,  and  receiving  at  inter- 
vals the  secretion  of  uumerous  lateral  diverticula  whicli  con- 
stitute the  vitellarium  (t'i).  A  pouch-like  diverticulunj  of  the 
atrium  serves  as  a  uterus,  and  the  single  median  oritice  (p)  of 
the  atrium  lies  near  the  posterior  end  of  the  body  behind  the 
mouth-opening. 


5.  Order  Folycladea. 

Th'>  Poh'clads  are  exclusively  marine  and  assume  various 
forms,  some  being  quite  elongated  while  otliors  are  Hat  h»af- 
like  ex})ansions.  Conn)ared  with  the  members  of  the  other 
orders  they  may  be  said  to  be  as  a  ruh»  large,  though  few 
reach  the  length  whicli  has  been  mentioned  for  sonu'  land 
Triclads.  The  mouth  varies  greatly  in  position,  as  in  the 
lihabdocnuls,  and  opens  into  a  spacious  pharyngeal  ])oucli 
containing  a  plicated  pharj'iix  (Fig.  73,  ph).  The  intestine 
consists  of  a  central  cavit}',  into  which  the  i)harynx  opens  at 
its  inner  end  ami  from  which  numerous  branches  (hence  the 
name  of  the  order)  i)ass  off  into  the  compact  parenchyma, 
where  they  branch  and  may  anastomose  witli  one  another  to 
form  a  network.  The  nervous  system  presents  a  somewhat 
similar  condition,  the  bilobed  brain  (ce),  usually  situated  near 
the  anterior  end  of  the  body,  giving  off  a  number,  usually  six, 
of  nerve-cords  which  become  lost  in  a  wide-meshed  network 
ramifying  through  the  body-tissues.  Eyes  are  usually  pres- 
ent, frequently  in  enormous  numbers,  and  furthermore  in  many 
forms  {Plnnocertt)  tentacles  arise  from  the  dorsal  surface  or 
else  from  the  margin  near  the  anterior  end  of  the  body.     As 


TYPE  PLA  TYIIELMINTUES. 


139 


vdi 


in  the  Tiiclads  otocysts  are  wnutiii^,  nor  have  oiliateil  hiteral 
depressions  been  described  as  occurriu}»  in  the  order. 

Little  is  kiiowu  coucerniii^  the 
excretory  systen).  Tlie  reproduc- 
tive system  ditters  from  tiiat  of 
the  other  orders  in  that  tlie  male 
and  the  female  a])])aratus  each 
possess  a  separate  o])eninjj;  ( ?■  and 
? ),  there  beiujj;  no  }j;(Miital  atrium 
common  to  botli.  lioth  apertures 
lie  behind  tlio  mouth-o])eniu^,  near 
the  ]>osterior  end  of  the  l)ody, 
the  male  ap])aratus  openinjj;  an- 
teriorly to  the  female.  The  former 
is  similar  in  structure  to  what  has 
been  described  for  the  Triclads. 
The  female  apparatus  ])ossess(»s 
no  vitellarium,  and  the  ovaries  {ov) 
are  very  numerous,  lyiuj^  in  the  ^^ 
hiteral  parts  of  the  body,  their 
various  ducts  unitin*^  to  form  wide 
canals  which  serve  as  uteri  {nt). 
Those  open  into  a  sin<j;le  tube,  the 
'iXKjina,  which  receives  the  secretion 
of  the  numerous  glands  isfj)  which 
form  the  shell-j^land 

in  some  forms  tluae  is  situated 
about  the  middle  of  the  ventral 
surface  of  the  body  a  muscular 
sucker  which  serves  as  an  or^an  of 
adhesion.  Since  the  prest;nc(i  or 
abscMice  of  this  orj^an  is  in  either 
case  associated  with  theoccurrtiuce 
of  other  important  strui^tural  ])ecu- 
liarities,  the  order  has  been  divided 
into  two  suborders—  the  (Joltjlea, 
provided  witli  a  sucker  ( 77///.«,vn/«-  (? .  2  =  malcaiMi  finrnhMmilccs. 
zoon^  Eurylepta),  and  the  Acofyha,  iu  which  it  is  absent 
{Planocera,  Leptophiiut). 


I-AHIAN,      /A'ptopliiiiti    alcinoi 

(lll'icl-  Lanqi. 

</</  --  at'CL'Ssory  frlantl. 

ce  --  cerebral  j^aiiniion. 

ov  =  ovary. 
p/i  =  pliiiiyii.v. 

St/  =  .slicli-giaiid. 

(f  :=  testis 

ut  -  iitrnis. 

vd  -  vas  deferens. 

vs  =  vesicula  seiiiiiialis. 


140 


INVEKTEDRA  TE  MORPHOLOG  Y. 


Reproduction  of  the  Turhellaria. — Non-sexual  reproduction 
is  not  characteristic  of  the  Turbellaria,  though  it  occurs  in  cer- 
tain Rhabdocoels.  lu  Microstoma  a  transverse  partition,  con- 
sisting of  two  closely- applied  lamellae,  forms,  extending 
from  the  outer  wall  of  the  body  to  the  wall  of  the  digestive 
tract,  which  it  constricts  slightly  without  dividing.  Later  a 
constriction  of  the  outer  surface  of  the  body  appears,  the 
two  lamelhe  of  the  partition  separate  slightly,  and  the  indi- 
vidual lying  behind  the  partition  develops  a  new  mouth  and 
pharynx  and  a  new  brain,  so  that  it  resembles  exactly  the 
anterior  individual  with  which  it  is  directly  connected,  by  the 
uninterrupted  digestive  tract.  Before  these  processes  are 
complete,  however,  they  are  repeated  in  e"  ^  of  the  two  indi- 
viduals, so  that  a  chain  of  four  imperfectly  eparated  individ- 
uals results,  and  by  further  repetitions  of  the  process  chains 
of  8,  16,  or  32  individuals  may  arise,  each  provided  with 
mouth,  pharynx,  and  brain,  the  anterior  individual  possessing 
the  original  structures,  and  all  connected  by  the  digestive 
canal  which  runs  uninterruptedly  through  the  entire  chain 
(see  Fig.  28).  Eventually  the  various  individuals  separate 
from  one  another  and  become  sexually  mature. 

The  sexual  method,  however,  plays  a  much  more  important 
part  in  the  life-histories  of  the  Turbellaria.  The  development 
of  the  three  lower  groups  has  not  as  yet  been  as  thoroughly 
investigated  as  is  desirable,  but  the  phenomena  which  occur 
in  the  Triclads,  and  especially  in  the  Polj'clads,  have  been  fol- 
lowed. The  Triclads  deposit  their  ova  in  chitinous  cocoons, 
which  contain,  besides  the  ova  proper,  large  numbers  of 
amoeboid  cells,  originating  in  the  vitellarium-pouches  of  the 
parent,  and  serving  as  food  for  the  young  embryo.  In  asso- 
ciation with  this  condition  of  a£fairs  many  peculiarities  of 
segmentation  and  growth  occur  in  the  Triclad  embryos,  all  of 
which  must  be  considered  as  secondary  adaptations. 

In  the  Polyclads,  however,  a  more  primitive  state  of  affairs 
occurs,  the  food-yolk  being  incorporated  with  the  protoplasm 
of  the  ovum,  a  more  or  less  distinct  irregular  segmentation 
resulting  from  its  telolecithal  arrangement  (p.  53).  The 
diploblastic  condition  arises  by  an  invagination  either  of  the 
embolic  or  epibolic  type,  but  at  an  early  period  of  the  segmen 


TYPE  PLATTHELMINTUEa. 


141 


I- 


tation  the  cells  wliicli  are  to  form  the  mesoderm  are  separated 
off  from  those  from  which  the  ectoderm  aud  eiidoderm  are  to 
be  derived,  so  that  even  before  the  invagination  all  the  three 
layers  are  represented.  This,  however,  is  to  be  regarded  as  a 
precocious  segregation  of  the  germ-lasers,  and  even  within  the 
limits  of  the  few  forms  whose  erabrology  is  known  consider- 
able variations  in  the  time  and  manner  of  the  differentiation 
of  the  mesoderm  occur.  The  result  of  the  invagination  is  in 
some  cases  a  solid,  bilaterally  symmetrical,  ciliated  embryo 
consisting  of  a  layer  of  ectoderm  enclosing  a  central  mass  of 
endoderm  and  mesoderm,  in  the  interior  of  which  a  cavity  ap- 
pears surrounded  by  the  endoderm.  A  depression  appears  on 
the  ventral  surface,  which,  deepening,  finally  unites  with  the 
enteron  and  forms  the  pharyngeal  pouch,  aud  gradually  the 
characters  of  the  adult  are  assumed. 

In  some  forms  whose  ova  are  provided  with  comparatively 
little  yolk  the  embryo  leads  from  an  early  period  a  free-swim- 
ming   existence,  and    in  accordance  with  this  a  specialized 
form  has  been  acquired  and  a  slight  metamorphosis  is  neces- 
sary for  the  conversion  of  this  larva  into  the  adult  condition. 
In  Stylochus  the   embryo  develops   into   what   is  known  as 
Goette's  larva,  a  bilateral  ciliated  structure  with  an  anterior 
and  posterior  tuft  of  strong  sensory  hairs,  while  from  the  ven- 
tral surface  on   either  side   of  the   mouth 
there  hang  down  two  ciliated  ear-like  lobes 
or  lappets.     In  another  form  (Thymnozoon) 
these  lappets   are   much    more   developed, 
passing  round  to  the  dorsal  surface  of  the 
body,  and  their  edges  are  drawn  out  into 
four  or  eight  lobes,  one   of  which   lies   in 
front   of   the   mouth   and    another   on  the 
dorsal  surface,  the  other  two  or  six  lying 
at  the  sides  of  the  body  and  being  arranged 
symmetrically   on   either   side.      It   seems f,o    thT^Lahva    of 
probable  that  this  larva,  known  as  Mailer's     Thymnozoon,    Mul- 
larva  (Fig.  75),  may  be  traced  back  to  a  con-     ^^''^     hnvva.     (after 
dition   such  as  that  described  in  Goette's 
larva,  the  two  lappets  of  that  form  having  united  in  front  of 
the  mouth,  while  their  lines  of  attachment  have  become  more 


142 


IN  VEHTEBKA TE  MOliPHOLOU  Y. 


and  more  oblique  uutil  what  were  origiually  the  posterior 
edges  of  the  lappets  meot  on  the  dorsal  surface.  The  edfj;es 
of  the  lobes  of  the  lappets  are  friui^ed  with  long  cilia,  and 
consequently  a  lobed  prseoral  band  of  cilia  is  produced. 
These  larvae  pass  into  the  adult  form  by  gradually  becoming 
more  and  more  flattened  dorsoventrally,  the  ciliated  lappets 
or  lobes  at  the  sama  time  growing  smaller  and  smaller  until 
they  finally  disappear. 

Relationships  of  the  Tnrbellaria. — A  relationship  of  the  Turbelhiriii, 
especially  of  the  Polyclads,  with  the  Ctenophores  has  been  advocated 
within  recent  years,  and  through  this  relationship  genetic  affinities  with 
the  Cnidaria  have  been  sought.  The  question  of  the  affinities  of  the  Cteno- 
phores has  already  been  discussed,  and  it  has  been  pointed  out  that  it  is 
probable  that,  instead  of  being  a  connecting  link  between  the  Cnidaria  and 
the  Turbellaria,  they  are  rather  highly  modified  Tnrbellaria  adapted  to  a 
pelagic  life.  In  this  sense  the  idea  of  a  genetic  affinity  between  the  Turbel- 
laria and  Ctenophores  may  be  correct,  though  it  seems  pro})able  that  the 
Polyclad  affinity  should  be  given  up  and  the  relationship  .sought  for  among 
AUoioca'Ian  forms. 

The  Ctenophore-Polyclad  theory  necessarily  viewed  the  Polyclads  as  the 
most  primitive  Turbellaria,  and  came  into  contact  in  this  way  with  the 
more  simple  organization  of  the  Aca;la,  AUoiofula,  and  Rhabdoca-hi.  a 
difficulty  which  was  avoided  V)y  assuming  that  tliese  were  degenerate 
groups  derived  from  Polycladan  ancestors.  No  good  grounds  for  such  an 
assumption  exist  however,  nothing  in  the  mode  of  life  suggesting  a  cai-.so 
for  degeneration  ;  and  until  embryological  evidence  of  degeneration  is 
obtained,  it  is  preferal^le  to  consider  their  simplicity  primitive. 

This  latter  view  is  strengthened  if  it  harmonizes  with  a  probable  phy- 
logeny.  It  has  already  beeti  pointed  out  that  the  solid  embryo  or  sterrula 
is  to  be  recognized  as  an  ancestral  form  of  the  Cnidaria.  With  such  an 
ancestral  form  the  Acoela  show  affinities  in  the  absence  of  a  differentiation 
of  the  central  mass  into  well-defined  endoderm  and  mesoderm.  The  local- 
ization of  a  definite  region  for  the  ingestion  of  nutrition  would  lead  to  the 
formation  of  a  mouili  in  the  Sterrula,  ju.st  as  it  has  done  in  the  riuijellata. 
The  differentiation  of  muscle-fibres  from  the  mesendodermal  cells  would 
naturally  f  )ll()\v  the  assumption  of  a  creeping  habit,  so  that  it  is  only  the 
possession  of  a  definite  nervous  system  imbedded  in  the  mesogloea  (in  which 
tissue,  however,  Cnidarian  characteristics  are  yet  discernible,  as  already 
pointed  out)  and  the  occurrence  of  a  complicated  reproductive  apparatus 
that  render  a  close  compari.son  with  the  Sterrula  difficult ;  but  even  the 
explanation  of  the  presence  of  these  structures  makes  fewer  demands  upon 
our  ideas  of  develo|)nieMtal  possibilities  than  does  the  assumption  that  the 
Acoela  owe  their  peculiarities  to  degeneration. 

Upon  this  view  of  the  phylogeny  the  Acaila  are  united  with  the  Ccjelen- 


TYPE  PLATYHELMINTIIE8. 


143 


tera  only  through  the  Sterrula  ancestor  common  to  both,  or  more  probably 
through  an  ancestor  in  which  the  mouth  had  developed,  as  well  as  a  slight 
differentiation  of  nuiscle-fibres,  but  in  which  no  hollowing  out  of  an  enteron 
had  yet  occurred.  This  appearing  in  a  primitive  acoelan  form  gave  rise  to 
the  Alloiocixla  from  whicli  two  divergent  lines  of  descent  arose,  one  leading 
to  the  lihabdocuils  and  the  other  to  the  Triclads  and  Polyclads. 

If  this  be  the  true  phylogeny  of  the  class,  some  evidence  of  it  ought  to 
be  found  in  the  embryological  history  of  some  of  the  higher  members  of 
the  group  in  .accordance  with  what  is  termed  the  Biogenetic  laiv,  which  is 
to  the  effect  that  an  individual  in  its  development  recapitulates  more  or 
less  accurately  its  phylogenetic  development,  or,  to  put  it  more  brieHy,  the 
ontogeny  is  a  recajiitidation  of  the  phylogeny.  Secondary  modifications, 
especially  in  the  form  of  the  abbreviation  or  omission  of  certain  stages, 
may  intervene  in  tlie  individual  development,  forming  what  are  teriued 
cenogenetic  mod ijicai ions,  but  notwithstanding  exceptions  produced  in  this 
way  the  law  is  of  general  application. 

In  Stylochus  the  young  larva  is  a  solid  body  without  any  enteron  and 
represents,  therefore,  an  Acoelan  stage  of  development ;  later  the  cen^ral 
mass  becomes  hollowed  out  to  form  an  enteron  whose  walls  are  not  at  lir.st 
clearly  marked  off  from  the  surrounding  parenchyma,  and  a  representa- 
tion of  the  Alloiocoelan  condition  results,  from  which  tlu;  Polyelad  condi- 
tion gradually  develops.  Consequently  in  Stylochus  the  ontogeny  indi- 
cates a  primitive  nature  for  the  Acarla,  and  agrees  with  the  phylogeny 
which  has  b(;en  outlined  above.  It  must  be  recognized,  however,  that  all 
reconstructions  of  the  phylogeny  of  the  Turbellaria  and  all  views  as  to 
their  athnities  to  the  Cnidaria  must  be  accepted  with  much  reservation, 
until  the  nnxch-needed  facts  as  to  the  developmental  history  of  the  Acuta 
and  Alloiocoela  are  available. 


II.  Class  Trematoda. 

The  Trematodes  or  Fluke-worms  are  throughout  parasitic 
either  upon  the  exterior  of  their  hosts  or  in  the  cavities  of 
their  body,  aud  in  correspondence  with  this  mode  of  life 
structures  are  developed  by  means  of  which  they  adhere  to 
their  hosts.  These  structur.js  are  of  two  kinds  ;  in  all  suclrrf^ 
are  present  consisting  of  cup- like  depressions  whose  walls  are 
richly  supplied  with  muscle-cells,  by  the  contraction  of  which 
a  vacuum  is  formed,  and  in  many  forms,  in  addition  to  these, 
chitinous  hooks  occur.  The  suckers  vary  in  number  from  (me 
{Monostomum)  or  two  [Distomum,  Fig.  70,)  to  several  {Poly- 
sfonium),  and  at  the  bottom  of  one  situated  at  the  anterior 
extremity  of   the   body  is   the   mouth-opening.     This   leads 


144 


IN  VEliTEBUA Tl£  MURPUOLOG  T. 


into  a  tubular  a'sopliagus  whose  walls   are  thickened   near 
its   anterior  end  to   form  a  muscular  pharyngeal  hidh  which 

functions  as  a  pump  for  the  ingestion 
of  the  nutritive  fluids  of  the  host.  At 
its  posterior  extremity  the  oesophagus 
branches  into  two  limbs  which  are  con- 
tinued backwards,  in  some  cases  giv- 
ing off  secondary  branches,  to  near 
the  posterior  end  of  the  body,  where 
they  either  end  blindly  or  unite  together 
in  the  middle  line  {Polystomum)  to  form 
a  loop. 

The  body  is  covered  by  a  distinct 
cuticle  secreted  by  the  ectodermal  cells, 
which  in  the  adult  may  undergo  a  con- 
siderable amount  of  degeneration,  or 
probably  in  some  cases  the  cuticle  is 
formed  in  part  by  the  transformation 
into  chitiu  of  the  ectoderm.    Spiny  ele- 

Fig.  7(i.-IHstrnnum  cyg-  ^^*^«°«  °^  *^®  ^^^^^^^^  ^^'®  P^*®«®"<^  ^^ 
n<nde«  (from  a  drawing  by  many  forms,  and  the  large  chitinous 
c.  lanoknbkck).  hooks  which  occur  in   many  ectopara- 

sitic  forms  are  but  further  developments  of  these  structures. 
Below  the  ectoderm  lies  the  usually  thin  basement-membrane, 
below  which  again  lie  the  circular  and  longitudinal  peripheral 
muscle-sheets,  and  between  the  intestine  and  these  muscles  is 
the  parenchyma  traversed  by  dorsoventral  muscle-bundles 
and  having  imbedded  in  it  the  various  organs. 

The  nervous  system  (Fig.  77)  consists  of  a  transversely 
elongated  ganglion  lying  dorsal  to  the  oesophagus — usually 
between  the  bottom  of  the  anterior  sucker  and  the  pharyngeal 
bulb.  The  ganglion  is  somewhat  swollen  at  each  extremity, 
indicating  its  origin  by  the  approximation  of  two  ganglionic 
masses,  and  from  these  thickenings  nerves  arise  which  pass 
both  forward  and  backward.  The  anterior  nerves  are  short 
and  slender,  and  supply  the  musculature  of  the  anterior 
sucker  and  the  sides  of  the  anterior  end  of  the  body,  while 
the  posterior  nerves  are  much  stronger  and  longer  and  vary 
from  two  to  six  in  number  ;  in  the  latter  case  four  run  along 


TYPE  PLATYHELMINTUE8. 


146 


tlie  ventral  surface  of  the  body,  two  on  each  side  of  the  mid- 
dle line,  the  other  two  having  a  more  dorsal  position,  while 
when  only  two  are  present  they  correspond  to  the  two  more 
median  ventral  nerves  of  this 
arrangement.  Sense-organs  are 
but  feebly  developed  as  a  rule, 
especially  among  the  endopar- 
asitic  forms,  but  in  some  ecto- 
parasites eyes  are  present  con- 
sisting usually  of  four  spots  of 
pigment  seated  upon  the  brain- 
ganglion  and  sometimes  provid- 
ed with  a  lens-like  structure. 


Fig.  77.— Nervous  System  op  Tre- 
MATODK,     Triiitomum     vioIab    (after 

Lanu). 


Fig.  78. — Excretory  System 
OF   Trematodb,    Distomum 
divergena  (after  Fraipont). 
A,  euiiie  system;    B,  tertniual 

fuuuels. 
/  =  fuDuel. 
fl  =  flume  «>f  cilia, 
n  =  main  truuk. 
ph  =  pharyngeal  bulb. 
8  =  uulerior  sucker. 
tc  =  terminal  cell. 
vs  =  ventral  sucker. 
vt  =  contractile  vesicle. 


The  excretory  apparatus  (Fig.  78)  consists,  as  is  usual  in 
the  Platylielminths,  of  two  longitudinal,  more  or  less  irregu- 
larly twisted  tubes  {n)  from  which  arise  the  funnel-bearing 
branches  (/).  A  peculiarity  of  the  Trematodes  is,  however, 
the  union  of  the  two  longitudinal  tubes  in  a  terminal  vesicle 
{vt)  which  opens  to  the  exterior  at  the  hinder  end  of  the  body 
by  a  single  pore. 

The  reproductive  system  is  exceedingly  complicated, though 
essentially  similar  to  that  of  the  higher  Turbellaria.     It  opens 


146 


INVERTEBRATE  MORPUOLOOY. 


to  the  exterior  by  two  pores  lyiug  close  together  on  the  ventral 
surface  rather  nearer  the  anterior  than  the  posterior  entl. 
The  male  apparatus  consists  in  the  Polystome(v  of  numerous 
closely-aggregated  testes,  or  elsc,  as  in  the  Distomew  (Fig.  70), 
of  only  two  situated  in  the  posterior  half  of  the  body ;  the 
ducts  from  the  testes  pass  forwards  towards  the  genital  pore, 
near  which  they  unite  to  form  a  sac-like  seminal  vesicle, 
from  whose  anterior  end  the  single  vas  deferens  is  continued 
on  towards  the  i)ore,  passing  in  the  latter  part  of  its  course 
through  a  muscular  protrusible  intromittent  organ,  the  cirrus. 
The  ovary  is  single,  and  its  duct  shortly  after  leaving  it 
receives  the  ducts  coming  from  two  yolk-glands  situated  one 
on  either  side  of  the  bodv,  and  is  surrounded  at  about  the 
same  region  by  a  shell-gland,  consisting  of  a  number  of  uni- 
cellular glands  arranged  in  a  radiating  manner  around  tiie 
oviduct.  Beyond  its  union  with  these  ducts  the  oviduct 
either  runs  almost  directly  to  the  genital  atrium,  opening 
into  it  in  close  proximity  to  the  cirrus,  or  else  pursues  a 
winding  contorted  course  through  the  parenchyma  and  serves 
as  a  uterus  or  ootijp,  within  which  the  ova  undergo  a  portion 
of  their  development. 


From  the  oviduct  in  the  region  where  the  ducts  from  tlie  vitellaria  and 
shell-gland  open  into  it  one  or  more  canals  may  arise  whose  significance  is  to 
a  certain  extent  problematical.  In  the  Distomeae  one  such  canal  occurs,  and 
when  a  seminal  receptacle  is  present  it  stands  in  mon;  or  less  close  relations 
to  this  canal,  known  as  Laurer's  canal,  which,  after  a  short  course,  opens  to 
the  exterior  on  the  dorsal  surface  of  the  body.  In  some  Folystomece  two 
canals  arise  from  the  yolk-ducts  and  pass  forwards  parallel  to  the  uterus 
to  open  by  a  number  of  pores  situated  on  the  margin  of  the  body.  The.se 
canals  have  been  termed  the  vagina,  and  in  some  forms  are  represented  by 
a  single  canal.  In  addition  to  the  vagina,  however,  another  canal  is  pres- 
ent which  has  been  .shown  \\\  Polystomnm  tiwd  Sphyrannra  to  o\iGn  into 
the  digestive  tract,  and  has  been  homologized  with  Laurer's  canal  of  the 
Distomeae. 

It  seems  pretty  certain  that  the  vagina  of  the  Polystomeae  functions  in 
copulation,  the  genital  orifice  of  one  Polystomum  having  been  observed  to 
come  into  contact  with  the  vaginal  openings  of  the  other  during  that  act. 
But  the  Laurer  canals  do  not  seem  to  have  any  such  function,  and  it  has 
been  suggested  that  they  may  serve  for  the  removal  of  surplus  yolk- 
material  produced  in  accordance  with  the  favorable  conditions  for  nutri- 
tion offered  by  the  parasitic  mode  of  existence  of  the  Trematodes. 


TYPE  VLATYUKLMimUES. 


147 


in 
to 
ct. 
ins 
k- 
ri- 


Two  orders  may  be  recognized  as  occurring  in  the  Tre- 
matoda. 

1.  Order  PolystomesB. 

The  PolystomesB  are  for  the  most  part  ectoparasites  and 
present  fewer  signs  of  degeneration  than  do  the  endopara- 
sitic  members  of  the  class.  The  apparatus  for  adhering  to 
their  hosts  is  usually  strongly  developed,  several  suckers 
usually  being  present,  as,  for  instance,  three  in  Tristomuiu 
and  seven  in  Polystomum  integerrimiim  (the  latter  parasitic  in 
the  urinary  bladder  of  the  Frog),  and  in  addition  a  number  of 
chitinous  hooks  ma}'  occur,  as  in  Gyrodactylus  and  Sphyranura 
(the  latter  parasitic  on  the  skin  of  Menohranchiis),  In  accord- 
ance,  too,  with  their  mode  of  life,  sense-organs  in  the  form  of 
eyes  and  probably  of  tactile  papilla?  on  the  skin  occur,  and 
furthermore  the  processes  of  development  are  much  simpler 
than  in  the  endoparasites,  as  will  be  seen  later. 

Some  peculiar  iiiionialifcs  occur  in  the  life-histories  of  some  of  the  'V;iy- 
stomeae,  as,  for  instance,  in  the  Gi/rodacti/lus,  which  lives  upon  the  gills  of 
the  Carp,  It  is  a  viviparous  form,  and  the  young  while  still  within  the 
body  of  the  i)arent  may  already  liave  become  mature  and  contain  young 
likewise,  which  again  may  contain  ova  in  course  of  development,  four  gen- 
erations being  thus  enclosed  one  within  the  other.  Diplozoon,  which  livi's 
likewise  on  the  gills  of  Cyprinoid  fishes,  is  i)eculiar  in  that  at  the  time  of 
sexual  maturity  two  individuals  become  fused  with  one  another  in  the 
form  of  an  X,  the  fertilized  ova  giving  rise  to  a  single  form  formerly 
known  as  Diporpa. 

2.  Order  Distomese. 

This  order  includes  endoparasites  which  show  a  mine 
marked  degeneration  than  do  the  members  of  the  preceding 
order.  Eyes  may  be  present  in  the  young  but  are  absent 
in  the  adult,  and  furthermore  a  very  complicated  metamor- 
phosis is  passed  through  in  the  development.  The  suckers 
for  adhesion  to  the  host  are  either  one  (Monostomiun)  or 
two  {Disiomum),  and  as  a  rule  no  chitinous  hooks  are  pres- 
ent. 

Among  the  more  interesting  members  of  this  order  are  Distomnm 
hepaticum,  a  large  form  measuring  2-3  cm.  in  length  and  inhabiting  the 


148 


INVEHTEDliA  TE  MOIWIIOLOO  Y. 


I 


bile-ducts  of  Sheep,  in  whicli  it  produces  what  is  termed  tlie  "  Rot,"  which, 
in  the  low-lying  pastures  of  En<,'Iand  and  the  Continent,  is  fre<iuently  the 
cause  of  tiie<lestnicti()n  of  large  numbers  of  sheep.  In  exceptional  cases 
it  has  been  known  to  occur  in  man.  In  Egypt,  however,  the  Fellaheen 
are  not  unfrequently  attacked  by  another  form,  Di,stonium  hvmutobium, 
which  is  peculiar  in  that,  contrary  to  the  rule,  the  sexes  are  se(>arated  in 
different  individuals.  The  margins  of  the  body  of  the  nuile  are  rolled 
inwards  on  the  ventral  surface,  forming  a  tube  within  which  the  more  slen- 
der female  lives.  Associated  in  pairs  in  this  way,  they  are  found  in  the 
blood  of  the  portal  vein  and  its  connections  and  pass  to  the  ureters  and 
bladder,  in  whose  mucous  membrane  they  deposit  their  ova,  thus  pro- 
ducing an  inflammation,  accompanied  by  suppuration,  of  these  organs. 

Development  of  the  Trematodes. — The  ova  of  Trematoda 
consist  of  two  ilistiiict  parts,  a  germ-cell,  the  product  of  the 
ovary,  surrouuded  by  a  mass  of  food- material,  the  secretion 
of  the  vitellaria,  the  whole  being  enclosed  in  a  shell  formed 
by  the  sheli-glaud.  In  the  Polystomete  the  development,  as 
a  rule,  is  entirely  carried  on  outside  the  body  of  the  parent, 
the  stalked  ova  being  attached  to  the  body  of  the  host,  though 
Gyrodocfylus  is  viviparous.  In  the  DistomesB,  however,  the 
reverse  is  the  rule,  the  ova  undergoing  a  certain  part  of  their 
development  in  the  uterus  of  the  parent,  and  leaving  the  egg 
shortly  after  its  extrusion  as  a  larva,  sometimes  ciliated, 
sometimes  provided  in  the  place  of  the  cilia  with  a  structure- 
less cuticle,  and  furthermore  in  these  endoparasites  there 
occurs  a  re'narVable  alternation  of  generations  of  the  kind 
already  referred  to  as  heterogony  (see  p.  61). 

The  heterogony  may  be  of  various  degrees  of  complexity. 
It  begins,  however,  in  all  cases  with  the  embryo  (Fig.  79,  A\ 
which  nuiy  be  a  free-swimming  ciliated  organism  provided 
with  a  short  pouch-slijxped  intestine  and  with  a  mouth,  and 
frequently  possessing  also  a  nervous  system  and  pigment 
eye-spot  as  well  as  excretory  tubes ;  in  other  cases,  however, 
as  stated,  the  embryo  is  destitute  of  cilia,  usually  in  this  case 
being  provided  with  one  or  more  spines  at  the  mouth-end 
of  the  body,  and  all  gradations  of  degeneration  of  the  eye- 
spot  and  nei'vous  system,  as  well  as  of  the  excretory  tubes 
and  digestive  system,  may  be  observed.  In  all,  however,  the 
space  between  the  more  or  less  developed  digestive  tract  and 
the  body-wall  is  occupied  by  numerous  unspecialized   cells 


TYPK  PL  A  TYlIKLMINrilKS. 


149 


(^c),  which  are  iu  reality  genu-cells  or  ova  capable  of  uuiler- 
goiij^r  a  parthenof^eiietic  developiueiit.  Eventually  this  larva 
makes  its  way  into  the  interior  of  an  animal  of  some  kinil, 
usually  a  Mollusk,  and  there  undergoes  a  further  develop, 
meut,  either  retaining:;  its  dij^estive  n[»paratus  and  elonj^atinj; 
somewhat  to  form  a  lieclia  (Fij^.  71),  JJ),  or  becoming  an  oval 


t-m 


Fig.  79. — A,  Ciliated  larva,  and  B,  Redia  of  Ditttomum  hepntieum  (after  Leuck- 

ARTl. 

d  =  intestine.  m  =  mouth. 

gc  =  germ-ceils.  r'  =  second  generation  of  Rediaj. 

sac  without  mouth  or  digestive  tract,  the  Sporocyst.  The 
Redia  is  a  much  more  highly  organized  form  than  the  Sporo- 
cyst and  is  frequently  capable  of  motion,  two  blunt  projec- 
tions near  the  hinder  end  of  the  body  serving  as  supports  in 
a  somewhat  similar  manner  to  the  sucker-like  feet  of  cater- 
pillars. It  adheres  to  the  wall  of  a  cavity  of  its  host,  from 
which  by  energetic  action  of  its  muscular  pharynx  it  is  able 
to  absorb  nutrition. 

From  this  stage  onwards  the  development  varies  in  com- 
plexity iu  various  forms.  It  is  simplest  in  Monostonium  muta- 
bile,  whose  ciliated  embryo,  while  still  free-swimming,  contains 
within  it  a  small  sexually  immature  Monosfomum,  and  after  it 
has  made  its  way  into  the  interior  of  its  Molluscan  host  the 
young  Monostomum  becomes  eucapsuled  in  the  tissues  of  its 
host.     The  mode  of  origin  of  this  immature  form  has  not  as 


mo 


IN VEHTKBUA  TE  MOIWIIOLOG  Y. 


1  . 


yet  beeu  observed,  but  there  is  uo  reason  for  doubtiug  that 
it  is  the  result  of  the  partheiio^euetic  developmeut  of  owe  of 
the  germ-cells  which  occur  iu  the  body-cavity  of  the  embryo. 
So  loug,  however,  as  it  reiuaius  iu  the  tissues  of  the  Mollusk 
it  undergoes  no  further  development;  it  can  only  reach  ma- 
turity in  a  second  host,  iu  this  case  some  water-bird  which 
swallows  the  Mollusk  and  its  eucapsuled  parasite,  when  the 
latter,  its  capsule  being  dissolved  by  the  digestive  juices  of 
the  bird,  is  set  free,  fastens  itself  to  the  wall  of  some  of  the 
cavities  of  its  host  and  becomes  sexually  mature. 

In  this  species  of  Trematode  but  two  hosts  are  required 

in  the  life-history  ;  iu  the  majority 
of  the  Distome«3  a  third  occurs,  an 
additional  stage  of  development  in- 
tervening between  the  liedia  or  Spo- 
rocyst  and  the  encapsuled  immature 
worm.  The  germ-cells  of  the  Kedia 
or  Sporocyst  while  in  the  interior  of 
the  Mollusk  develop  into  a  form 
resembling  an  immature  Distome, 
but  provided  with  a  mobile  muscular 
tail  whose  axis  is  formed  by  a  fibrous 
rod  resembling  somewhat  in  appear- 
ance the  Vertebrate  uotochord.  Such 
an  organisir.  is  known  as  a  Cercaria 
(Fig.  80),  and  when  fully  developed 
the  Cercaria  brood  leaves  the  body 
of  the  parent  liedia  or  Sporocyst, 
makes  its  exit  from  the  tissues  of 
Fig.    bo.— Cercaria    armata  the    Mollusk    and    leads    for    a    time 

a  free-swimming  existeuce.  Eventu- 
ally the  Cercaria  mak;^s  its  way  into 
the  body  of  a  second  li  st,  .  .<,illy 
like  the  first    a   M'  and   there 

becomes    encapsu  u   the  tis^    js, 

losing  at  the  same  me  i^  tail,  and 
it  reaches  its  maturity  ol  y  after  the 
Mollusk  has  been  swallowed  by  the  definitive  host,  as  was 
the  case  in  3Ionostomum. 


(after  Schwarze). 

as  =  Hiiterior  sucker. 

cv  =  coutractile  vesicle. 

d  =  iutestiue. 

n  =■  uepliridial  tube. 
ph  =  pharynx. 
sp  =  spine. 

V8  =  ventral  sucker. 


TYPE  PLATYUELMlNTima. 


101 


A  still  further  complexity  is  fouud  iu  the  Liver  Fluke, 
Dislomum  hepaiicum.  Iu  this  form  the  froe-swiminiuj,'  embryo 
makes  its  way  iuto  the  tissues  of  a  small  suail  ami  tlieio  bo- 
comes  cou verted  iuto  u  Sporocyst.  The  germ-culls  uf  the 
Sporocyst  j?ive  rise  by  their  devolopmeut,  not  to  Ctucariiu, 
as  iu  the  usual  cases,  but  to  llediio,  and  these  may  }^ive  rise 
uuder  certaiu  conditions  to  a  second  brood  of  liediie  (Fij^.  7i>, 
B,  /').  Duriufj;  the  summer,  however,  the  llediie  produce  Cer- 
cariie,  which,  having  their  host,  swim  about  for  a  short  time, 
and  tiually  encyst  themselves,  not  necessarily  in  a  second 
Mollusk,  but  on  grass  or  any  other  object  with  which  they 
may  come  iu  contact,  the  tail  at  the  same  time  being  lost. 
If,  now,  these  encysted  forms  are  swallowed  by  a  sheep,  the 
young  Distome  makes  its  way  to  the  bile-ducts  of  the  host, 
where  it  becomes  mature. 

Tlie  following  schema  by  R.  Hertwig  will  show  the  relationships  of  these 
different  methods  of  development : 


S1HP1.K  Mode. 

Usual  Mode. 

COMPI.ICATKD    MOOK. 

£n 

Emiiryo. 

Water. 

Embryo. 

Water. 

Embryo. 

Water. 

fc.2 
OS 

Sporocyst 
or  Kedia 

I.   HlPHt 

(Mollusk) 

Sporocyst 
or  Reilia 

I.  Host 
(MulUmk) 

Sporocyst 

I.  Host 
(Mollusk) 

Rediae 

tl 

g 

a 
_o 

*^ 

g 

a 

a 

Cercariao. 

Water. 

s 

1 

V 

V 

0 
a 

CereariaB. 

Water. 

1 

a 

Enuapsuled 
Uistoine 

I.  Host 

Encapsnied 
Distome 

II.  Host. 

Encapsnied 
Disiomo* 

II.  Host  or 
on  Krass.  etc. 

Mtttuie 
Distome 

H.  Host 

Mature 
Distome 

III.  Host 

Mature 
Distome 

III.  Host 

In  a  few  Distomeae  a  simplification  more  extensive  than  that  repre- 
sented in  the  first  columns  of  the  «f^hema  occurs,  as  for  instance  in  thi; 
genus  Uolostomum,  whose  embryo,  after  j.;r^king  its  way  into  the  body  of 
tiie  first  host,  seems  to  be  gradually  metamorphob^d  into  the  immature 
Distome,  without  any  alternation  of  generations, 

A  very  peculiar  life-history  is  found  in  Distomum  macrostomum,  which 
is  parasitic  in  insect-eating  birds.  The  Sporocyst  is  found  in  a  snail,  and 
is  peculiar  in  that  it  assumes  a  branching  form,  the  branches  forming  a 
network  among  the  tissues  of  the  host.  In  the  ends  of  filaments  of  the  net- 
work young  Distomes  devslop  without  the  intervention  of  aCercaria  stage, 
and  by  their  development  and  its  own  growth  the  terminal  branches  be- 
come of  a  considerable  .size,  two  of  them  extending  into  the  tentacles  of 


152 


IN  VERTEBRA  TE  MORPUOLOO  Y. 


tl»o  snail,  which  thus  bccoino  enormously  distended.  The  club-shaped 
structures  so  fornied  are  al)Uiidantly  supplied  with  luusele-libres,  and  by 
rif?orous  inoveinents  tiually  burst  the  distended  wall  of  the  tentacle,  and 
separating  from  tiie  Sporoeyst  fall  to  the  j?round.  There  they  move 
al)out,  resembling  an  insect  larva  in  general  appearance,  a  resemblance 
increased  by  banded  markings  of  green  and  white,  which  render  Ihem  very 
conspicuous,  and  they  are  apt  finally  to  be  snapped  up  by  some  bird,  in 
whose  digestive  tract  the  young  Distomcs  are  set  free  and  become  mature. 
There  ca*»  be  little  question  but  that  the  simple  metamorphosis  of  the 
l\>/i/stoinec'  represents  the  original  metlio<l  of  development  of  the  Trematoda, 
the  heterogony  characteristic  of  most  J)iMomeie  being  a  secondary  acquisi- 
tion developed  in  accordance  with  their  end«)parasitism.  An  idea  of  the 
mode  in  which  this  alternation  of  generations  has  been  brought  about  is 
furnished  by  such  forms  as  GyroUacti/lit^,  in  which  the  development  of  the 
ovum  takes  place  within  the  body  of  the  parent,  the  young  in  their  turn 
developing  embryos  before  being  born  (see  p.  147).  This  acceleration  of 
sexual  maturity,  accompanied  by  parthenogenesis,  has  brought  about  the 
condition  seen  in  the  Spoiocyst  or  liedia,  which  are  embryos  provided  with 
ova  capable  of  parthenogenetic  development.  Thus  fundamentally  the 
heterogony  is  a  {)a)dogenesis  (see  p.  00),  and  may  be  compared,  in  a  gen- 
eral way,  with  the  formation  of  a  hydroid  colony  by  the  budding  of  a 
medusa  larva. 

III.  Class  Cestoda. 

Like  the  Trematoda  the  members  of  this  class  are  para- 
sites, but  are  throuj^hout  eiulopurasiteH,  ami  present  a  much 
greater  ilegeueratioii  of  structure  thau  is  found  in  the  Disto- 
mejc,  accompanied  by  peculiarities  of  development  differing 
somewhat  from  what  occurs  in  these  forms.  The  Cestodes 
or  Tapeworms  lack  all  trace  of  a  digestive  tract  and  of  a 
mouth,  living  in  their  mature  state  attached  to  the  wall  of  the 
digestive  tract  of  tlieir  host,  and  immersed  in  the  nutritive 
fluids  contained  in  the  intestine. 

In  some  Torms,  sucli  as  Qtryophylln'us  (Fig.  81,  A),  para- 
sitic in  the  intestine  of  Cyprinoid  fishes,  the  similarity  to  a 
Tretnatode  is  very  striking,  except  in  the  absence  of  suc-kers 
for  adhesion  and  of  a  digestive  tract.  The  worm  consists  of  a 
somewhat  dihited  head,  succeeded  by  a  narrower  portion  which 
may  be  termed  a  neck  and  gradually  enlarges  to  the  rather 
cylindrical  body,  which  contains  a  single  set  of  reproductive 
organs.  In  Ligida,  which  is  found  in  the  intestine  of  aquatic 
birds,  there  is  likewise  an  al>sence  of  suckers,  but  the  repro- 


I 


i 


TYPE  PLATYIIELMINTUKS. 


153 


i 


I  " 


(liictive  organs  are  present  in  several  sets  succeeding  one  an- 
other, witliout  any  external  inilioatious  of  a  reduplication  of 
parts.  In  Truvnophorm  the  multiplication  of  the  sets  of  re- 
productive organs  is  indicated,  however,  externally  by  indis- 
tinct constrictions  of  the  body,  an  indication  of  a  tendency 


Fig.  81. — A,  CarycphyllmuH  vuitabili»  (after  Stkin)  ;  B,  Tcrnia  saginata  (after 

Leucraht)  ;  C,  anterior  end  of  T.  HuginaUi  (after  Lkuckakt). 

od  =  oviduct.  rd  —  vas  duferviis. 

ov  =  ovary.  vi  =  yolk-glauds. 

t  =  testis.  vid  =  yolkduct. 

V8  =  vesiculn  suininalis. 

for  the  individual  to  separate  into  a  number  of  parts,  each 
])ossessing  a  certain  amount  of  individuality.  This  tendency 
reachas  its  highest  development  in  such  forms  as  lioihrio- 
cephdbis  and  Tamia  (Fig.  81,  /i),  which  consist  of  an  anterior 
portion,  the  Scolex  (Fig.  81,  (>),  provided  with  organs  of  adhe- 
sion in  the  form  of  suckers,  accompanied  or  not  by  chitiuous 


154 


IN VERTEliliA  TE  MOlcPUOLOQ  Y. 


hooks  and  followed  by  a  varying  number  of  segments  or  pro- 
(jlottides,  each  possessing  a  set  of  reproductive  organs  and  cap- 
able of  separating  from  its  fellows,  maintaining  for  a  time  an 
independent  life.  The  proglottides  towards  the  hinder  end 
of  the  chain  or  slrobilu  are  the  most  advanced  in  development, 
and  one  after  another  dro[)  oil'  and  pass  to  the  exterior  of  the 
host's  body  with  the  faeces ;  more  anteriorly  the  proglottides 
are  sexually  immature,  and  still  nearer  the  scolex  they  are  to 
be  found  in  various  stages  of  formation.  In  fact  the  hinder 
end  of  the  scolex  may  be  regarded  as  a  zone  of  growth,  new 
])roglottides  being  successively  formed  at  this  region.  The 
process  of  proglottid  formation  resembles  not  a  little  what 
has  been  described  as  the  non-sexual  re])r()duction  of  the  Dis- 
comedusa},  the  scolex  corresponding  to  the  ])arent  Scyphos- 
toma  and  the  proglottides  to  the  Ephyra%  the  entire  aggre- 
gation in  both  cases  being  termed  the  Strobila. 

The  exterior  of  the  body  of  a  Cestode  is  formed  by  a  cuti- 
cle without  any  trace  of  cellular  structure,  and  is  perhaps  to 
l)e  regarded  as  a  basemeut-nif-mbrane,  the  ectoderm,  originally 
present,  having  disappeared.  The  cuticle  varies  much  in 
thickness,  and  is  throughout  traversed  by  fine  pores  which 
allt)W  of  the  absorption  into  the  body  substance  of  the  nutri- 
tive fluids  in  which  the  Tapeworm  lives,  either  directly  or 
by  permitting  the  passage  to  the  exterior  of  tine  protoplasmic 
])rocesses  from  the  subjacent  tissue.  Special  developments 
of  the  cuticle  in  the  form  of  chitinous  hooks  are  frequently 
present,  arranged  in  some  Tjenias,  for  example,  in  a  double 
circle  upon  a  prominence,  the  rosteUum,  at  the  apex  of  the 
scolex,  and  forming  a  very  efficient  means  of  attaching  the 
worm  to  the  wall  of  the  intestine  of  its  host.  Beneath  the 
cuticula  there  is  to  be  found  a  very  thin  muscular  layer,  the 
peripheral  musculature,  but  the  main  bulk  of  the  muscula- 
ture consists  of  those  tibres  which  traverse  the  parench3'ma. 
These,  especially  the  longitudinal  and  transverse  ones,  are 
massed  into  strong  bauds,  the  former  lying  usually  exterior 
to  the  latter,  and  both  enclosing  a  central  mass  which  is  trav- 
ersed by  weaker  bundles  of  dorso- ventral  muscles,  and  con- 
tains the  reproductive  apparatus. 

lu  couuectiou  with   the  muscular  system  may  be  men- 


TYPE  PLATYIIELMINTIIES. 


155 


tioned  the  suckers  which  frequently  occur  upon  the  scolex, 
and  serve  with  the  hooks,  when  these  are  present,  to  attach 
the  pai'asite  to  its  host.  In  Tivnia  these  suckers  are  four  in 
number,  and  have  the  form  of  eircuhir  depressions  whose 
walls  are  richly  supplied  with  niuscle-tibres,  while  in  Bothrio- 
cephahts  they  have  the  form  of  elongated  grooves,  situated  on 
the  edges  of  the  somewhat  flattened  scolex. 

As  might  be  expected  from  the  great  development  of  the 
muscles,  a  well-detiued  nervous  system  is  present.  It  consists 
of  a  brain  lying  imbedded  in  the  tissues  of  the  anterior  por- 
tion of  the  scolex,  evidently  composed  b}'  the  union  of  two 
ganglionic  masses  and  giving  rise  to  two  main  nerve-cords, 
which  pass  backwards  through  the  entire  length  of  the 
strobila  without  interruption  (Fig.  82,  n).  So,  too,  the  excre- 
tory system  (Fig.  82,  we)  extends  through  the  entire  strobila 
uninterruptedly.  It  consists  of  two  uephridial  tubes,  which 
in  the  anterior  part  of  the  scolex  may  be  united  by  a  cross 
branch,  as  they  are  at  the  posterior  edge  of  each  proglottid, 
and  open  to  the  exterior  by  a  pore  situated  at  the  centre  of 
the  posterior  edge  of  the  last  proglottid.  As  each  proglottid 
separates  from  the  chain,  a  new  pore  forms  in  the  one  pre- 
ceding it,  which  becomes  the  terminal  one,  so  that  an  opening 
for  the  system  is  always  present. 

Tbe  reproductive  system  (Fig.  82)  possesses  a  complexity 
similar  to  what  has  been  described  for  the  Trematoda,  and 
hermaphroditism  prevails  throughout  the  class.  In  the  stro- 
bilar  Cestodes  each  proglottid  contains  a  complete  set  of 
organs,  both  male  and  female ;  the  testes  (Fig.  82,  fe)  are 
usually  very  numerous,  consisting  of  small  spherical  masses 
scattered  through  the  i)arenchyma,  each  being  provided  with 
a  small  duct,  which  after  a  short  course  unites  with  similar 
ducts  coming  from  other  testes,  all  finally  uniting  to  a 
single  vas  deferens  ('"</),  which  opens  to  the  exterior  after 
passing  through  a  muscular  organ,  the  cirrus-sac,  by  the  con- 
traction of  which  its  terminal  portion,  often  provided  on  its 
inner  surface  with  barbed  hooks,  is  protruded  to  the  exterior 
as  an  iutromittent  organ  or  cirrus  (cj.  The  female  apparatus 
varies  somewhat  in  its  arrangement.  In  the  majority  of  forms 
the  ovary  is  a  bilobed  organ  (ov),  lying  near  the  i)osterior 


I 


166 


IN  VERTEBRA  TE  MORPIIOLOG  Y. 


I 


end  of  the  proglottid.  The  oviduct  soon  after  leaving 
the  ovary  unites  with  the  yolk-duct  {vid)  coming  from  the 
albuminous  vitellarium  (m),  which  consists  of  a  number  of 
glands  scattered  through  the  parenchyma  similarly  to  the 
testes.     At  the  point  of  union  with  the  yolk-duct  the  oviduct 


Fig.  82.— Proglottid  op  Taenia  filicollis  (after  Krakmbr). 
e  =  cirrus.  te  =  testis. 

n  =  uerve.  ut  =  uterus. 

ne  =  excretory  canal.  va  =  vagiua. 

ov  =  ovary.  vd  =  vas  duferens. 

eg  =  sliell-gland.  vi  =  yollt-gland. 

vid  =  yolk-duct. 

enlarges,  receiving  at  the  enlargement  the  secretion  of  a  num- 
ber of  unicellular  glands  composing  the  shell-gland  {mj). 
From  this  enlargement  two  tubes  arise  :  one,  the  vagina  (va), 
runs  almost  d'vectly  forward  to  open  into  a  chamber,  the 
genital  atrium,  which  contains  also  the  cirrus-sac  and  com- 
municates directly  with  the  exterior;  while  the  other,  the 
uterus  {ut),  after  a  somewhat  convoluted  course  opens  inde- 


TYPE  PLATYUELMINTIIEa. 


157 


■ 


pendently  to  the  exterior  a  little  bebiud  the  genital  pore. 
The  vugiua  serves  as  a  duct  for  the  spermatozoa  during 
copulation,  and  corresponds  with  the  canals  opening  at  the 
sides  ot  the  anterior  end  of  the  body  in  the  Polystomere  (see 
p.  146),  while  the  uterus  serves  for  the  retention  of  fertilized 
and  mature  ova.  In  Bothriocephalus  the  opening  of  the  genital 
pore  is  in  the  middle  line  of  one  of  the  surfaces  of  the  })ro- 
glottid ;  in  Tctnia,  however,  it  occurs  on  the  lateral  margin  of 
the  proglottid,  and  in  some  cases  each  proglottid  may  have  an 
opening  in  each  of  the  lateral  margins,  there  being  a  duplica- 
tion in  such  cases  of  the  genital  ducts.  Furthermore,  in  the 
Ttenias  the  vitellarium  is  much  less  voluminous  than  in 
Bothriocephalus ^  and  produces  an  albumen-like  secretion  in- 
stead of  yolk-cells,  and  in  addition  the  uterus  has  no  special 
opening  to  the  exterior  and  is  relatively  small,  though  it  may 
become  fairly  voluminous  by  its  walls  being  pushed  out  into 
pouch-like  sacculations  by  the  contained  ova. 

Uevdopment  of  the  Cestoda. — Accompanying  the  differences 
iu  the  arrangement  of  the  reproductive  apparatus  ditterences 
in  the  development  are  found  in  the  two  groups.  In  Hothrio- 
cephalus  (taking  this  form  again  as  the  example  of  the  one 
group)  the  egg  is  richly  provided  with  yolk-cells,  among 
which  the  germ-cell  lies  imbedded.  The  embryo  leaves  the 
egg  in  the  form  of  a  spherical  ciliated  body,  provided  with 
six  chitinous  hooks,  arranged  more  or  less  distinctly  in  j;airs. 
After  swimming  about  for  a  time  the  cilia  and  their  cells  are 
thrown  off,  and  the  six-hooked  embryo  makes  its  way  into  the 
body  of  the  first  host,  where  it  becomes  enclosed  in  a  thin 
cyst,  within  which  it  develops  directly  to  a  scolex.  If  this 
be  swallowed  by  the  second  host,  the  worm  fastens  itself  to 
the  walls  of  the  digestive  tract,  and  soon  develops  to  the 
sexually  mature  strobila. 

In  the  Tienias,  however,  the  ova  are  much  smaller,  the 
yolk-cells  being  replaced  by  an  albumen-like  substance, 
relatively  small  in  amount,  and  the  embryos  when  they  hatch 
out  are  destitute  of  cilia,  resembling  the  six-hooked  embryo 
of  Bothriocephulm  after  it  has  lost  its  ciliated  covering  (Fig. 
83,  A).  In  this  condition  it  makes  its  way  into  the  prinmry 
host,  in  whose  tissues  it  becomes  encysted,  and  develops  in 


ir)8 


IN  VEliTEBltA  TE  MOIiPIIOLOG  Y. 


'  1 

I 


some  forms,  such  as  the  Ta-nia  cucumerina  of  the  dog,  whose 
primary  host  is  the  Dog-tick,  into  a  Cysticercoid.  This  resem- 
bles a  scolex,  wliose  head  has  been  witlidrawu  into  and  enchased 
by  the  body.and  when  it  is  swaUowed  by  tlie  secondary  host,  the 
dog  in  the  instance  cited,  the  head  is  pushed  out,  fastens  itself 
to  the  wall  of  the  digestive  tract,  and  begins  to  grow  and  form 
l)r<)glottides.  In  other  cases,  however,  the  posterior  part  of  the 
scolex  into  which  the  head  is  retracted  becomes  enormously 
swollen  by  the  accumulation  of  fluid  within  it,  forming  a  large 
vesicle,  into  the  interior  of  which  the  head  projects,  having 
become   completely   invaginated.      Such   a   form   as   this   is 


Fio.  83. — A,  six-liooked  embryo  of  Tcenia ;  li,  ditiii^rain  of  CysUcercua ;  G, 
diugrain  of  Cwnurus;  U,  diagmiu  of  Eckinococcus. 

termed  a  Cysticerctis  (Fig.  83,  B) ;  when  it  is  swallowed  by  the 
secondary  host  the  head  evaginates,  and  the  cyst  remains  for 
some  time  attached  to  the  hinder  end  of  the  scolex,  but  later 
disappears,  and  the  formation  of  the  proglottides  occurs. 
Further  modifications  arise  by  the  formation  in  the  wall  of  the 
cyst  of  not  only  one  but  several  invaginated  heads,  forming 
the  Coemtrm  (Fig.  83,  C) ;  or  even  secondary  cysts  may  arise 
from  the  inner  wall  of  the  original  vesicle,  and  each  of  them 
may  develop  several  heads,  forming  what  is  known  as  the 
Echinococcus  (Fig.  83,  D). 

Several  of  the  Cestoda  are  especially  interesting  from  a  medical  stand- 
point, inasmuch  as  they  arc^  para.sitic  in  man  either  during  the  adult  or  the 


TYPE  PLAT rJlKLMlIf TUBS. 


159 


larval  stage.  Among  these  may  be  mentioned  Bothritvephalus  lattis,  which 
occurs  in  the  human  intestine,  where  it  nuiy  reach  a  length  of  as  much  as 
12  metres,  in  such  cases  consisting  of  many  tliousaiul  proglottids.  These 
may  readily  be  recognized  by  tlie  convolut»!d  uterus,  and  by  the  open- 
ings of  the  reproductive  organs  on  the  median  Mne  of  one  of  the  flat  sur- 
faces, wiiile  the  head  is  characterized  by  being  flattened,  and  provided 
on  the  margin  with  two  elongated  suckei-s.  The  ova  give  rise  to  a  ciliated 
larva  which  becomes  transfornuul  into  the  six  hooked  embryo,  this  latter 
making  its  way  into  the  tissues  of  certain  fish,  wliieh  serve  as  the  first  host. 
Man  becomes  infected  with  the  worm  by  eating  improperly  cooked  or  .salted 
tish,  tiu;  Pike  being  the  more  usual  primary  host,  tiioiigh  this  part  may  also 
be  played  by  other  forms. 

The  genus  Tanki  furiii.shes  two  human  i)arasites.  The  genus  is  char- 
acterized by  the  head  being  provided  with  four  circtdar  suckers,  as  well  as 
in  some  cases  with  one  or  more  crowns  of  hooks  ;  tlie  genital  ])ore  is  situ- 
ated upon  the  margin  of  the  proglottids,  and  tlie  uterus  is  a  straight  tul)e 
with  a  varying  number  of  lateral  transverse  pouches. 

Tania  .•iolinni  is  by  far  the  most  frecjueiit  tapeworm  of  man,  and  may 
reach  a  length  of  8-3^  metres,  and  consist  of  800-900  j)roglot tides.  Tlie 
head,  in  addition  to  the  four  suckers,  is  provided  with  a  rostellum  bearing 
a  double  crown  of  from  20-28  hooks.  The  proglottids  are  about  .'>  mm. 
broad  and  10-12  ram.  long,  and  the  uterus  has  7-M  stout  lateral  pouches. 
The  Cyst  ice  t'ru,s  state  of  this  worm  occurs  in  the  nniscles  of  i>igs,  whence 
man  becomes  infected  by  eating  improperly  cooked  or  salted  pork.  It 
measures  8-10  mm.,  and  possesses  when  imbedded  in  tin*  mu.scjes  an  ellip- 
tical shape,  its  long  axis  being  parallel  with  the  long  axis  of  the  nniscle 
fibres.  In  addition  to  its  occurrence  in  swine  muscle,  however,  it  has  also 
been  found  occasionally  in  man,  encysted  in  the  mu.scles,  brain,  or  eye. 
The  source  of  the  infection  of  man  is,  in  many  cases  at  least,  cre.ss,  lettuce, 
and  such  articles  of  food  which  have  been  watered  with  li<|uid  manure 
containing  the  fertilized  ova  of  the  worm.  The  six-hooked  embryo  encysis 
itself  in  the  tissues  named,  and  man  becomes  the  intermediate  host  of  the 
worm. 

Tanfa  sayhiata,  also  known  as  T.  medioeanellata,  is  of  less  frerpient 
occurrence  than  T.  solium,  from  which  it  is  easily  distinguished  by  its 
greater  length,  7-8  metres,  and  by  the  greater  number  of  pj-oglottids. 
1200-i;W0.  The  head  has  no  rostellum  or  crown  of  hooks,  and  the  ]>ro- 
glottids  are  recognizable  by  their  size,  measuring  5-7  mm.  in  breadth  an<l 
18-20  mm.  in  length,  and  also  by  the  lateral  l)ranches  of  the  uterus  being 
slender  and  20  or  30  in  number.  The  Oysticercus  occiu's  in  the  muscles  or 
occasionally  in  other  organs  of  cattle,  improperly  cooked  beef  being  the 
source  of  infection  for  num. 

In  addition  to  being  occasionally  the  intermediate  host  of  T.  solium, 
man  may  also  be  the  host  of  the  Kchinococcus  of  T.  echiuocoams,  a  small 
worm  alMiut  4  mm.  in  length  and  with  only  three  proglottids,  which  occurs 
in  its  adult  state  iu  the  intestine  of  the  dog.     The  ova  may  be  received  into 


UIO 


IN VKHTEBRA  TE  MOIiPHOLOO  T. 


the  human  digest  ive  tract  by  fondling,  and  especially  by  kissing,  infected  pet 
dogs,  and  the  six-hooked  embryo  maizes  its  way  to  the  liver,  lungs,  brain, 
or  other  organs,  where  it  becomes  encysted,  producing  tumors  which, 
e.s|)ecially  in  the  liver,  may  reach  a  great  size  and  a  weight  of  from  10  to, 
ill  some  cases,  15  kiiogra mines. 

Domestic  animals  arc  also  apt  to  be  infected  with  Ccstodes  in  addition 
to  those  already  mentioned,  occasionally  with  fatal  results.  This  is  es- 
pecially the  case  with  sheep,  in  whose  intestine  T.  expausa  may  develop  in 
such  numbers  as  to  occlude  the  lumen,  and  cause  death,  especially  in  young 
iambs.  A  Coenurus  also  occurs  occasionally  encysted  in  the  brain  of  sheep, 
producing  a  disease  known  from  its  symptoms  as  the  "staggers,"  which 
may  likewise  result  fatally. 

The  Relatioitships  of  the  Cestodes. — In  considering  the  affinities  of  the 
Cestoda,  the  nature  of  the  strobila,  so  far  as  its  individuality  is  concerned, 
must  be  inquired  into.     Two  views  upon  this  point  are  open.     The  older 
one  regards  the  Cestode  as  a  colony,  considering  each  proglottid  an  indi- 
vidual equivalent  to  the  scolex,  and  the  process  of  strobilation  one  of 
reproduction  by  budding.     On  this  view  the  strobila  is  exactly  comparable 
to  the  Scyphostoma  strobila,  the  scolex  corresponding  to  the  Scyphostoma 
base  and  the  proglottids  to  the  Ephyrse.     There  is  undoubtedly  much  to  be 
said  in  favor  of  such  a  view  which  regards  the  reproduction  of  the  Cestoda 
as  a  process  of  alternation  of  generations,  but  at  the  same  time  it  must  be 
recognized  that  the  buds  or  proglottids  are  not  reduplications  of  the  parent 
bud  as  is  the  case  with  Microstoma,  where  the  budding  individual  has  the 
adult  form.    In  the  Scyphostoma  strobila  the  buds  do  differ  from  the 
parent  which  gives  rise  to  them ;  but  the  Scyphostoma  is  a  larva  which 
gives  rise  by  budding  to  the  adult  form,  and  is  comparable  rather  to  the 
Cysticercus  than  to  the  scolex  of  the  Cestode.     Non-strobilating  Scyphos- 
tomas  become  medusa^,  but  the  scolex  never  becomes  a  proglottid,  and 
the  latter  cannot  be  considered  the  terminal  stage  of  the  life-history  in  the 
same  sense  as  a  medusa  is.    The  nervous  system  of  the  entire  Cestode 
strobila  centres  in  the  brain  of  the  scolex,  the  various  proglottids  never 
developing  independent  brains,  the  reproductive  organs  being  practically 
the  only  organs  which  are  reduplicated  in  successive  buds. 

According  to  the  second  view  the  strobila  is  an  individual,  and  the 
strobilation  is  regarded  as  a  culmination  of  the  reduplication  of  organs 
seen  in  many  forms,  but  more  especially  in  the  Nemerteans  (q.v.).  This 
view  receives  strong  support  from  the  occurrence  of  such  forms  as  Cari/- 
ophyllveus,  JAi/ula,  and  TricBuophorus,  described  on  a  preceding  page,  in 
which  may  be  seen  successive  gradations  of  strobilation,  beginning  with  a 
simple  reduplication  of  the  reproductive  apparatus  in  Lignla,  this  redupli- 
cation being  accompanied  in  Trice nophorns  by  a  tendency  for  the  body  to 
constrict  into  parts,  each  of  which  contains  one  of  the  sets  of  reproductive 
organs. 

The  choice  between  these  two  views  hinges  upon  the  question  of  indi- 
viduality.   The  individuality  of  either  Lhjnlu  or  Triiinophorus  can  hardly 


I  i    I 


TYPE  PLA  TYIIEI.MISTUES. 


161 


be  questioned,  and  there  is  no  reason  for  rej,'jirdin<;  a  T<nii<i  for  instance 
as  an  individual  l)elonginfj  to  a  liigher  grade  than  eitiier  of  these — a  view 
which  the  first  and  older  tlieory  implies,  since  it  regards  a  Tanin  as  a 
colony  of  eijiiivalent  individuals.  Such  a  form  as  Cunjoiihijlhriis  is  an 
aggregate  of  individualities  of  a  lower  grade,  organ-individuals  ;  and  just 
as  the  cell-individuals  composing  these  may  divide,  so  the  organs,  or 
rather  the  embryonic  masses  of  cells  destined  to  give  risf  to  I  horn,  may 
bud,  producing  a  reduplication  of  organs.  This  reduplication  may  occur 
in  one  or  more  organs  ;  in  the  Aea'la  among  the  Turbellaria  it  att'ects  only 
the  testes,  in  the  Alloiocoila  it  affects  both  ovaries  (the  vitellaria  being 
originally  parts  of  the  embryonic  mass  which  gives  rise  to  the  ovaries)  and 
testes,  and  in  the  lihabdoca'la  it  affects  only  the  ovaries.  In  theCestodes 
the  entire  reproductive  apparatus  is  reduplicated  in  this  niannei,  a  scries 
bci.ig  produced,  and  secondarily  a  tendency  for  each  member  of  the 
series  to  be  capable  of  separation  from  its  fellows  has  come  about  owing  to 
the  greater  certainty  it  gives  for  the  perpetuation  of  the  species.  A  cer- 
tain amount  of  individuality  of  the  progloftids  is  thus  brought  about,  but 
at  the  same  time  the  process  of  strobilation  cannot  accurately  be  termed  a 
process  of  non-sexual  reproduction  by  budding,  since  the  proglottid  indi- 
viduals are  not  quite  of  the  same  grade  of  morphological  individuality  as 
CaryopliyllcEUS,  which  the  seolex  represents.  Both  views  are  correct  to  a 
certain  e.xtent  :  the  strobilation  is  a  budding  off  of  individuals  from  the 
seolex,  but  of  individuals  of  a  lower  grade  ;  and  the  entire  strobila  is  in 
reality  an  individual  comparable  to  Caiyophyllipus  or  a  Trematode. 

Considering,  then,  the  strobila  as  a  metamere-individual,  what  are  the 
affinities  of  the  Cestodes  ?  They  seem  to  have  l^een  derived  from  Trema- 
todes,  the  simpler  forms  without  I'eduplication  of  the  reproductive  organs 
being  capable  of  being  regarded  either  as  Trematodes  without  a  digestive 
tract  or  as  Cestodes  without  any  indications  of  strobilation.  If  this  be  true, 
indications  of  their  affinity  should  appear  in  the  life-history  in  accordance 
with  the  biogenetic  law.  One  interesting  form  deserves  mention  in  this 
respect — Archigetes,  which  occurs  in  certain  Annelids.  It  is  a  Cestode 
without  reduplication  of  organs  and  provided  with  a  tail,  similar  in  a  gen- 
eral way  to  that  of  a  Cerearia.  Certain  facts  in  its  life-history  seem  to 
indicate  that  Archiyetes  is  comparable,  not  to  an  adult  Cestode,  but  to  a 
Cysticercus  which  has  become  sexmxUy  mature,  and  it  migl  •  be  expected 
that  similarities  to  the  Trematode  Cerearia  might  Ixi  fow  in  Cysticerci. 
Recently  such  similarities  have  been  shown  to  exi^  «rtain  Cystieer- 

coids :  a  tail-like  appendage,  which  later  separai.  ^  und  degenerates,  has 
been  described  as  occurring  at  this  stagt;  of  (he  development ;  and  fin-tlMsr- 
more  it  has  been  suggested  that  the  cavity  of  the  (\vsticercoid  into  which 
the  head  is  invaginated  maybe  equivalent  to  the  Trematode  intestine,  later 
on  becoming  obliterated  by  the  growth  of  the  parenchyma.  The  evidence 
at  present  available  points,  then,  to  a  derivation  of  the  Cestode  from  the 
Trematodes,  and  from  Trematodes  in  which  the  Oeroaria-stage  had  already 
been  established. 


1G2 


7iV  VEHTEBHA  TE  MOJiPUOLOG  7. 


:    ( 


— I-  CO 


IV.  Class  Nemestina. 

The  three  preculiu^'  classe.s  show  luurked  evideuces  of 
jj;eiietic  attiuity,  tlie  chunictei  istic  ditt'ereuces  of  Htructuie  iu 
the  Treiiiiitode.s  uiid  (Jestoiles  beiujj;  due  to  the  paiusitic 
liuhits  of  these  forms.  The  Neiuerteaus,  ou  the  other  hand, 
tliouj:[h  appaieutly  traciu*^  dt'sceut  from  a  Turbelhiriaii-Hke 
ancestor,  show  a  marked  advance  in  structure,  and  must  be 
regarded  as  organisms  of  a  considerably  higher  grade  thuu 
the  otlier  Phityhehninths. 

They  are  for  the  most  part  marine,  though  a  few  forms 

inhabit  fresh  water  or  even  damp  earth, 

and    are    usually    elongated   ribbon-like 

"«    forms,  reaching  a  length,  iu   some  cases, 

of   several  centimetres.    The  body  is  ex- 

ternally  unsegmented,  though  a  more  or 

less  marked  metamerism  of  the  internal 

organs,  due  to  their  repetition  at  definite 

intervals,  may  be  present.     The  ectoderm 

of  the   body  resembles  that  of  the  Tur- 

bellaria  in  being  throughout  ciliated,  and 

rests  upon  a  basement  membrane,  which 

in   some   cases   contains   cells.      AVithiu 

the  membrane  are  a  varying  number  of 

muscle    l.'iyers,    differentiations     of    the 

outermost   portions  of   the    mesodermal 

tissue,  which  iu  the  form  of  a  parenchyma 

Occupies  the  interval  between  them  and 

Fn;.84.-ANTKRi<mPon-  ^^^^  digestive  tract.     This  (Fig.  84,  d)  is 

TioN  «>F  ISEMKKTKAN.    au  ahuost  straight  tube,    except   iu    the 

ce  =  cciobml  ganglion,    jrenus  MuhcohhVa,  and  is  pushed  out  ou 

eg  =  Hliafed  funnel.        ^^^j^  ^-j^  j,j^^    sac-like    pouches,    which 

d  =  intestine.  .  ' 

oe  =  tyi's.  ^^'^    arranged    in    some    cases    with     a 

ov  =  ovary.  regularity    of    succession    almost   meta- 

jir  =  proi)o.sci8.  meric.      It  opens  to  the  exterior  at  the 

anterior  end  of  the  body  by  the  mouth, 
a  short  non-muscular  oesophagus  inter- 
vening bet'veen  the  iutestiue  proper  and  that  opening ;  and 
at  the  othor  end  of  the  body  is  a  second  communication  with 


rju 


ov 


rm  =  retractor  muscle  of 
proboscis. 


TYPK  PLA  TYllKLMIi\THKS. 


i(;3 


the  exterior,  the  nnm,  au  openinf»  uurepresentod  in  othor 
Phityht'Iiiiinths.  The  tlij^eHtive  tnict  is  iio  hmj^er  h  blind 
sue,  but  has  the  form  of  a  tube,  as  in  all  the  liij^ht'r  types. 

In  the  anterior  end  of  the  body,  above  tiie  di<^<'stive  tratit, 
is  a  structure,  the  prohoscis  (Fig.  84,  pr),  «>ss<intially  ptundiar 
to  the  Nenierteaus,  although  indications  of  such  an  organ  are 
to  be  found  in  the  UhuMiHwhi.  It  consists  of  a  closed  tube, 
the  pwhoscifi  tilieath,  with  njuscuiar  walls,  imbedded  in  the 
body  pareuchynni  and  extending  backwards  in  some  cases 
almost  to  the  end  of  the  body,  and  within  it  lies  the  proboscis, 
also  a  tube,  united  to  the  wall  of  tin;  sheath  near  its  anterior 
end  and  in  fact  closing  it  at  that  region.  From  this  liuo  of 
attachment  the  ))robosci,.  stretches  back  in  the  cavity  of  the 
sheath,  the  space  between  it  and  the  walls  of  the  sheath  being 
tilled  with  fluid.  It  is  a  simple  invagination  into  the  cjivity 
of  the  sheath  of  the  external  body-wall,  whose  musculature 
as  well  as  ectoderm  are  continuous  with  that  of  the  proboscis. 
From  the  tip  of  the  invagination  a  baud  of  muscle  fibres, 
forming  the  retractor  muscle  (rm)  of  the  j)roboscis,  passes  to 
the  wall  ot  the  bod}'.  By  the  contraction  of  the  muscular 
walls  of  the  sheath  the  fluid  contained  in  its  cavity  forces  the 
])roboscis  to  be  evaginated  sometimes  with  sufficient  force 
to  tear  itself  loose  from  its  line  of  attachment ;  but  should 
this  accident  not  happen,  the  proboscis  can  be  reiuvaginated 
by  the  contraction  of  its  retractor  nmscle.  The  function  of 
this  organ  is  doubtful.  In  some  cases  it  is  undoubtedly  a 
weapon  of  ofience  and  defence  ;  but  it  seems  not  improbable, 
from  its  rich  nerve-suj)ply  and  from  the  probable  function  of 
its  prototype  in  the  lihabdoccela,  that  in  some  cases  at  least 
it  may  be  a  tactile  organ. 

A  well-developed  nervous  system  is  always  present,  though 
it  may  show  in  some  cases,  as  CaritwUd,  the  })rimitive  char-icter 
of  being  still  imbedded  in  the  ectoderm  or  else  lying  immedi 
ately  beneath  it.  In  other  cases,  however,  as  ('erchrnlnhi.s,  it 
is  enclosed  in  the  muscles  of  the  body-wall  or  may  even  be 
completely  within  them,  imbedded  in  the  parenchyma.  It 
consists  in  its  most  usual  form  of  two  ganglionic  masses 
(Fig.  84,  ce)  from  which  short  nerves  pass  forwards  and  which 
are  united  by  two  transverse  commissures,  one  of  which  passes 


164 


INVmiTKBRATE  MOliPHOLOaT. 


I    f 


over  or  iu  front  of  the  (i>.sopliu^uH,  while  the  other  arches 
from  Olio  ^uii^lioiiic  muss  to  the  other  over  tlio  proboscis 
nheuth.  Each  {^aiigliou  is  bih>bo(1,  the  siiiullur  posterior 
lobe  beiii|j;  iu  soiiu)  cases  uuited  to  the  larger  «)iio  by  a  rela- 
tiv(>ly  thiu  baud  of  uerve-tissue  so  that  it  ai>pears  to  be 
almost  imlepemleut.  From  tlie  larger  lobe  of  each  siile  a 
nerve-cord  passes  towards  the  posterior  eud  of  the  body, 
where  the  two  may  unite  to  form  au  arch  i)assiiig  over  the 
posterior  i)art  of  the  iutestiue.  Iu  addition  to  these  a  third 
uerve  origiiiatiug  from  the  commissure  ])assing  over  the  pro- 
boscis sheath  aud  ruiiuiug  backwards  iu  the  mediuu  dorsal 
liiH'  is  frequently  present  as  well  as,  iu  some  forms,  another 
uerve  runuiug  along  the  dorsal  wall  of  the  proboscis  sheath, 
to  which  it  sends  branches.  In  many  forms,  such  as  Cerehrat- 
tdm,  a  tine  jilexus  of  uerve-tissue,  lying  between  the  muscle 
layers  of  the  body-wall,  unites  the  three  main  nerve-cords, 
some  of  the  strands  of  the  plexus  being  sometimes  larger 
than  the  others  and  forming  circular  commissures  between  the 
nerve-cords ;  in  Tetrastemmd  and  Ainphi'porits,  for  example, 
these  circular  commissures  may  be  strongly  developed  aud 
have  au  almost  metameric  arrangement,  the  general  plexus 
being  in  sudi  cases  wanting. 

E^'es  (Fig.  84,  oc)  are  present  in  some  forms  occasionally 
iu  considerabh;  numbers,  but  are  frequently  wanfing,  and  oto- 
cysts  occur  but  seldom.  The  lateral  ciliated  grooves  which 
occur  on  the  sides  of  the  head  of  some  lihabdoccjcla  reach  iu 
the  Nemerteaus  a  high  development  (erf),  in  some  forms,  e.g. 
Cerehrtituhis  and  l^etrnsffnnnn,  becoming  ciliated  funnels  of  some 
length,  whose  inner  onds  are  imbedded  in  the  substance  of  the 
]M>sterior  lobe  of  the  brain.  Au  olfactory  function  has  been 
assigned  to  these  organs,  though  some  authors  have  consid- 
ered them  mainly  respiratory. 

The  excretory  system  consists  of  a  lougitudiual  canal  (m 
each  side  in  the  auterior  portion  of  the  body,  sometimes  re- 
placed by  a  network  of  canals,  which  opens  to  the  exterior  by 
one  or  more  ducts  leading  to  pores  situated  on  the  margin  of 
the  body.  In  some  cases  these  lateral  ducts  and  the  pores 
may  be  quite  numerous  aud,  like  the  intestinal  ])ouches  and  the 
circular  nerve-coraraissures,  may  have  a  somewhat  metamerio 


T  rn/S  PL  A  T  YIIELMINTlIKa. 


165 


arraugemeut.  Tho  vurious  termiual  bruuclies  of  the  nepliridial 
tul)08  nro  cluh-HliuptMl  uiitl  closed,  a  flamn  of  vWwx  projecting; 
fr:)iii  tlie  closed  eii<l  into  the  lunieu  of  the  tulx'.  The  canals 
and  tubes  are  lined  with  ciliated  cells,  and  are  therefore  inter- 
cellular and  not  intracellular,  i.e.,  do  not  perforate  cells,  ditlVir- 
in^  in  this  respect  from  the  uephridia  of  other  IMaty hel- 
minths. 

The  blood-vascular  system  is  peculiar  to  the  Nenicrtea 
nmon<;  Platyhelminths,  and  consists  in  the  simi>le  forms,  such 
as  CarineUa,  of  two  lateral  vessels  which  anteriorly  open  into 
lacunar  spaces  without  definite  walls.  In  the  more  highly 
organized  forms,  however,  three  longitudinal  trunks,  two 
lateral  and  one  dorso-mediau,  are  ])resent  with  definite  and 
sometimes  muscular  walls,  and  unite  in  a  J_-shaped  manner 
at  the  posterior  enil  of  the  body,  while  in  front  they  may 
either  open  into  a  system  of  lacuna>,  or,  as  iu  Tetrastemma^ 
unite  with  each  other  as  they  do  posteriorly,  a  perfectly  closed 
system  thus  resulting.  Transverse  connecting  branches  be- 
tween the  dorsal  and  lateral  vessels  occur  in  regular  succes- 
sion, a  metamerism  being  again  suggested.  The  blood-vessels 
and  lacunte  contain  a  tluid  in  which  float  round  or  elliptical 
corpuscles,  which  in  sonn^  of  the  higher  forms  have  a  reil 
color,  due  to  the  presence  of  hfenioglobin.  No  heart  or 
special  contractile  organ  is  present,  the  Ijlood  being  driven 
through  the  vessels,  without  any  detinite  direction,  by  the 
movements  of  the  body. 


Tho  oc(!nrrnnce  of  a  blood-vasculiir  system  in  the  Nottierteans  and  its 
cliaract«r  in  \\w  lowest  members  of  tlie  f^roiip  siii^j^csts  a  mode  of  origin  for 
the  system  which  a^'rees  well  with  what  may  he  deduced  from  tMiibryolojjical 
ol)servalioMS  on  other  forms.  It  may  be  suppo.^sed  that  in  the  primitivo 
Nemerteans  a  system  of  spaces  tilli'd  with  lliiid  existed,  in  which  cells  derived 
from  the  parenchyma  floated.  Thest*  spaces  would  represent  a  simple 
ccelom,  and  were  lacunar  iti  character,  lackinj;  definite  walls,  the  cir- 
culation of  the  fluid  they  contained  bcinf;  very  irregular.  In  time  tho 
spaces  along  the  sides  of  the  body  might  arrange  themselves  iu  a  linear 
manner,  and  might  acquire  definite^  wall.s,  the  rest  of  tho  spaces  remaining 
hicunar,  when  a  condition  resembling  that  in  CarineUa  would  ensue,  tho 
arrangement  found  in  higher  forms  resulting  from  the  conversion  of  the 
remaining  lacunar  spaces  into  vessels  with  deflnite  walls. 

According  to  this  view  the  blood-vascular  system  is  to  be  regarded  as 


166 


INVEHTKliUA TE  MOliPHOLOG Y. 


in  reality  u  portion  of  tho  cceloni  separated  off  for  a  special  purpose,  and 
other  insljinces  bearing;  the  same  sijinitieanee  will  be  noticed  later. 

The  leproiliK^tivo  system  differs  from  that  of  Iho  other 
Platyheli!iii»ths  iu  its  much  greater  siiuplicity,  no  vitelhiria  or 
shell-gluiiil  beiiij^  present,  and  furthermore  the  Nemerteans 
are  ahuost  witliout  exception  of  separate  sexes.  Tlie  ovaries 
(Fij^.  84,  ov)  or  testes  an^  {)res(Mit  in  considerable  numbers, 
one  lying  iueach  interval  bt  tween  two  lattual  diverticula  of  the 
intestine,  so  that  they  j)artake  in  their  airraugement  of  tl\e 
more  or  less  pronounced  metamerism  of  that  organ.  Between 
the  intestine  and  tlu^  genital  masses  there  is  in  some  forms  a 
distinct  cavity,  or  co'lomic  space,  and  at  the  time  of  maturity 
a  separate  communication  with  the  exterior  forms  for  each 
ovary  or  testis. 

The  class  Nemertina  nuiy  be  divided  into  four  orders, 
whose  chief  characteristics  nuiy  bt^  briefly  stated,  having  been 
for  tho  most  part  already  described. 


1.  Order  Faleeonemertini. 

To  this  order  belong  the  genus  (Juriwlhi  and  allied  forms, 
all  characterized  by  structural  peculiarities  wliitrh  are  to  be 
reganhid  as  primitive.  The  lateral  ciliated  organs  are,  as  in 
the  llhabdocu'la,  nnue  grooves,  not  being  continiuMl  inwards 
to  the  brain  in  tlu^  form  of  a  funnel ;  and  furthermore  the 
n(^rvous  system  is  either  imbedded  in  the  ectoderm  or  lies 
ininHuliately  below  it.  To  these  characters  may  \n\  add(Ml  the 
more  ov  less  lacunar  nature  of  the  blood  vascular  syst(Mn,  and 
the  communication,  in  some  cases,  of  the  uephridia  with  it. 

2.   Order  Schizonemertini. 

In  the  Schizonemertini  the  ciliated  funnels  are  well  devel- 
oped, and  the  nervous  system  is  imbedded  in  the  muscular 
layers  of  tlu5  body-wall  ;  and  though  the  mu've-cords  are  still, 
as  in  the  preceding  order,  united  by  a  })lexus,  nevertheless 
there  are  indications  of  a  developnu^nt  of  commissural  con- 
necting nerves.  Tho  blood  vascular  system  is  still  lacunar 
anteriorly,  though  posteriorly  three  well-defined  vessels  are 
present.     The  genus  (Jcrehnitiilns  belongs  here. 


TYPK  PLA  TYIIELMINrilES. 


167 


3.  Order  Hoplonemertini. 

This  order,  which  iueludes  th(3  j^einna  THraattemma  aud 
AntphiporuN  lueutioned  ubovo,  lias,  liko  the  j)rectHliii}^  orch'r, 
ciliatod  I'uiuiels  as  lateral  organs,  and  tlio  uorvous  system  lies 
completely  withiu  the  muscular  layer  of  the  body-wall  and 
the  uerve-cords  are  united  by  transverse  commissures,  the 
plexus  being  wanting.  Tlu^  blood  vas<rular  system  is  Tu  closeil 
series  of  tubes,  not  communicating  with  lacunar  spaces.  The 
most  st)iking  characteristic  of  the  or(h;r  is,  howtiver,  the 
structure  of  the  probost^is,  which  is  armed  near  its  posterior 
(thjit  is,  while  ir.^ivginated)  end  by  one  or  more  dagger-lik*^ 
sj)ines  or  stylets.  'J'he  most  ]»osteiior  })ortion  is  not  capable 
of  being  (^vaginated,  and  its  walls  are  glandular,  secreting  a 
poisonous  fluid  which  is  })oured  into  the  more  anterior  por- 
tion of  the  tube,  bathing  the  stylets  and  thus  being  carritul 
into  the  wouiul  which  may  be  mad(»  l)y  the  forcibly  evagiuKtod 
})roboscis  with  the  stylets  coming  into  contact  with  the  body 
of  the  prey  or  enemy. 


\ 


4.  Order  Malacobdellina. 

This  order  contains  a  single  genus,  MnliiciilnlcVa^  which  is 
found  in  the  manthi-cavity  of  nnirine  Lamellibrjinchs,  such  as 
the  common  Mussel  and  CMam.  It  resign bles  the  lloploiionvr- 
tiiii  in  many  particulars,  but  is  destitute  of  latt>ral  ciliated 
organs,  and  its  ])rol)oscis  poss(»sses  no  stylets.  The  intestine 
is  a  convoluted  tub(5  without  lateral  d'\er'icula,  and  the  hind 
end  of  the  body  is  ])rovi«led  with  a  sucker. 

DeriiopiiK'tit  of  f/ic  yt'iiiirtf>iii.  —  \\\  some  Ncmcrloaiis,  such  as  Tctni- 
,sft  innxi  and  Mofurohife/hi,  Ww  yoiin;?  wunn  Icavrs  the  cijjj:  in  (lie  form  of  a 
cyliiidrical  ciliated  larva,  iiHiially  provided  at  t In;  extreiiiilies  of  flu*  body 
with  l)inicli('s  of  loiij^er  cilia,  wliicli  may  possibly  he  sensory  in  funelinti, 
and  f^radiially  eliaiiji<'s  without  any  marked  melamorpliusis  into  the  adiill 
form.  The  mouth  opens  up-  m  the  ventral  surface  of  the  body  into  a  re- 
tort-shaped dij^estive  trat't  wliieli  in  early  stages  possesses  no  anus — I  Ids 
structure  only  appearing?  mueli  later.  In  many  forms,  however,  a  peculiar 
metamorphosis  occurs  duriu);  the  transformation  of  the  larva,  known  from 
its  first  deseriber  as  /h.sor's  /arm,  into  the  adult.  On  the  ventral  surface 
of  the  body  there  api)car  four  invaginations <»f  the  cctodurni.  two  situated  in 


168 


INVEliTBBRATE  MORPHOLOGY. 


lii 


front  of  the  mouth  and  two  behind  it,  whieh  gradually  separate  from  the 
ectoderm  to  form  four  single-layered  plat(?s  lying  immediately  beneath  it. 
By  a  subsequent  growth  and  fusion  of  these  plates  a  new  ectodermal  cover- 
ing is  formed  enclosing  the  internal  organs,  and  on  its  completion  the  orig- 
inal larval  ectoderm  is  thrown  off.  In  some  species  a  somewhat  more  com- 
plicated process  occurs.  The  larva,  known  as  the  IHlidium  (Fig.  85),  has 
the  shape  of  a  helmet  from  w  hose  rim  two  ear-like  lappets  hang  down,  be- 


Fio.  85.  —  I'Uidiniu  IwAKVA  (after  Sai.knskv). 
ap  =  apical  plate.  m  =  mouth.  «  =  iiii,'estive  sac. 

twccn  which  lies  the  mouth-opening  (/»),  while  at  the  npex  of  the  helmet 
tliere  is  an  ectodermal  thickening  (<//>),  nervous  in  charactter,  from  which 
piMJeets  a  bunch  of  strong  .sensory  cilia.  As  in  the  De.sor  larva  four  in  vagi- 
nal ions  of  the  ectoderm  of  the  ventral  surface  occur,  whicdi,  however,  .sepa- 
rate from  the  larval  ectoderm  Jis  four  hollow  .sacs  which  unite;  together, 
their  inner  Willis  tiiickeiiing  to  form  the  ectoderm  ol  the  young  Ncniertcan, 
while  the  outer  walls  Iteconu)  thin  and  form  what  is  termed  the  (tinnion  sur- 
rouiuling  a  cavity  within  which  lies  the  young  worm.  During  thc^  process 
of  fusion  of  the  four  sacs  theenteron  ( /S)  and  a  portion  of  the  mesoilerm 
of  the  Pi/idium  an;  enclo.sed  and  give  risi;  to  the  digestive  tract  and  uie.so- 
derui  of  the  young  worm,  which  later  breaks  through  the  amnion  and 
Filidium  wall  to  become  free. 

The  si.i'ii  Ilea  nee  of  this  metamor|>hosis  is  decidedly  ob.scure.  Some 
authors  n;gard  it  as  more  primitive  than  the  direct  method  of  development, 
on  the  ground  that  the  f'ifh/iinii  with  its  lappets  pres(Mits  general  simi- 
larities to  the  .Mlillerian  laiva  of  the  Tolyclades  and  is  derived  phylogeneti- 
cally  from  such  a  form,  being  therefore  more  ancestral  in  it»  wliaracters 
than  the  simpler  larva-.  It  must  be  recognized,  however,  that  there  i.s  no 
indication  of  metamorpiiosis  in  the  l'oly(;lad  larva',  and  furthermore  that 


TYPE  PLA  r Y1IELMINTIIE8. 


169 


tlie  Xenierteans  perhaps  show  greater  siinihirities  to  Alloiocoplan  Turbel- 
lariii  than  to  Polychids.  Perhaps  an  exphmation  of  the  process  is  to  be 
found  in  the  sloughing  of  the  cetocU^rnt  and  the  formation  of  new  ciliated 
cells  which  is  seen  in  the  hirva  of  a  Pahfonenierteau,  Vephalothrix,  the 
metamorphosis  of  Desor's  larva  and  of  the  Piliiiiiim  being  a  greater  and 
more  complicated  ecdysis  derived  from  the  simpler  one. 

Some  interesting  evidence  as  to  the  morphological  signiticauce  of  the 
anus  is  to  be  derived  from  a  study  of  its  development  in  the  Nemerteans. 
it  is  an  opening  which  has  been  considered  by  some  to  have  arisen  l)y  the 
closure  in  tiie  middle  tA  an  elongated  slit-like  blastopore— the  two  ends,  iiow- 
ever,  remaining  open  to  lorm  respectively  the  mouth  and  anus;  and  it  has 
been  thought  that  the  direct  transformation  of  the  blastopore  into  the  per- 
manent mouth  in  some  cases,  and  in  otliers  into  the  permanent  amis, 
receives  on  this  theory  an  explanation.  The  phenomenon  of  the  closure  of 
the  bla.stopor(!  in  the  niiddhi  does  actually  occur  in  the  Annelid-like  Tra- 
cheate  Peripalus.  and  in  many  forms  both  mouth  and  anus  stand  in  close 
ontogenetic  relationship  to  tlie  blastopore.  In  the  Nemerteans  are  repre- 
sented the  most  lowly  organized  animals  wiiich  possess  botli  mouth  and 
anus,  and  accordingly  it  snight  be  expected  that  in  them  the  original  rela- 
tionships will  be  most  clearly  .seen.  The  young  Nemertean  pos.se.sses  no 
anus.  It  resembles,  so  far  as  its  digestive  tract  is  concerned,  an  Alloioco'lan; 
it  is  only  relatively  very  late  in  its  life-hi.story  that  the  anus  appears,  and 
then  ill  a  region  of  the  liody  which  has  no  relation  whatever  to  the  original 
blastopore.  This  fact  should  carry  considerable  weight  with  it,  especially 
as  ill  the  majority  of  forms  the  anus  is,  in  comparison  with  the  mouth, 
of  relatively  late  formation.  It  setJins  not  improbable  that  primitively  it 
had  no  n.'Iatiou  with  the  blastopore,  and  whore  such  relations  do  occur 
they  are  entirely  secondary. 

The  indication  of  metamerism  seen  in  the  Xemorteans  needs  no  further 
discussion  after  what  has  been  said  on  p.  4;J  with  reference  to  similar  pecul- 
iarities in  the  Turbellarians. 


SUnKl\(.I)()M    MF.TAZOA. 


TYPE  PLA  TYIIEI.MIXTIIES. 


I.  Class  Ti'UBKLLARiA.— Ectoderm  ciliated;  no  anal  opening. 

1.  Order  Anthi — Mouth  present,  but  no  digestive  tract.     noiivolnUt. 

2.  Order  J/Zo/ochVu. — Digestive  tract  present;  space  between  it   and 

liody-wall  occupied  by  itareiichyina.     }f()iii)tns,  /'/tii/iii,stoin(t. 

3.  Order    /k'Aa/<f/n<,v/7f/.-  Digestive    tract    straight     rod-    or   sac-like; 

space  between    it  and    Itody-wall  not    tilled  with  parenchyma. 
Micro.stoina,  Mrsostintnu  Prorhijiivhns,  Viirtex. 

4.  Order  7V/c7f<(/m.— Digestive  tract  luanehed,  thr(!e  principal  limbs 

giving  ri.s(!  to  secondary  branches;  male  and  female  reproductive 


170 


INVERTEBRATE  MORPHOLOGY. 


organs  with  common  opening.     Qunda,  PJauaria,  Phagocata, 
Dendroccelum,  Jkletloura,  Bipalinm,  tSyucwliUinm. 
6.  Order  Pulycladva. — Digestive  tract  branched,  the  primary  branches 
being  numerous;  male  and  female  organs  having  separate  open- 
ings, 
(a)  With  terminal  sucker  (6'y<y/ea).    Thysanozoon,  Eurylepta. 
(6)  Without  sucker  {Acotylea).     Planocera,  Leptoplatui,  iSty- 
lochtts. 
11,  Class  Trematoda. — Ecto-  or  endoparasites;    ectoderm  not  ciliated; 
with  digestive  tract  and  suckere. 

1.  Order  Polydomea:. — Suckers  more  than  two;  development  direct; 

usually  ectoparasites.    Polystomum,  Sphyranura,  Tristomum, 
Gyrodaatylns. 

2.  Order  Distomeir. — Suckers  one  or  two;  development  indirect;  usu- 

ally endoparasitic.     Distomuin,  Monostomttm. 

III.  Class  Cestoda.— Endoparasites;  ectoderm  without  cilia;  no  digestive 

tract  or  mouth;  usually  strobilated.     T(Ftii<i,  Bothriocephalus^ 
Curyophyllmis,  Liyuhi,  TiUrnophoruH,  Arrhhjetes. 

IV.  Class  Nemektina. — Ectoderm  ciliated;   not  parasitic;  anus  present; 

with  i)rotrusible  proboscis. 

1.  Order  Pala;ouemertini.—\Ai\iiri{\  ciliated  funnels  shallow;  nervous 

system  imb< dded  in  ectoderm;  proboscis  without  stylets.     Cari- 
ui'Va. 

2.  Order  Schizouemertini. — Ijatcral  ciliated  funnels  deep;  nervous 

system  imbedded   in  muscle-layer;   proboscis  without  stylets. 

Cerebratultts. 
8.  Order  Hoplonemerttni . — Lateral  ciliated  funnels  deep;  nervous 

system   within   muscle-layer;   proboscis  with  stylets.      Tetru- 

stemmn,  Aiiipltiporus. 
4.  Order  MalacohdelUna. — No  lateral    ciliated    funnels;    proboscis 

without  stylets.    Malacobdella. 


\ 


LITERATURE. 


TXTiniKr.I.AlUA. 

L.  von  OrafF.     Monaympfiif  der  Tnrhe/fdriin  :  I.  lih(ilnhriiiiilei).  Leipzig,  18b2 

L.  Ton  Oraff.     Die  Organimtion  dcr  Tui'luUtirin  arnla.     Leipzig,  189L 

L.  BOhmig.     UnterHiichungcn  i'llur  rhnhdotuiifii  Turhllarieii,  IL     Zeitschr.  fUr 

wiss«'nsch.  Zooloufie,  i,\,  1890. 
W.  M.  Woodworth.     C'liiitrilnitions  to  the  Morpholony  of  the  TitrbeUaHa,  L    Bul- 

h'tiii  of  the  Musoiim  of  Comp.  Zool.,  xxi,  189L 
A.Lang.     Die  Polyrhtd  n.     Fiiunn  u.  Flora  dt's  (iolfes  von  N«*apcl.     MojH)gr., 

XI,  IHSt. 
W.  M.  Wheeler.     Synriiliitiiitn  pcUuridi/iii,  it  ntir  mari/w  Triclud.     Jour.  i«l 

MorjiholoKv,  IX,  1894. 


TYPE  PLATYUELMINrilES. 

TREMATODA  AND  CE8TODA. 


171 


B.  iMckart.    Die  Parasiten  des  Memchen.    2te  Aufl    Bd    T      r  •     • 

Heidellier^r,  187«-is89.  *   ^-      Leipzig  u. 

A.  Lang.      UuterHuduunjni  zur  vergkichenden   Anatomic  nnrJ   TV,,     -     . 
J^nven»ff«tem,  der  TurbeUarien   II  u    Mr      mm  Ilistologie  den 

C.  Clana.     ;f«,.  morphologiHchen  vnd  phylonenetischn  n.„.fi  ;        .      r. 

^ur.nk^-pers.    Arbeiten  a.  d.  Zoof  InZXI^  ^nAm"'         """' 

NEMEUTINA. 

W.  C.  McInto.h      A  Mmograph  of  th,  British  Auudids     Part  T  AT        . 
Loudou,  l«7a_74  -^'"tutua.    fait  I.  Ifemerteans. 

0.  Bttrger.     iJntersiiifiKiKJcn  i/her  ili,'i,..,i  •  ,,- 

Zeitscbr.  fur  wisseJh!  Z^^Z^.  ''"  ''"'  ^^'«'''-^'-«. 


172 


IN  VEHTEBBA  TE  MOBPUOLOG  Y. 


CHAPTER  VIII. 


TYPE  NEMATHELMINTHES. 


r ) 


The  Nemathelmiutlis  are,  like  tlie  members  of  the  preced- 
inj;  type,  characterized  by  the  form  of  the  body,  which  is 
cyliudrical  and  usually  elougated  or  even  thread-like,  whence 
the  popular  terms  liouud-worms  or  Tiiread-wornis  which 
are  frequently  applied  to  them.  The  ectoderm  is  ctjvered  by 
a  thick  layer  of  cuticle  which  it  secretes,  and  in  connection 
with  which  spines,  bristles,  or  hooks  may  be  developed  at 
various  parts  of  the  body.  There  is  no  ti'ace  of  segmentation 
or  reduplication  of  organs,  with  the  exception  that  in  some 
forms  the  circular  nerve-commissures  uniting  the  longitudinal 
cords  may  succeed  each  other  with  tolerable  regularity  ;  the 
cuticle,  it  is  true,  especially  when  thick,  is  ringed  by  numer- 
ous grooves  succeeding  one  another  at  short  intervals,  but 
this  cannot  be  interpreted  as  an  indication  of  metamerism, 
but  is  more  pnibably  a  provision  to  counteract  the  rigidity  of 
the  cuticle  aiul  to  give  a  considerable  amount  of  mobility  to 
the  body.  The  Nemathelniiuths  accordingly  have  the  same 
grade  of  individuality  as  a  simple  Platyhelmiuth,  such  as  an 
Alloioco}lau,  ami  arc  to  be  regarded  as  metamere  individuals. 

One  iniportuut  ditlerence  of  structure  which  these  worms 
show  from  the  Phitvhebninths  is  the  ])resence  of  a  cajjacious 
cwlom,  tlic  interval  between  the  digestive  tract  and  the  mus- 
culature of  tlu!  body- wall  not  being  tilled  up  by  parenchyma- 
tous mesoderm,  but  being  a  simple  undivided  cavity  in  which 
lie  tlu;  reproductive  orgaus.  These  latter  are  simple,  the 
animals  being  as  a  rule  bisexual,  and  there  is  no  separation 
of  the  female  organ  into  ovary  and  vitellarium.  Structures 
of  an  excretory  nature  occur  in  one  of  the  two  classes  into 
which  the  type  is  divisible,  but  a  blood  vascular  system  is 
entirely  wanting. 

The  habit  of  life  varies  greatly  in  the  various  members  of 


i 


iiiiiiiiiiiiril- 


It- 

h 

e 

ju 

s 


TYPE  NEMATIlELMINTHEa. 


173 


the  group.  In  the  class  Neraatoda  many  forms  live  freely  in 
the  sea,  fresh  water,  or  damp  earth,  while  others  are  parasitio 
during  a  part  of  their  ]iv(;s,  and  others  a^^ain  are  parasites 
j)ractieally  throuj^hout  their  whole  existence.  The  Acautho- 
cephala  are  without  exception  parasitic. 

I.  Class  Nematoda. 

The  Nematodes  are  distinj^uished  from  the  members  of 
the  second  class  by  the  presence  in  nearly  all  cases  of  a  dis- 
tinct digestive  tract,  usually  with  mouth  and  anus,  and  by 
the  absence  of  a  retractile  proboscis  furnished  with  hooks 
at  the  anterior  end  of  the  body.  The  arrangement  of  the 
muscles  of  the  body-wall  are  also  peculiar  inasmuch  as  longi- 
tudinal muscles  only  are  present  (Fig.  80,  m),  which  instead  of 
forming  a  closed  sheath  are  interrupted  along  four  longitudi- 


FiG.  80.— TuANSVEUBE  SECTION  OF  Ascnrut  InmhrieoioM  at  the  Level  ok 

Pharynx  (froin  hkbtwk.i 
c  =  <'ti!)cle..  n»  =  loriirrnidiuiil  muHehra. 

d  =  (lorsiil  H».  •  =  latt-ml  liiu;. 

h  —  hypuderiuis.  r  —  vtmtral  liue. 

to  =  Dcpb  rid  i  mill. 

nal  lines  {d,  v  iiud  x),  or  in  some  (ra*»#»m  ailoug  a  smgle  ventral 
line,  in  the  former  ca*K'  there  bfii.M_:  u»«r  lourntudinuil  buinllcs 
of  muscles  extending  tlie  length  -  tlJoe  b*»dy.  lu  the  struc- 
ture of  most  of  the  organs,  howe-wer,  eonsiderable  vairiatiou  is 
found,  and  it  will   be   moHi  (Muiv^in^nt.  t*)   (»tM'ribe  them  us 


174 


INVEUTKniiA TK  MOIWIIOLOG  Y. 


they  are  fouutl  iu  each  of  the  two  ortlers  iuto  which  the  class 
may  be  divided. 


r^i". 


(e 


1.  Order  Eunematoda. 

This  order  contains  the  majority  of  the  Nematoda,  and 
all  its  members  are  furnished  with  a  mouth  and  anus  and  a 
functional  digestive  tract.     The  mouth  is  in 
some  cases  at   the   bottom   of  a  funnel-like 
depression  which  may  be  armed  with  spines, 
special  developments  of  the   cuticula  which 
covers  the  body.     This  is  throughout  cylin- 
drical in  shape,  except  that  in  the  nmles  of 
some  species  it  exj)Hnds  at  the  posterior  ex- 
tremity into  a  relatively  large  funnel-shaped 
structure  with  thin  walls,  the  bursa  (Fig.  87), 
at  the  bottom  of  which  lies  the  opening  of  the 
cloaca,  a  cavity  into  which  the  intestine  and 
the  male  reproductive  organ  (jpen.     Beneath 
the    ringed   cuticle   lies  the  ectoderm  \hypo- 
tlcrmis)  which  secretes   it,  and  beneath  this 
the    muscular   layer   which   consists  only  of 
longitudinal  muscle-tibres,  diilerentiatious  of 
the  outer  ends  of  large  cells  whose  undiffer- 
entiated inner  ends  project  into  the  cwlom,  so 
as  almost  to  obliterate  it  in  some  cases.     The 
muscle-tibres  do   not,  however,  form  a  com- 
sp     plete    continuous    shealh    surrounding    the 
C(L'l<)m,  but  are  intt^rrupted  along  four  longi- 
tudinal  lines,    two   lateral,    one  dorsal,    and 
one  ventral  (Fig.  8(5).     The   ccelom  contains 
the  intestine  and   reprcnluctive    organs,  and 
is  peculiar  in  that  it  is  not   bounded    by  a 
limiting    cellular    membrane    or    peritoneal 
lining,  being  simply  a  si)ace  comparable  to 
the  co'lomic  cavities  of  the  Khabdocoela  or 
the  blood-sinuses  of  i\\o  Nemerteans. 

The  digestive  tract  is  a  straight  tube  traversing  the  body 
from  one  extremity  to  the  other,  opening  posteriorly  iu  the 


Fi«.  87.  —  Ascari« 
nif/rorenoKit  Mai.k 
(uftiM-  Ij-.i'(;k.»kt). 
i  =  iiitestiiic. 
ph  =  iiliarynx. 
«p  =  si>icules. 
te  =  testis. 


TYPK  NEMA TIIKL.VI\T1IKS. 


175 


female  directly  to  the  exterior,  in  the  mules  into  a  cloaca 
commou  to  it  aud  to  the  male  orj^aii  of  reproduction.  Its 
anterior  part  is  a  muscular  cosophaj^us  lined  with  cuticle  di- 
rectly continuous  with  that  coverinj^  the  surface  of  the  hody, 
while  posteriorly  it  is  a  delicate  tube  composed  of  a  sinj^de 
layer  of  cells,  not  being  surrounded  by  any  mesodermal  mus- 
cular tissue. 

The  excretory  system  is  not  as  yet  fully  understcjod.  It 
a[ipears  to  cousist  of  a  pair  of  tubes,  for  which  no  ctdlular 
lining  has  as  yet  been  made  out,  which  lie,  one  on  each  side, 
in  the  thickened  hypodermis  of  the  lateral  lines.  In  the  an- 
terior portion  of  the  body  tlu^y  unite  to  form  a  single  tube 
which  opens  to  the  exterior  in  the  median  ventral  line  not  far 
behintl  the  brain  (Fig.  88,  13). 

This  latter  consists  of  a  ring  or  nerve-collar  surrounding 
the  anterior  part  of  the  oesophagus  on  which  lateral  masses  of 
ganglion-cells  occur  and  which  gives  rise  to  two  njain  nerves, 
one  of  which  runs  back  in  the  median  dorsal  line,  while  the 
other,  which  in  some  forms  api)ears  to  be  double,  li«;s  in  the 
median  ventral  line.  Other  nerves  pass  forwards  from  the 
nerve-ring  to  the  anterior  part  of  the  bod}',  and  in  addition  to 
the  dorsal  and  ventral  nerve-cords  two  lateral  nerves  i)ass 
backwards  a  short  distance,  while  circular  commissures  con- 
nect the  two  niain  nerve-cords,  those  of  the  two  sides  of  the 
boily  not,  however,  being  opposite  each  other,  so  that  they  do 
not  suggest  a  pseud«)-metanierism  so  strongly  as  the  similar 
commissures  of  the  Hoplonemertini.  Special  sense-organs 
are  as  a  rule  absent,  though  a  few  forms  possess  ej-es. 

The  reproductive  organs  are  exceedingly  simple.  In  the 
male  they  are  represented  by  a  single  convoluted  tube,  lined 
in  its  upper  part  by  the  mother-cells  of  the  spermatozoa  and 
dilating  below  into  a  seminal  vesicle,  to  which  succeeds  a 
short  ejaculatory  duct  which  oj)ens  into  the  cloaca.  The  walls 
of  this  latter  cavity  are  frequently  invaginated  to  form  two 
small  sacs  in  each  of  which  lies  a  chitinous  spicule  capa- 
ble of  being  ])rotruded  from  the  cloacal  opening  and  serving, 
with  the  bursa,  as  co{)ulatory  organs.  The  female  organs,  on 
the  other  hand,  consist  of  a  pair  of  convoluted  tubes,  each  of 
which  dilates  into  a  uterus  and  unites  with  its  fellow  to  form  a 


176 


IN  I  'KU TEliHA  TK  MOlt Pill) LOO  Y. 


< 


siugle  tube,  tlie  vagina,  sometimes  with  muHcular  walls,  which 
opens  to  the  exterior  iu  the  ventral  mid-lino  some  distance  iu 
front  of  the  anus.  Ah  a  rule  the  sexes  are  separate,  her- 
maphroditism occurrin<^  only  in  a  few  isolated  cases. 

Mjiny  Nematodes  are  free  thron^hout  their  entire  exist- 
ence, livinj^  in  the  sen,  fresh  water,  or  damp  earth,  and  fre- 
quently possessin;^  eyes.  Others  are  found  in  some  domestic 
l)roducts,  such  as  the  vinej^ar-eel  {AntjitiUtda),  found  iu  vino- 
gar  and  sour  paste  ;  while  «)thers,  again,  are  parasitic  ou 
plants,  such  as  Tylenchm,  which  lives  ui)on  the  young  grains 
of  wheat  and  in  some  ca.ses  produces  very  serious  damage  to 
crops,  and  Heteroihrn,  which  is  quite  as  injurious  to  root- 
crops.  More  interesting,  however,  are  a  number  of  forms 
occurring  as  parasites  in  animals,  many  affecting  man,  iu  some 
cases  producing  serious  results. 

Life-histories  of  the  Ennematoda. — The  free-living  forms  show  no 
peculiarities  of  dovelopinent,  the  immature  animal  developing  directly 
from  the  egg.  Among  the  parasitic  forma,  however,  interesting  variations 
from  direct  development,  due  to  a  change  of  host,  occur,  a  well-marked 
heterogony  occasionally  being  found.  An  example  of  this  is  seen  in  Rhab- 
ditis  iiiifrovenosa,  which  at  one  stage  of  its  existence  lives  in  damp  earth, 
the  females  being  viviparous  and  producing  young  which  make  their  way 
into  the  lungs  of  frogs,  where  they  assume  a  form  which  led  them  to  be 
assigned  to  the  genus  Asearis,  and  where  they  become  mature.  At  tliis 
stage  they  differ  from  the  Hhabditis  forms  in  Iwing  hermaphrodites,  and 
from  the  eggs  d«!posited  by  them  the  Rliabditis  generation  again  results. 

From  a  medical  .standi)oint  one  of  the  most  important  forms  is  Trichina 
spir(dis,  which  occui-s  encapsuled  in  the  mu.scles  of  various  warm-blooded 
aninuiis,  such  as  man,  the  i)ig,  rat,  ..  )use,  and  occasionally  in  the  fox.  cat, 
and  rabbit.  The  capsules  are  oval  and  al)out  0.6  mm.  in  length,  and  occa- 
sionally have  a  white  color,  due  to  the  (lepo.sition  of  calcareous  matter  iu 
the  wall.  In  the  interior  of  the  capsule  lies  coiled  up  an  immature 
Trichina,  which  may  retain  its  vitality  in  this  condition  apparently  during 
the  lifetime  of  its  host.  Should,  for  instance,  improperly  cooked  or  salted 
pork  which  contains  such  capsules  be  eaten  by  man,  the  capsuh*  Ivconies 
dissolved  by  the  digestive  juices  and  the  young  Trichina  is  .set  free  .a  the 
small  intestine  and  in  the  course  of  a  f(!W  days  becomes  sexually  mature. 
Each  female  may  deposit  in  the  intestine  as  many  as  1000  eggs,  from 
which,  in  the  second  or  third  week  aft(?r  infection,  young  Trichinw  meas- 
uring alxjut  0.01  mm.  hatch  out  and  at  once  pioctsed  to  bore  through  the 
walls  of  the  intestine,  producing  a  more  or  less  violent  inflammation 
according  to  the  degree  of  infection.     They  wander  through  the  connective 


TYPK  NKMA  TIIKLMISTIIKS. 


177 


tissue  and  finnlly  reach  t!ie  iniiscU's,  especially  of  tho  neck  and  diaphragm, 
into  which  they  tM>i-(>,  produeiiiK  a  defeneration  of  the  tissue  upon  which 
they  feed.  In  the  coui-mc  of  the  third  luonth  after  infection  they  encyst 
tlieniselvus  in  the  muscle-tissue,  and  intlaniniatory  changes  pntduced  in  the 
connective  tissue  in  their  iuwuediatt'  vicinity  result  in  the  formation  of  a 
sivoiid  cyst -wall  around  them  (Fig.  XH,  A).  If  the  intestinal  intiammat  ion 
and  the  succeeding  muscular  inllammation  have  not  prove<l  fatal  Ut  the 
host,  the  danger  is  past,  the  eneapsulcd  Trhhimr  undergoing  no  further 
development  in  the  nuisclcs. 

Other  forms  which  occur  in  num  are  Asmris  /iimhrh'nhlcs,  the  round- 
worm (Fig.  ny,  li),  a  large  form,  of  which  the  female  nu'asurcs  40  cm.  in 
length  and  the  nuile  25  cm.,  and  which  hears  some  resemblance  in  shape  to 
an  earth-worm,  (Krynrisvetmhnhiiin,  asmaller  form,  1  cm.  in  length,  which 
inhabits  the  rectum  esiwcially  of  youiij;  children,  and  TrtrhnrephdlusiUspur 
(Fig.  8H),  which  measures  IJ-.I  cm.  in  length  and  is  charjutcrizcil  by  the 
anterior  half  of  tho  Ixnly  being  exceedingly  slender,  the  worm  boring  into 
tho  intestinal  wall,  especially  in  the  neighborhood  (»f  the  ca'cum,  by  this 
slender  portion,  the  hin<ler  thicker  por- 
tion hanging  freely  in  the  wall  of  the 
intestine.  Tho  presence  of  these  three 
forms  nuiy  be  recognized,  independ(>ntly 
of  the  finding  of  the  actual  worm,  by 
their  ova,  whoso  re.spcctivo  characters  dif- 
fer very  greatly.  So  far  as  is  known  tho 
development  of  these  forms  is  direct  and 
there  is  no  intermediate  host,  but  the  ova 
are  taken  into  the  body  with  the  food. 
The  exact  manner  of  infection  is,  however, 
obscure. 

In  addition  to  the  forms  which  have 
been  mentioned  there  are  a  few  which  are 
more  especially  frcHpient  in  tropical  cli- 
nuites.  Doc/i mills  iluodcmilis  is  a  small 
form  about  1-3  cm.  in  length,  with  .strong 
teeth  or  blunt  spines  in  the  mouth  region, 
which  fastens  itself  to  the  wall  of  tho 
siuidl  intestine  and  lives  upon  the  blood 
of  its  host,  producing  anu;mia.  Its  ova 
develop  in  stagiuint  water  or  damp  earth, 
and  probably  num  becomes  directly  in- 
fected. It  has  long  been  known  in  tho  tropics,  producing  the  disease 
known  as  Chlorosis  ayi/ptiaca,  but  may  also  affect  mitiers  or  workers  iit 
tunnels,  having  appeared  endemically  in  tli(!  workers  ou  the  St.  Got  hard 
tunnel,  whence  it  has  since  spread  somewhat  in  Germany,  especially  among 
workers  in  clay.  Fihiria  midineusis  is  limited  entirely  to  the  tropics  and 
is  a  very  slender  worm  nearly  1  metre  in  length  which  lives  in  the  connec 


Fui.  88.— .4,  Trichiim  tiicyslud 
in  muscle;  B,  auleiior  exlrim- 
iiy  of  AscuHh  lumhricoidva 
from  the  veutral  surface,  show- 
ing the  two  veutral  oral  pupillu; 
and  the  excietory  pore  (i)oih 
uftfi- LeiTCKAHT);  C,  Trichoceph' 
aliia  di»par  (after  Owen), 


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178 


INVERTEBRATE  MORPHOLOOT. 


tive  tissue  beneath  the  skin,  producing  ulcers,  at  the  bottom  of  which  the 
worm  lies  coiled  up.  The  ova  develop  in  water  and  the  embryos  pass 
probably  into  small  Crustacea,  which  are  swallowed  with  drinking-water. 
Filaria  sanguinis  hominis,  also  solely  tropical  in  its  distribution, 
receives  its  name  from  the  fact  that  it  lays  its  ova  in  the  blood  of  man, 
which  may  thus  swarm  with  countless  numbers  of  small  worms.  These 
make  their  way  to  the  exterior  of  the  body  by  the  kidneys,  producing 
haemorrhages  or  minute  abscesses  in  that  organ  and,  as  the  result  of 
these,  milky  or  bloody  urine. 


:|i 


2.  Order  Gordiacea. 

This  order  includes  the  families  of  the  Gordiidce  and  Mer- 
mithidce,  long  slender  thread-like  worms,  which  diflfer  from  the 
Eunematoda  in  several  important  respects.  They  occur  in 
their  mature  state  in  fresh  water ;  in  their  immature  stages, 
however,  they  are  parasitic  in  insects.  In  the  adult  Gordius 
the  mouth  is  usually  closed  by  an  overgrowth  of  the  cuticle, 
and  the  anus  is  lacking  in  Mermis.  The  musculature  of  the 
body-wall  consists  only  of  longitudinal  fibres  (Fig.  89,  w),  wh'ch 


Fig.  89.  — Tkansvbrse  Section  op  Oordiua  (after  Vejdotskt). 
eu  =  cuticle.  n  —.  nerve-cord. 

d  =  intestine.  pe  =  peritoneum. 

?iy  =  bypodermis.  ut  =  uterus. 

m  =  longitudinal  muscles.  ov  =  oviduct. 

differ  in  their  arrangement  from  those  of  the  Eunematoda  in 
being  interrupted  only  in  the  mid-ventral  line.  The  coelom 
is  lined  by  a  peritoneal  epithelium  {pe)  lying  beneath  the 


TYPE  NEMATUELMINTUES. 


179 


muscle-cells,  and  is  divided  into  two  lateral  chambers  by  a 
mesentery  (w)  running  the  entire  length  of  the  body  and  con- 
sisting of  two  layers  surrounding  the  intestine  {d),  and  inserted 
into  the  body-wall  dorsally  and  ventrally,  their  outer  surfaces 
being  lined  by  a  continuation  upon  them  of  the  peritoneal 
epithelium. 

No  excretory  system  has  been  as  yet  discovered.  The 
nervous  system  consists  of  a  gangl:  mic  ring  surrounding  the 
oesophagus,  from  which  a  number  of  nerves  pass  forward, 
while  a  single  nerve-cord  {n)  passes  backwards  in  the  mid- 
ventral  line,  dilating  at  the  posterior  end  of  the  body  into 
a  ganglionic  mass. 

The  reproductive  organs  consist  in  the  female  of  a  series 
of  ovaries  {ov)  attached  one  behind  the  other  to  each  mesen- 
tery above  the  intestine.  In  the  mesenteries  two  tubes  {ut) 
pass  backwards  which  receive  some  of  the  ova  and  function 
as  uteri,  near  the  hind  end  of  the  body  bending  ventrally  to 
open  into  the  cloaca,  whose  wall  is  invaginated  to  form  a 
single  seminal  receptacle.  The  testes  have  not  yot  been 
found,  but  two  seminal  vesicles,  corresponding  to  the  uteri  of 
the  female,  occur  and  open  likewise  into  the  cloaca,  which  in 
the  male  is  evertible  and  serves  as  a  copulatory  organ. 

The  Affinities  of  the  Nematodes.— HhQ  relationships  of  the  Nematodes  are 
exceedingly  obscure.  Their  unsegraented  character  and  the  character  of 
the  nervous  system  seem  to  ally  them  more  closely  with  the  Platyhelminths 
than  with  higher  forms,  but  the  relationships  to  any  of  the  known  Platy- 
helminths must  be  exceedingly  remote.  The  parasitism  which  occurs  so 
frequently  in  the  group  is  to  be  considered  as  secondary,  since  so  many 
forms  lead  a  free  life  and  peculiarities  of  structure  can  hardly  be  attributed 
to  degeneration.  The  Gordiacea  stand  on  a  higher  plane  than  the  Eu- 
nematoda,  as  shown  by  the  possession  of  a  mesentery  and  the  arrangcs- 
mentof  the  reproductive  organs  and  nervous  system,  which  bear  some  sim- 
ilarities to  those  of  the  Annelids,  but  their  Nematode  characteristics  are 
most  pronounced.  Perhaps  the  ancestors  of  the  Nematodes  are  to  be  found 
in  the  yet  unknown  intermediate  forms  between  the  Platyhelminths  and 
Annelids,  a  view  which  would  account  for  their  similarities  in  certain 
respects  to  both  these  groups. 

II.  Class  Acanthocefhala. 

This  class  contains  a  number  of  parasitic  forms  which 
occur  more  especially  in  the  digestive  tract  of  fishes,  though 


180 


IN  VERTEBRA  TE  MORPIIOLOO  Y. 


rm 


also  found  in  Maimmaiia  and  in  exceptional  cases  in  man.     A 
great  uniformity  of  structure  exists  throughout  all  the  species, 
so  that  they  are  all  referable  to  a  single  genus,  Echinorhynckus. ' 
The  body  (Fig.  90)  is  cylindrical  and  as  a  rule  not  very  long, 

and  a  marked  distinction  from  the 
Nematodes  is  found  in  the  retractile 
proboscis  {pr)  occurring  at  the  anterior 
end  of  the  body.  It  is  a  cylindrical 
prolongation  of  the  anterior  portion  of 
thebody  and  is  provided  with  a  number 
of  chitiuous  hooks  by  means  of  which 
it  adheres  to  the  intestinal  wall  of  its 
host.  The  proboscis  may  be  invagi- 
nated  into  a  double-walled  muscular 
proboscis-sheath  by  whose  contraction 
it  may  again  be  protruded,  a  strong 
retractor  muscle,  extending  from  the 
tip  of  the  proboscis  to  the  base  of  the 
sheath,  serving  for  the  invagination  ; 
and  from  the  base  of  the  sheath  re- 
tractor muscles  {rm)  pass  to  the  body- 
walls  and  serve  to  hold  the  sheath  in 
position.  No  traces  of  a  digestive  tract 
occur. 

The  body  is  covered  upon  the  out- 
side by  a  thick  cuticle  secreted  by  the 
subjacent  hypodermis,  which  is  a  rather 
Fig.  00.— Male  Ecldnorhyn-  thick  laj'er  consisting  of  a  protoplasmic 
c/A«s  (aft*..- lkuckaiit).        matrix  in  which  nuclei  are  scattered 

but  in  which  no  cell-outlines  are  to  be 
distinguished.  Beneath  the  cuticle  the 
matrix  has  a  fibrillar  character,  and 
near  its  inner  surface  it  is  hollowed  out 
into  a  network  of  anastomosing  canals 
of  which  mention  will  be  made  later. 
Beneath  the  hypodermis  lies  a  basement-membrane  within 
which  are  two  layers  of  muscle-cells,  having  the  same  epi- 
thelio-muscular  character  as  those  of  the  Nematodes,  the 
fibres  of  the  external  layer  having  a  circular  direction,  while 


g  =  glands. 
I  =  lemniscus 
p  =  penis, 

pg  =  proboscis  ganglion 
pr  =  proboscis. 
rm  =  retractor  muscle. 
I  =  testis. 


g<l&S*iS»*>- 


TYPE  NEMATHELM1NTHE8. 


181 


lie 
le 


those  of  the  iuuer  layer  have  a  lougitudiual  course.  The 
botly-wall  encloses  a  well-marked  coelom,  not  lined  by  a 
special  peritoneal  epithelium,  but  which  contains  the  repro- 
ductive organs  and  is  traversed  by  the  retractor  muscles  of 
the  proboscis-sheath. 

The  nervous  system  consists  of  a  ganglionic  mass  {pg) 
lying  within  the  proboscis-sheath  which  sends  forward  nerves 
for  the  supply  of  the  walls  of  the  sheath  and  of  the  retractor 
muscle  of  the  proboscis.  Posteriorly  two  lateral  nerve-cords 
extei.d  backwards  along  the  sides  of  the  body,  and  in  male 
individuals  are  connected  near  ♦^he  posterior  extremity  with  a 
ganglion  lying  beneatli  the  reproductive  ducts  and  from  which 
nerves  pass  to  the  genital  apparatus. 

The  system  of  lacunar  canals  which  form  a  network  in  the 
lower  layers  of  the  hypodermis  is  probably  excretory  in 
function.  The  canals  are  found  throughout  the  entire  hypo- 
dermis, both  in  the  proboscis  and  in  the  body-wall,  in  the 
latter  there  being  indications  of  two  larger  lateral  trunks. 
From  the  point  of  junction  of  the  proboscis  with  the  body- 
wall  two  muscular  sacs,  the  lemmsci  (I) ,  hang  down  into  the 
coelom.  The  cavity  which  they  contain  communicates  with  a 
circular  lacuna  which  surrounds  the  base  of  the  proboscis 
and  with  which  the  lacunae  of  the  proboscis-hypodermis  like- 
wise communicate,  this  system  of  the  proboscis-lacunre  and 
the  lemnisci  being  shut  off  from  the  system  of  the  body-wall 
by  a  partition  extending  from  the  basement-membrane  to  the 
cuticle.  The  lemnisci  have  been  regarded  as  possible  repre- 
sentatives of  a  digestive  tract,  but  it  seems  more  probable 
that  they  are  reservoirs  for  the  reception  of  the  fluid  con- 
tained in  the  lacunae  of  the  proboscis  when  it  is  driven  from 
them  during  invagination. 

The  reproductive  organs  are  much  more  complicated  than 
those  of  the  Nematodes.  The  sexes  are  separate,  the  male 
individuals  being  usually  smaller  than  the  females.  The 
ovaries  are  paired  bodies  enclosed  within  a  muscular  ligament 
attached  anteriorly  to  the  base  of  the  proboscis-sheath  and 
posteriorly  to  the  reproductive  duct.  At  an  early  stage  of 
their  development,  however,  the  ovaries  split  up  into  masses 
which  float  about  in  the  ccelom  together  with  large  numbers 


■ 


182 


INVERTEBRATE  MORPHOLOGY. 


of  separated  ova.  They  pass  to  the  exterior  by  a  complicated 
system  of  ducts,  the  most  anterior  portion  of  which  is  a  wide 
funnel-shaped  structure,  the  hell,  to  whose  wall  the  ligament  is 
attached  and  which,  by  a  rhj'thmical  expansion  and  contrac- 
tion, engulfs  the  ova  and  ova-masses  floating  about  in  the 
ccelom.  From  the  lower  end  of  the  bell  they  escape,  the  ova- 
masses  to  be  returned  to  the  ccelom,  while  the  fertilized  sepa- 
rate ova  pass  into  a  short  tube,  the  oviduct,  which  opens  below 
into  a  muscular  uterus,  which  finally  communicates  with  the 
exterior  at  the  posterior  end  of  the  body. 

The  male  apparatus  consists  of  usually  two  testes  (Fig. 
90,  t)  enclosed  within  the  ligament,  which  is  attached  below 
to  the  wall  of  the  evertible  bursa.  From  each  testis  a  duct 
passes  backwards,  the  two  soon  uniting  to  the  single  vas  de- 
ferens, which,  after  receiving  the  ducts  of  some  unicellular 
glands  {gl),  opens  into  the  bursa  at  the  tip  of  a  muscular  j^ewis 
{jp).  The  bursa  when  everted  is  a  somewhat  funnel-shaped 
structure  at  the  bottom  of  which  is  the  penis,  the  edge  being 
furnished  in  some  forms  with  hooks  by  means  of  which  it 
serves  as  a  copulatory  organ. 

The  life-history  of  the  Acanthocephala  includes  a  change  of  host.  The 
larvae  are  found  in  the  body-cavity  of  Crustacea  or  insects,  and  reach  ma- 
turity only  when  the  intermediate  hosts  are  swallowed  by  the  proper  final 
host.  The  largest  species  of  Echinorhynchus  is  the  E.  gigas,  whicli  occurs 
in  the  intestine  of  the  pig ;  the  intermediate  host  of  this  form  is  the  June 
bug  {Melolontha). 

Nothing  can  as  yet  be  stated  with  any  certainty  concerning  the  relation- 
ships of  tlie  Acanthocephala.  They  are  usually  associated  with  the  Nema- 
todes, to  which  they  certainly  present  similarities,  but  no  intermediate 
forms  bridging  the  gap  between  the  two  classes  are  yet  known,  and  the 
embryological  histoi'y  throws  little  light  upon  the  question. 

SUBKINGDOM  METAZOA. 
TYPE  NEMA  THELMINTHES. 

I.  Class  Nematoda. — With  digestive  tract ;  without  proboscis  furnished 
with  chitinous  hooks. 
1.  Order  Eiineiriatoda. — Musculature  of  body- wall  interrupted  along 
the  lateral  line  ;  no  mesentery  ;  no  peritoneal  epithelium.  An- 
guilhila,  Tylenchus,  Heterodera,  Trichina,  Ascans,  Oxyuris, 
Trichocephalus,  Dochmius,  Filaria. 


TYPE  NEMATHELMINTHE8. 


183 

eoraiusj^ZL  ""'''''''  '"'  P«"'°"«-'  epithelium 

II.  Class  ACANTHOCEPHALA-Witbout    digestive    tract  •    with         k      • 
armed  with  recurvpd  r.hiH«        u    ,  '    ^""    P'oboscis 

^c;i^;^o.%,.c•/.^  ''''^'''  P''^^«^'"«  throughout. 


A 

L. 
J. 


LITERATURE. 

2te  Aufl.    (In  course  of  publica- 


Berlin,  1866. 
Buda-Pesth,  1880. 


B.  Leuckart.-2?e^  Paramen  des  Menschen. 
tion.) 

i^auUm-MomgrapIde  der  Nematottn. 
XLvr,  1888.  -^-eitsclir.   fUr  wissenscb.  Zoologie, 


: 


184 


INVERTEBRATE  MORPHOLOGY,     n. 


1      ! 


CHAPTER  IX. 

ORDER    ECHINODERA;    CLASS    CH^TOGNATHA ;    CLASS 
ROTIFERA  ;  ORDER  GASTROTRICHA  ;  DINOPHILUS. 

This  chapter  includes  a  description  of  a  number  of  forms 
whose  affinities  are  at  present  rather  doubtful  and  which 
show  similarities  sometimes  to  the  Nemathelminths  and 
sometimes  to  the  Annelida.     Instead,  however,  of  assigning 

them  to  one  or  the  other  of  these  types, 
it  has  been  thought  advisable  to  consider 
them  in  a  separate  chapter  and  each 
group  independently,  indicating  briefly 
their  most  probable  affinities. 

Order  Echinodera. 

The  order  Echinodera  includes  a 
number  of  small  organisms  all  marine  in 
habitat,  and  all  referable  to  a  single 
genus,  Echinoderes  (Fig.  91).  The  body 
varies  in  length  from  somewhat  less  than 
1  mm.  to  almost  0.1  mm.  according  to  the 
species,  and  tapers  somewhat  posteriorly, 
terminating  in  one  or  two  prolongations 
or  cerci,  while  anteriorly  there  is  a  pro- 
FiG.  91.  —  Echinoderes  boscis  armed  with  strong  setsB  which 
DujardinU  (after  cla-  ^^   invagiuated  within  tlie  anterior 

PARKDB  from  Hatscbkk).  •'  ^  " 

portion  of  the  body,  and  serves  as  an  or- 
gan of  locomotion  as  well  as  for  the  prehension  of  food. 
The  outer  surface  is  covered  by  a  layer  of  chitin  which  is 
divided  into  distinct  metameric  rings,  the  number  of  which, 
eleven,  is  constant  for  all  known  species,  and  which  are  pro- 
vided in  some  species  with  definitely-arranged  setae.  No 
cilia  are  present.  Beneath  the  chitinous  rings  lies  the  ecto- 
derm, which  shows   indications   of  metamerism  also,  being 


ORDER  ECIIINODERA. 


185 


erior 
11  or- 
ood. 
li  is 
ich, 
pro- 
No 
cto- 
ing 


thickened  beueath  the  interval  between  two  successive  rings  ; 
it  consists  of  a  granuhir  layer  of  protoplasm  in  which  scat- 
tered nuclei  occur.  Beueath  the  ectoderm  lies  a  somewhat 
incomplete  layer  of  longitudinal  muscles,  which  become  spe- 
cialized anteriorly  into  separate  bundles  for  the  retraction  of 
the  proboscis ;  in  each  metamere  two  dorso- ventral  muscle- 
bundles,  one  on  each  side  of  the  middle  line,  are  also  found. 
A  relatively  spacious  body-cavity  in  which  various  organs 
lie  occurs,  but  no  lining  peritoneal  epithelium  or  mesenteries 
have  been  observed. 

The  digestive  tract  begins  with  the  mouth,  which  lies  at 
the  bottom  of  the  invaginated  proboscis  and  opens  by  the  in- 
tervention of  a  short  tube  into  a  muscular  pharynx  into  the 
anterior  portion  of  \yhich,  four  glands,  either  salivary  or  poi- 
sonous in  function,  pour  their  secretion.  The  pharynx  com- 
municates posteriorly  with  a  sac-like  stomach,  upon  which 
follows  a  short  straight  intestine  opening  to  the  exterior  at 
the  posterior  end  of  the  body  between  the  terminal  cerci. 

Two  elongated  pear-shaped  bodies  lying  in  the  coelom  in 
about  the  middle  region  of  the  body  have  been  described  as 
excretory  organs.  They  are  closed  at  the  free  end,  their 
cavity  is  ciliated,  and  they  open  to  the  exterior  on  the  dorsal 
surface  near  the  margin  of  the  body.  The  reproductive 
organs  are  cylindrical  sacs  which  are  provided  with  ducts 
opening  to  the  exterior  on  the  terminal  segment ;  all  the 
species  whose  reproductive  organs  have  been  studied  are 
bisexual. 

Four  cellular  masses  lying  above  the  pharynx  seem  to 
represent  the  nervous  system,  though  no  nerves  passing  from 
them  have  been  discovered  ;  nor  do  any  special  sense-organs 
exist. 

The  affinities  of  these  forms  is  highly  problematical,  especially  since 
nothing  is  known  of  their  development.  The  metamerism  indicated  by  tlie 
chitinons  rings,  the  thickenings  of  the  ectoderm,  and  the  dorso-ventrul 
muscles  suggest  an  affinity  with  the  Annelids,  while,  on  the  other  hand,  in 
the  chitinous  covering,  and  the  occurrence  of  a  longitudinal  musculature 
only,  similarities  to  the  Nematodes  may  be  found.  The  excretory  organs 
may  perhaps  be  compared  with  the  larval  nephridia  of  the  Annelids,  and 
the  existence  of  but  a  single  pair  of  them,  together  with  the  absence  of 
any  metameric  arrangement  of  nerve-ganglia,  favors  the  idea  that  tlie 


186 


IN  VERTEBRA  TE  MORPIIOLOO  Y. 


CO 


Ecliinodera  are  not  to  be  considered  as  being  truly  nictanieric,  indications 
of  metamerism  which  are  found  being  altogether  secondary  and  without 
phylogenetic  significance.     Until,  however,  something  has  been  ascertained 

rogardinglheir  embryological  history  nothing  can 
be  positively  stated  as  to  their  affinities.  It  is 
worthy  of  notice,  however,  that  in  some  particulars 
they  resemble  the  Gastrotricha,  and  it  is  not  im- 
probable that  their  nearest  allies  are  lo  be  found 
in  that  order,  which  on  its  part  is  related  to  the 
Rot  if  era  (see  p.  189). 

Class  CHiBTOGNATHA. 


The  Clieetognatha  constitute  a  small 
group  of  forms  separable  into  two  genera, 
Sagitta  and  /Spadella.  All  the  members 
of  the  group  are  marine,  and  are  elonga- 
ted in  form,  the  sides  of  the  body  being 
furnished  with  one  or  two  pairs  of  lateral 
expansions  or  fins,  to  which  is  added  a 
caudal  fin.  The  anterior  portion  of  the 
body  is  somewhat  enlarged  so  as  to  form 
a  head,  and  on  either  side  of  the  mouth 
are  a  number  of  strong  chitinous  bristles 
movable  by  means  of  special  muscles 
and  serving  the  purpose  of  jaws. 

The  ectoderm  consists  of  several 
layers  of  flattened  cells  giving  rise  in  the 
head  region  by  secretion  to  chitinous 
plates  which  serve  for  the  attachment 
of  the  muscles  which  move  the  jaw- 
bristles.  Both  the  lateral  and  the  caudal 
fins  are  ectodermal  expansions  consisting 
of  a  homogeneous  lamella  covered  by 
one  or  two  layers  of  ectodermal  cells. 
They  possess  no  muscle-fibres  and  are 
passive  in  locomotion,  which  is  per- 
formed by  the  contraction  of  the  longitudinal  muscles 
producing  rapid  lateral  movements  of  the  posterior  part  of  the 
body.  The  genus  Sagitta  possesses  two  lateral  fins,  while  in 
Spadeda  (Fig.  92)  but  one  large  one  is  present.    Below  the 


Pig.  9^.— Spadella  ceph- 
aloptera    <after  Hbrt- 
wio). 
ce  =  cerebral  ganglion. 
i  =  intestine. 
0  =  olfactory  organ. 
oc  =  eye. 
ov  =  ovary. 
t  =  testis. 


••2iiiJiiJ.»«i<tV«>' 


CLASS  CUu^TOONATIIA. 


187 


are 


ectoderm  lies  a  well-defiued  busemeut-membrane,  and  below 
this  are  the  muscles  of  the  body-wall,  which  are,  as  a  rule, 
lougitudinal  iu  their  directiou,  aud  are  interrupted,  as  iu  the 
Nematodes,  aloug  four  lougitudinal  lines,  one  dorsal,  one  ven- 
tral, and  two  lateral.  In  one  species  of  Spadella  there  is  on 
the  inner  side  of  the  longitudinal  muscles  a  thin  layer  of  trans- 
verse muscles,  but  usually  only  longitudinal  fibres  are  present, 
except  in  the  head,  when  there  are  a  number  of  special  muscle- 
bundles  for  the  movement  of  the  jaw-bristles. 

Within  the  musculature  of  the  bod^'-wall  is  the  spacious 
coelom,  lined  throughout  by  a  delicate  lay^.r  of  cells  constitut- 
ing the  peritoneum,  and  divided  into  three  chambers  by 
transverse  partitions,  one  of  which  lies  just  behind  the  head, 
while  the  other  is  towards  the  hind  end  of  the  body.  The 
peritoneal  epithelium  lines  the  surfaces  of  these  dissepiments, 
aud  in  the  trunk  and  tail  regions  is  reflected  in  the  mid- 
dorsal  and  ventral  lines  towards  the  centre  of  the  body,  form- 
ing a  mesentery,  surrounding  the  intestine  and  dividing  the 
coelom  into  lateral  compartments. 

The  mouth  lies  on  the  ventral  surface  of  the  head  and 
opens  into  an  oesophagus  surrounded  by  a  single  layer  of 
muscle-fibres  having  a  dorso-ventral  direction  and  passiug 
above  and  below  into  the  general  musculature  of  the  head. 
After  being  narrowed  in  passing  through  the  anterior  dissepi- 
ment the  digestive  tube  again  expands  (Fig.  92,  i),  and  is  sup- 
ported throughout  the  trunk  region  by  the  mesentery.  In 
this  region  it  is  a  simple  straight  tube,  unprovided  Avith  mus- 
cle-fibres, and  terminates  in  an  anal  opening  situated  ven- 
trally  at  the  junction  of  the  trunk  and  tail  regions,  not  being 
continued  into  the  latter. 

Neither  an  excretory  nor  a  blood  vascular  system  is  pres- 
ent. The  nervous  system  lies  for  the  most  part  imbedded  in 
the  ectoderm,  and  consists  of  two  principal  ganglionic  masses, 
of  which  one,  the  cerebral  or  supraoesophageal  ganglion  (ce), 
lying  in  the  head  region,  is  situated  in  the  ectoderm  of  the 
dorsal  surface  of  the  body  and  has  a  somewhat  hexagonal 
outline,  giving  off  five  pairs  of  nerves,  one  pair  passing  back- 
wards as  commissures  to  unite  with  the  ventral  or  sub- 
oesophageal  ganglion,  lying  also  in  the  ectoderm  a  little  iu 


ill 

ill 


1!    . 


188 


INVEHTEBllA  TE  MORPUOLOG  Y. 


frout  of  the  middle  of  the  trunk  region  of  the  body.  This 
gungliou  gives  off  numerous  nerves,  among  which  are  two 
principal  nerve-cords  passing  backwards  and  giving  off  along 
their  entire  length  tiner  nerves  which  branch  and  finally  lose 
themselves  in  a  tine  ectodermal  nerve-plexus  throughout 
which  ganglion-cells  are  scattered.  In  addition  to  these 
ectodermal  portions  three  pairs  of  ganglia  are  found  in  the 
head  region  at  the  sides  of  the  oesophagus,  the  largest  gan- 
glion on  either  side  being  united  with  the  supraoesophageal 
ganglion  by  a  commissure.  From  the  supraoesophageal 
ganglia,  behind  the  commissures  to  the  ventral  ganglion,  a  pair 
of  nerves  pass  backwards  to  the  two  eyes  {oc)y  which  lie  com- 
pletely imbedded  in  the  ectoderm  of  the  dorsal  surface  of  the 
head,  each  consisting  of  three  biconvex  lenses  imbedded  in  a 
central  pigment  mass  and  surrounded  on  their  outer  surfaces 
by  a  retina  composed  of  an  outer  layer  of  cubical  cells,  a 
middle  layer  of  cylindrical  cells  with  large  nuclei,  and  an 
inner  layer  of  rod-like  structures  arranged  perpendicularly  to 
the  surface  of  the  lenses.  Behind  the  eyes  lies  a  circular  band 
of  fine  columnar  ciliated  cells  (o),  which  is  supplied  by  a  pair 
of  nerves  arising  from  the  supraoesophageal  ganglion  be- 
tween the  optic  nerves.  The  function  of  this  organ  is  doubt- 
ful, though  it  has  been  considered  olfactory.  Scattered  some- 
what regularly  over  the  body  are  numerous  round  or  oval 
eminences  consisting  of  a  number  of  central  spherical  cells 
arranged  in  two  rows  and  bearing  rod-like  bristles.  These 
are  enclosed  in  a  sheath  of  cylindrical  cells  and  below  come 
into  contact  or  are  continuous  with  terminal  nerve-branches. 
These  sensory  hillocks  are  supposed  to  be  tactile  in  function 
and  resemble  not  a  little  the  lateral  sense-organs  of  certain 
Annelids  (see  p.  210). 

The  Chsetognatha  are  without  exception  hermaphrodite. 
The  ovaries  (ov)  are  cylindrical  bodies  lying  in  the  trunk  re- 
gion of  the  body,  one  on  each  side  of  the  digestive  tract,  and 
upon  the  outer  side  of  each  is  a  tubular  oviduct  which  ends 
blintlly  anteriorly  and  opens  posteriorly  at  the  sides  of  the 
body  near  the  dissepiment  between  the  trunk  and  tail  regions 
of  the  body.  There  is  no  communication  apparently  between 
the  cavity  of  the  oviduct  and  the  ovary  or  coelom,  and  the 


CLASS  ROTIFERA. 


189 


manner  in  which  the  t)va  make  their  escape  is  yet  iiuknowu. 
Both  ovaries  ami  oviducts  are  enclosed  within  a  fold  of  peri- 
toneum (mesentery)  exteudin}^  from  the  sides  of  the  body. 
The  testes  (/)  are  situated  behind  the  posterior  dissepiment, 
i.e.,  in  the  tail  region  of  the  body,  and  consist  of  a  streak  of 
cells  on  each  side  in  the  peritoneal  covering  of  the  body-wall. 
From  these  streaks  masses  of  immature  spermatozoa  separate 
and  Hoat  about  in  the  ccelom  of  the  tail  segment,  and  when 
mature  make  their  escape  through  canals,  each  of  which  com- 
municates with  the  ccelom  by  means  of  a  tine  ciliated  opening, 
and  near  its  opening  to  the  exterior  at  the  side  of  the  body  is 
dilated  into  a  seminal  vesicle. 

The  erabryological  history  of  Sagitta  throws  -i  .  liglit  upon  the  affinities 
of  these  forms.  In  structure  they  recall,  especially  in  the  arrangement  of 
their  musculature,  the  Nematodes,  and  esp<;.  iully  tluj  Gordiuuese,  but  at  the 
same  time  show  many  similarities  to  the  lo.ver  marine  Annelids,  as  for  in- 
stance in  the  origin  of  the  spermatozoa  from  the  "vall  of  the  ccelom,  and  in 
the  similarity  of  the  vasa  deforentia  to  nephridial  canals.  The  occurrence 
of  dissepiments  also  suggests  affinities  to  the  Annelids,  but  it  does  not 
seem  that  these  structures  indicate  a  segmentation  of  the  body,  since  the 
arrangement  of  the  nervous  system  points  to  the  conclusion  that  the  Chae- 
tognaths  consist  of  a  single  segment.  From  the  evidence  at  present  open 
to  us  it  would  seem  that  the  Chcetogtiatha  are  more  nearly  related  to  the 
Annelids  than  to  the  Nematodes,  but  the  relationship  must  be  regarded  as 
a  rather  remote  one,  and  it  seems  hardly  fitting  to  include  Sagitta  and  its 
allies  among  the  Annelida. 


Mte. 
re- 
md 
ids 
the 
ms 
fen 
he 


Class  Rotifera. 

The  Rotifers  or  "  Wheel-animalcules "  are  microscopic 
Metazoa  which  are  widely  distributed  both  in  salt  and  fresh 
water.  They  are  unsegmented  forms  with  a  well-developed 
coelom,  and  are  somewhat  oval  in  form  as  a  rule,  the  an- 
terior end  of  the  body  being  surrounded  by  one  or  two 
bands  of  cilia  whose  rapid  movement  produces  the  appear- 
ance of  a  wheel,  and  has  suggested  the  popular  name  for 
the  group.  The  posterior  t  :id  of  the  body  is  frequently  pro- 
longed into  a  usually  extensible  so-called  foot,  which  in  some 
cases  {Ladmdaria)  is  furnished  with  adhesive  glands,  and  is 
used  as  a  point  of  fixation,  though  the  majority  of  forms  swim 
about  freely  or  attach  themselves  only  temporarily,  the  foot 


i 


190 


INVEBTEBRATE  MOBPHOLOOY. 


0 


N 


having  in  such  cases  the  form  of  a  sucker  (Philodina),  or  ter- 
miuating  in  two  movable  lamella),  BracMonus  (Fig.  93),  or  else 
being  entirely  wanting  {Asplanchna). 

The  body,  with  the  exception  of  the  anterior  portion  or 
trochal  disk,  which  bears  the  bands  of  cilia,  is  enclosed  in  a 

Qg  chitinous  cuticle,  occasionally 
comparatively  thick  and  firm, 
forming  a  case,  the  lorica,  into 
which  the  softer  parts  may  be 
^  withdrawn,  and  frequently  pre- 
senting a  delicate  sculpturing 
GI  or  prolongations  into  spines. 
A  few  forms  (Floscularia)  se- 
crete a  gelatinous  case  within 
Qy  which  they  live,  foreign  parti- 
cles being  sometimes  added 
to  the  secretion  ;  a  species  of 
Melicerta,  for  instance,  building 
a  case  for  itself  of  pellets  man- 
ufactured from  foreign  bodies 
and  arranged  in  oblique  or 
spiral  rows  and  cemented  to- 
gether by  the  gelatinous  secre- 
tion. 

The  trochal  dish  which  oc- 
Brachionus  urceolaris  (after         •       Ji  ■     •  i     p  ji 

cupies  the  anterior  end  of  the 

body  is  but  rarely  circular  in 

outline ;    more    usually    it    is 

lobed  at  its  margins  and  may 

even   be    separated    into    two 

parts.     The  margin  of  the  disk 

is  surrounded  b}'  one  or   two 

bands  of  cilia  which  follow  the 

lobatious,  when  two  bands  are 

present    one     being     entirely 

prflBoral  and  the  other  postoral  in  its  position,  so  that  the 

mouth  lies  between  the  two  on  the  ventral  side  of  the  disk. 

Various  differences  of  arrangement  of  the   bands  are  found 

in  diflferent  species,  one  of  them,  the   preeoral,  being  some- 


FiG.  93 

Eckstein  I. 
Br  =  uerve-gaugliou. 

cv  =  cou tractile  vesicle. 
Gl  =  (lis^cstive  gland. 

M  =  muscle. 
Ma  =  iiiastax. 

N  =  nepliiidial  canal. 

0  =  eye. 
Oc  =  ocellus. 
Ov  =  ovary. 
8p  =  calcar. 


,Ji^te,fflgJfi& 


CLASS  ROTIFERA. 


191 


or 


Ik. 


times  discontinuous,  as  in  Brachionus,  or  reduced  to  a  few 
isolated  patches,  as  in  Asplanchna. 

It  is  a  question  whether  the  forms  with  a  double  band  of  cilia  or  those 
in  which  it  is  single  represent  the  more  primitive  arrangement.  It  may  be 
supposed  that  originally  there  was  but  a  single  band  which  later  became 
double,  but  it  seems  more  probable  that  the  double  condition  is  the  more 
primitive,  from  the  fact  of  its  frequent  occurrence  and  also  since,  when 
a  single  band  is  present,  it  seems  to  represent  in  some  cases  the  pra?- 
oral  band  and  .in  others  {Floscularia)  the  i)ostoral  one.  Such  a  condition 
of  affairs  can  be  most  plausibly  explained  on  the  assumption  that  originally 
two  bands  were  present,  and  that  in  some  forms  the  pra^oral  one  gradually 
gained  pre-eminence  in  its  development,  the  postoral  one  disappearing  pari 
passu,  while  in  the  Flosculariidae  the  reverse  was  the  case. 

Beneath  the  cuticle  lies  the  ectoilerni,  consisting  of  a  layer 
of  cells  whose  outlines  cannot  be  distinguished,  and  within 
this  comes  the  musculature  of  the  bod}',  which  does  not,  how- 
ever, form  a  more  or  less  continuous  layer  beneath  the  skin, 
but  consists  of  aggregations  of  muscle-tibres  into  bundles 
which  traverse  the  body-cavity  in  various  directions,  some 
running  longitudinally  and  forming  retractors  of  the  foot  and 
of  the  trochal  disk,  while  others  have  a  circular  direction.  The 
ccelom,  in  Avliich  the  muscle-bundles  and  the  various  organs  lie, 
is  not  lined  by  a  special  peritoneal  layer  of  cells,  but  may  be 
traversed  by  a  greater  or  less  number  of  delicate  fibrils 
arising  from  amoeboid  cells  and  representing  undifferentiated 
mesoderm. 

The  mouth  lies  near  the  ventral  border  of  the  trochal  disk, 
the  ciliated  bands  serving  to  produce  currents  which  con- 
verge toward  the  mouth-opening,  and  so  carry  to  it  food-parti- 
cles, which  are  then  carried  through  the  ciliated  oesophagus 
to  the  pharynx,  whose  walls  contain  a  somewhat  complicated 
comminuting  apparatus,  the  mastax  (Fig.  93,  ma),  consisting 
of  two  calcareous  bodies,  the  mallei,  of  varying  shape,  and 
sometimes  also  of  a  median  body,  the  incus.  By  the  action  of 
musclefci  attached  to  the  mallei,  these  parts  of  the  apparatus 
can  be  brought  into  contact  with  each  other,  and  with  the 
incus  when  this  is  present,  the  food-particles  being  thus 
comminuted.  From  the  pharynx  the  food  passes  through  a 
ihorter  or  longer  tube  lined  with  chitiu,  which  is  to  be 
regarded  as  a  continuation  of  the  pharynx,  to  the  stomach, 


I       < 


■  1  ■' 

I  i       i 


192 


INVERTEBRATE  MORPHOLOGY, 


usually  a  globular  cavity,  whose  wall  is  formed  by  a  layer  of 
ciliated  cells  contaiuiug  fat-globules  and  various  other  par- 
ticles, probably  absorbed  food-particles,  these  cells  being 
covered  externally  by  a  layer  of  connective  tissue.  Into  the 
stomach  there  opens  from  either  side  the  duct  of  a  gland  (gl), 
whose  secretioii  is  probably  digestive  in  function  and  which 
may  be  termed  a  digestive  gland  from  its  resemblance  to 
similarly  located  glands  in  other  invertebrates.  The  stomach 
opens  below  into  the  shorter  or  longer  intestine,  whose  walls 
are  lined  by  ciliated  cells  ;  and  this  in  turn  communicates  with 
the  terminal  cloaca,  which  receives  in  some  cases  the  termi- 
nations of  the  excretory  tubules  and  may  be  contractile. 
The  cloaca  opens  to  the  exterior,  usually  on  the  dorsal  sur- 
face, near  the  base  of  the  foot,  though  in  some  forms  which 
live  within  a  case  the  intestine  bends  forward  upon  itself,  so 
that  the  cloacal  opening  lies  further  forward. 

The  nervous  system  consists  of  a  relatively  large  ganglionic 
mass  (Br)  lying  on  the  dorsal  side  of  the  oesophagus,  from 
which  nerves  pass  anteriorly  to  the  trochal  disk,  and  posteriorly 
to  supply  a  dorsal  sensory  papilla,  the  calcar  (Sp).  In  addi- 
tion to  this,  two  pairs  of  posterior  nerves  have  been  described, 
one  of  which  passes  to  a  sense-organ  situated  on  each  side  of 
the  body  in  its  posterior  third,  while  the  other  pair  runs  back- 
wards on  each  side  of  the  middle  line  to  near  the  posterior 
end  of  the  body,  giving  oflf  branches  to  the  musculature  as 
it  goes.  Among  the  sense-organs  eyes  {O)  are  very  generally 
present,  varying  in  number  from  one  to  several,  and  situated  in 
the  region  of  the  supraoesophageal  ganglion,  with  which  they 
are  connected.  They  consist  of  patches  of  red,  brown,  or 
black  pigment  with  which  sensory  or  retinal  cells  are  asso- 
ciated, and  which  are  in  some  cases  covered  by  a  refracting 
lens  formed  as  a  special  cuticular  thickening.  Other  sense- 
organs  to  which  a  tactile  function  has  been  ascribed  con- 
sist in  their  simplest  form  of  one  or  several  cells  bearing 
stiff  cilia.  A  pair  of  such  organs  is  usually  present,  one  on 
each  side  immediately  above  the  ganglion  of  the  lateral 
nerves,  and  anteriorly  in  the  mid-dorsal  line  just  behind  the 
trochal  disk  a  third  occurs,  the  cahar  (Sp),  which  frequently 
is  situated  upon  the  extremity  of  a  tubular  extensible  process 


CLASS  ROTIFBRA. 


193 


as 


they 


tiug 
ise- 
jon- 
j-iug 
on 
)i-al 
Ithe 

]tiy 

less 


of  the  body-wall,  supplied  with  muscles  for  its  retraction,  and 
to  which  nerve-fibres  pass  from  the  supraoesophageal  ganglion. 
In  a  few  forms,  such  as  Melicerta,  the  calcar  is  double. 

No  blood  vascular  system  exists,  but  a  well-developed 
excretory  apparatus  [N),  resembling  that  of  the  Turbellaria, 
is  present.  It  consists  of  two  lougitudinal  tubes,  one  on  each 
side  of  the  body,  from  which  arise  a  varying  number  of  finer 
lateral  branches,  each  of  which  terminates  in  a  fuunel  closed 
by  a  flame-cell,  as  in  the  Turbellaria.  Anteriorly  the  two 
tubes  may  be  united  by  a  transverse  connecting  tube,  and 
posteriorly  they  may  unite  together  to  form  a  contractile 
bladder  which  opens  into  the  cloaca,  or  in  some  cases  may 
open  directly  to  the  exterior. 

The  female  reproductive  apparatus  consists  of  a  relatively 
large  ovary  {Ov)  which  in  some  cases  at  least  consists  of  a 
vitellarium  portion  and  an  ovary  proper,  the  whole  being 
surrounded  by  a  thin  membrane  a  backward  prolongation  of 
which  forms  an  oviduct  opening  into  the  cloaca. 

The  preceding  description  of  the  structure  of  a  Kotifer  is 
that  of  such  a  form  as  is  most  frequently  met  with.  It  was 
for  a  long  time  believed  that  these  were  hermaphrodite,  but 
no  trace  of  a  testis  could  be  found.  It  was  later  found,  how- 
ever, that  they  were  all  females,  and  the  males  of  several 
species  have  been  discovered,  differing  decidedly  in  size  and 
structure  from  the  females,  and  besides  being  usually  rather 
rare  in  their  occurrence.  They  are  considerably  smaller  than 
the  female,  and  possess  like  it  eyes,  nerve-ganglion,  muscles, 
and  excretory  system ;  but  the  ciliated  band  of  the  trochal 
disk  is  single,  and  the  digestive  tract,  with  the  exception  of  the 
cloaca,  is  reduced  to  a  solid  band  of  tissue.  The  single  testis 
occupies  the  greater  portion  of  the  body-cavity,  and  the  short 
vas  deferens  opens  into  the  cloaca,  passing  through  an  evertible 
intromittent  organ.  This  marked  difference  of  form  of  the 
male  and  female  individuals  of  the  same  species  constitutes 
a  phenomenon  known  as  sexual  dimorphism.  An  explanation 
of  the  usual  numerical  preponderance  of  the  females  over  the 
males  is  to  be  found  in  the  fact  that  under  favorable  condi- 
tions the  females  produce  ova  capable  of  developing  parthe- 
nogenetically,  and  giving  rise  in  all  cases  to  females.    A  series 


till 


194 


INVERTEBRATE  MORPHOLOGY. 


of  generations  reproducing  by  these  so-called  "  summer  ova  " 
may  thus  succeed  each  other  without  the  intervention  of  a 
male.  Under  certain  conditions,  however,  certain  females  pro- 
duce "  summer  ova  "  of  a  smaller  size  than  usual,  which,  devel- 
oping parthenogenetically,  give  rise  to  the  males.  In  addition 
to  these  two  forms  of  "  summer  ova,"  some  species  produce  a 
third  kind  of  egg,  the  so-called  "  winter  ovum,"  which  differs 
from  the  summer  ova  in  containing  more  yolk  and  in  being 
enclosed  within  a  stout  resistant  shell.  It  seems  probable 
that  these  ova  develop  only  after  fertilization. 

There  are  two  Rotifers  which  deserve  ii  special  description  on  account  of 
their  having  served  as  a  basis  for  phylogenetic  speculation.    One  of  these, 

Trochosphcera  (Fig.  94),  is  spherical 
in  shape  ;  a  band  of  cilia  runs  round 
the  equator  of  the  sphere,  not  encirc- 
ling it  completely,  however,  but  leav- 
ing an  unciliated  region  on  the  dorsal 
surface.  Anteriorly  this  band  passes 
above  the  mouth-opening,  which  is 
bounded  below  by  a  very  small  post- 
oral  band  and  opens  into  a  pharynx 
provided  with  a  mastax  (Ma),  from 
which  the  stomach,  with  digestive 
glands,  passes  towards  the  centre  of 
the  body  and  there  bends  at  right 
Fig.  9i.—Trochosp7uera  oequatorialis  angles  to  open  through  the  intestine 


(after  Semper). 

A  =  anus. 

CI  =  cloaca. 
Ms  =  excretory  tube. 

M  =  mouth. 
Ma  =  mastax. 
mu  =  muscle. 

N  =  uerve-ganglion. 

n  =  nerve. 
o  =  ovary. 

So  =  sense-organ. 


into  a  cloaca  (CI)  which  receives  the 
excretory  tubules  (Ex)  and  the  oviducts 
and  opens  to  the  exterior  at  the  lower 
pole  of  the  sphere  (A).  The  brain  (If) 
lies  above  the  pharynx  and  sends 
nerves  to  the  two  eyes  situated,  one 
on  each  side,  below  the  equatorial 
band  of  cilia,  and  also  to  a  small 
sensory  papilla  (So),  probably  the 
calcar,  lying  on  the  dorsal  surface. 


this  nerve  (n)  encircling  the  anterior 
half  of  the  sphere,  and  running  in  a  plane  at  right  angles  to  that  in  which 
the  ciliated  band  lies. 

The  other  form  belongs  to  the  genus  Hexarthra  and  differs  from  other 
Rotifers  principally  in  the  occurrence  of  six  hollow  processes  of  the  body, 
arising  from  the  ventral  surface  and  arranged  in  pairs  diminishing  in  size 
from  before  backwards.  Each  is  terminated  by  a  bunch  of  stiff  bristles  or 
setae,  and  all  are  supplied  with  muscles  whereby  they  can  be  rapidly  swept 


ORDER  GASTROTRICHA. 


195 


*f 


backwards  in  the  manner  of  a  paddle  and  so  serve  as  locomotor  organs,  pro- 
ducing a  quick  jerky  movement  quite  different  from  the  steady  progression 
caused  by  the  cilia  of  the  trochal  disk.  In  another  nearly-related  form, 
Pedalion,  six  processes  are  also  present,  but  are  arranged  somewhat  dilfer- 
ently  from  those  of  Hexarthra,  the  largest  one  arising  from  the  ventral 
and  another  from  the  dorsal  surface,  while  the  other  four  are  lateral  in 
position,  two  occurring  on  each  side. 

The  Affinities  of  the  Rotifera. — Several  views  have  been  advanced  as 
to  the  affinities  of  the  Rotifers,  especially  as  regards  their  relationships  to 
higher  forms ;  these  opinions  will  not,  however,  be  fully  considered  here, 
but  merely  indicated,  attention  being  directed  first  to  the  relationships  in 
which  the  Rotifers  stand  to  organisms  lower  in  the  scale.  In  this  connec- 
tion the  excretory  system  becomes  of  no  little  importance  on  account  of  its 
resemblance  to  that  of  the  Turbellaria,  a  resemblance  which  is  further 
emphasized  by  the  nervous  system, — consisting  of  the  simple  brain,  from 
which  posteriorly-directed  nerve-cords  arise, — by  the  combined  ovary  and 
vitellarium,  and  by  the  absence  of  a  blood  vascular  system.  Here,  how- 
ever, the  resemblance  ceases,  and  the  presence  of  an  anal  opening  to  the 
digestive  tube  marks  the  Rotifers  as  standing  on  a  higher  level  than  the 
Turbellaria.  It  seems  probable,  however,  that  the  similarities  do  indicate 
the  ancestry,  and  that  the  Rotifera  have  been  derived  from  the  Turbellarian 
type. 

Another  possibility  which  has  been  suggested  is  to  the  effect  that  they 
are  derived  from  the  form  represented  by  the  Trochophore  larva  of  the 
Annelida  (see  p.  213).  The  principal  argument  for  this  view  is  found  in  the 
arrangement  of  the  trochal  cilia,  which,  in  the  occurrence  in  many  cases  of 
both  praeoral  and  postoral  bands,  certainly  resembles  not  a  little  that  of 
the  Trochophore  larva.  It  must  be  remembered,  however,  that  the  similar- 
ity in  the  arrangement  of  tlie  cilia  is  not  quite  perfect,  and  that  it  may  be 
without  phylogenetic  significance,  having  been  acquired  independently  in 
the  Rotifers  and  in  the  Trochophore  larva  ;  and  furthermore  it  is  noticeable 
that  in  one  important  character  at  least  a  marked  difference  is  found,  the 
nervous  ganglion  lying  in  the  Rotifers  behind  in.stead  of  before  the  pra?oral 
band  of  cilia.  The  most  that  can  be  said  at  present  is  that  the  Rotit'eis 
show  closer  structural  affinities  to  the  Turbellaria  than  to  any  other  group, 
and  that  it  is  probable  that  they  represent  the  culmination  of  a  line  of 
development  originating  in  that  group,  and  furthermore  that  it  is  possible 
that  they  represent  the  ancestral  annelid  form  indicated  by  the  Troclio- 
phore  larva. 


Her 

y, 

ze 
or 
pt 


Order  Gastrotricha. 

The  Gastrotricha  are  minute  forms,  few  exceeding  0.2  mm. 
in  length,  which  occur  in  fresh  water  and  have  an  elongated 
form  flattened  somewhat  on  the   ventral   surface,   tapering 


196 


Ili  VERTEBRA  TE  MORPHOLOQ  T. 


i 


ee— 


posteriorly  to  end  usually  in  one  or  two  cereal  processes, 
and  anteriorly  show  a  dilatation  succeeded  by  a  more  or 
less  well-marked  narrow  region,  the  two  giving  rise  to  a 
bead  and  neck.  The  body  is  covered  upon  the  outside  by  a 
cuticle,  which  may  be  smooth  as  in  the  genus  Jchthydium,  or 
take  the  form  of  overlapping  scales  as  in  Chcetonotm  (Fig.  95), 
sometimes  bearing  spine-like  prolongations.  Along  the  ven- 
tral surface  two  bands  of  cilia  run  from 
the  posterior  part  of  the  head  region 
almost  to  the  hind  end  of  the  body, 
and  in  addition  to  these  patches  of 
cilia  are  found  upon  the  ventral  surface 
and  on  the  sides  of  the  head,  some  of 
which  are  undoubtedly  sensory  in  func- 
tion. Beneath  the  cuticular  covering 
lies  the  ectoderm  in  the  form  of  a  layer 
of  protoplasm  in  which  no  cell  outlines 
can  be  perceived,  but  which  contains 
numerous  scattered  nuclei.  A  pair 
of  longitudinal  muscle-bands  lie  be- 
neath the  ectoderm  on  the  dorsal  sur- 
face, and  other  bauds  traverse  the 
coelom  in  an  antero-posterior  direction. 
Transverse  and  circular  muscles  are, 
however,  absent.  A  distinct  coelom  is 
present,  the  greater  portion  of  which 
is  occupied,  however,  by  the  internal 
organs  ;  it  is  not  lined  with  a  peri- 
toneal epithelium,  nor  are  any  mesen- 
teries present. 

The  mouth  is  situated  at  the  an- 
terior extremity  of  the  body  and  opens 
into  a  muscular  oesophagus  (o?),  which 
opens  in  turn  into  the  cylindrical 
stomach  (t).  To  this  succeeds  a  short  intestine  opening  to 
the  exterior  at  the  posterior  extremity  of  the  body. 

No  blood  vascular  system  is  present,  but  the  excretory  sys- 
tem consists  of  a  single  pair  of  much-convoluted  tubes  (nephr) 
which  terminate  at  one  end  in   a  closed  ciliated  "funnel," 


Fig.  95.—Ch(etonottu  maxi- 
mu8  (after  Zulinka). 
eg  =  iierve-gnngliou. 
dr  =  glauds. 
i  —  intestine. 
m  =  longitudinal 
muscles. 
ntephr  =  nepliridia. 
oe  =  (Bsopbugus. 
on  =  ovury. 


ORDER  OASTROTRICHA. 


197 


while  ut  the  other  they  open  ou  the  veutral  side  of  the  body 
to  the  exterior.  The  reproductive  system  (ov)  consists  of  two 
groups  of  germ-cells  lying  in  the  posterior  part  of  the  body, 
one  ou  each  side  of  the  digestive  tract,  but  no  oviduct  has  been 
definitely  made  out  to  exist.  With  regard  to  the  testes  some 
uncertainty  exists,  an  oval  body  lying  in  the  same  region  of 
the  body  as  the  ovaries,  but  beneath  the  intestine,  having  been 
described  as  such  an  organ,  though  the  identification  is  open 
to  question.  If,  however,  the  body  in  question  be  the  testes, 
the  animals  are  hermaphrodite.  As  in  the  case  of  the  female 
organ  no  ducts  have  been  observed  leading  from  the  testes, 
and  nothing  is  known  as  to  the  method  by  which  the  sexual 
products  are  extruded. 

The  nervous  system  (n)  consists  of  a  large  ganglionic  mass 
which  lies  above  the  oesophagus  in  the  head  region,  and  from 
the  posterior  border  of  which  two  processes,  one  on  each  side 
of  the  middle  line,  are  directed  posteriorly  and  dorsally, 
perhaps  representing  the  origin  of  a  pair  of  nerves,  while  the 
postero-external  angles  of  the  ganglionic  mass  are  continued 
backwards  to  near  the  posterior  extremity  of  the  body  to 
form  the  lateral  nerves.  Certain  of  the  elongated  cilia  found 
on  the  head  no  doubt  function  as  sense-organs,  coming  into 
intimate  connection  at  their  bases  with  the  cells  of  the  supra- 
oesophageal  ganglion ;  in  addition  to  these  sense-organs  eyes 
have  also  been  described  as  occurring  in  some  species,  either 
in  the  form  of  simple  patches  of  pigment  lying  in  the  integu- 
ment above  the  brain,  or  else  of  such  patches  provided  with 
lens-like  structures. 

The  affinities  of  the  Qastrotriclia  seem  almost  certainly  to  be  with  the 
Rotifera,  many  of  the  structural  features  being  exceedingly  similar  in  the 
two  groups.  The  principal  differences  are  to  be  found  in  the  an-angement 
of  the  cilia  and  in  the  structure  of  the  nephridia.  With  regard  to  the 
former  it  seems  not  iiiii)robable  that  in  the  arrangement  seen  in  the  Oastro- 
tricha  a  relic  of  a  more  primitive  uniform  ciliation  is  presented,  and  that 
in  this  particular  as  well  as  in  the  greater  simplicity  of  the  digestive  tract, 
and  in  the  general  form  of  the  body  and  life-habits,  the  Gastrotricha 
approach  more  nearly  an  ancestral  Turbellarian  form  than  do  the  Rotifera. 
The  nephridia  depart  much  more  widely,  however,  from  the  Turbellarian 
condition  than  do  those  of  the  Rotifera— a  fact  which  argues  against  the 
more  primitive  character  of  the  Gastrotricha,  as  does  likewise  the  absence 
of  ducts  for  the  reproductive  organs.     Whether,  therefore,  the  Gastrotricha 


198 


INVERTEBRA  TE  MORPUOLOO  T. 


I: 


are  to  be  considered  as  representing  the,  ancestral  forra  from  which  both 
they  and  the  Rotifera  have  descended  more  nearly  than  the  latter  group, 
or  whether  they  are  modifications  of  the  Rotifer  type  of  structure  and  have 
had  for  their  ancestors  forms  which  were  Rotifer-like  in  structure,  it  is 
difficult  to  say ;  though  the  balance  of  evidence  seems  to  tip  in  favor  of 
tiio  former  view. 

Attention  should  be  called,  however,  to  a  possible  affiliation  of  the  Qas- 
trotric?ia  with  the  Echinodera,  If,  as  has  been  suggested  (p.  186),  the  seg- 
mentation of  the  latter  has  no  piiylogenetic  significance,  it  is  not  difficult 
to  trace  similarities  of  structure  in  the  two  groups,  the  principal  differ- 
ences being  connected  with  external  parts.  It  is  by  no  means  improbable 
that  the  Gastrotricha,  Rolifera,  and  Echinodera  form  a  series,  each  of  the 
groups  being  of  equivalent  rank,  and  related  to  each  other  somewhat  as 
are  the  Turbellaria,  Trematoda,  and  Cestoda. 

Genus  Dinophilus. 

The  genus  Dinophilm  includes  some  small  marine  organ- 
isms all  of  which  are  referable  to  a  small  number  of  species. 
The  body  (Fig.  96)  is  cylindrcal  and  consists  of  a  head  segment 

followed  by  from  5-7  trunk  segments  (the 
number  varying  according  to  the  species), 
each  of  which  bears  a  ring  of  cilia,  inter- 
rupted ventrally  by  a  uniform  ciliation  which 
covers  the  entire  ventral  surface.  The 
head  is  likewise  provided  with  a  ring  of 
cilia  which  is  usually  double,  one  of  the 
constituent  bands  passing  in  front  of  the 
mouth  and  the  other  behind  it,  the  area  in- 
tervening between  these  two  bands  being, 
in  one  species  at  least,  occupied  by  smaller 
cilia.  The  musculature  of  the  body-wall 
is  but  weakly  developed,  though  both  an 
external  layer  of  circular  fibres  and  an  in- 
ternal one  of  longitudinal  fibres  may  be 
found,  both  layers  being  absent  in  one 
species  in  the  dorsal  region.  The  coelom 
is  traversed  by  a  network  of  branching  cells, 
there  being  no  special  peritoneal  layer,  and 
no  musculature  in  the  walls  of  the  intestine. 
The  mouth  is  situated  on  the  ventral  surface  at  the  junc- 
tion of  the  head  and  first  trunk  segments,  and  leads  into  a 


Fig.  QQ.—Dinophiiua 
gyrociliatus  (after 
Mayer). 

ne  =  nephridium. 

ov  =  ovary. 

ag  =  salivary  gland. 


ii*^ 


0  Elf  us  DINOPUILVa. 


199 


8, 

id 
e. 


a 


wide  ciliated  oesopbagus,  beneath  which  lies  a  muscular  pro- 
boscis contained  in  a  special  sheath  and  protrusible  throuj^h 
the  mouth-opening.  Behind  the  oesophagus  is  aproventi'iculus, 
a  small  thick-walled  ciliated  cavity,  into  which,  at  its  junction 
with  the  oesophagus,  a  pair  of  salivary  glands  (sg)  pour  their 
secretion.  Behind,  the  proventriculus  communicates  with  a 
cylindrical  stomach,  upon  which  follows  the  short  straight  in- 
testine, terminating  in  the  anus  at  the  posterior  end  of  the 
body. 

There  is  no  blood  vascular  system.  An  excretory  system 
is  present,  consisting  in  J),  gyrociliatus  and  D.  tivniatus  of  five 
pairs  of  nephridia  (wc)  which  open  externally  on  the  sides  of 
the  body  and  terminate  in  the  coelom-spaces  in  a  funnel  con- 
taining a  flame-like  bunch  of  cilia.  Whether  a  direct  commu- 
nication between  the  lumen  of  the  nephridial  tubes  and  the 
coelom  exists  in  all  cases  has  not  been  definitely  ascertained, 
but  a  similarity  of  structure  to  the  Platyhelmiuth  type  of 
nephridium  is  shown  by  the  flame-like  bunch  of  cilia  and  by 
each  nephridium  being  composed  of  a  series  of  perforated  cells. 
The  reproductive  organs  are  separated  in  different  individu- 
als ;  and  in  one  species,  D.  gijrociliatuSy  a  marked  sexual  dimor- 
phism similar  to  that  occurring  in  the  Botifera  exists,  the 
male  being  much  smaller  than  the  female  and  possessing  only 
the  ciliated  ring  of  the  head  and  the  ventral  ciliation ;  and 
furthermore  the  digestive  tract  and  the  principal  sense- 
organs  are  entirely  wanting.  The  reproductive  elements  {ov) 
are  shed  into  the  coelom-spaces  and  find  their  way  to  the  ex- 
terior in  some  species  at  least  by  means  of  the  most  posterior 
pair  of  nephridia,  which  in  the  male  of  D.  tceniatus  are  trans- 
formed into  seminal  vesicles  and  are  connected  with  an  intro- 
mittent  organ  situated  in  the  posterior  segment. 

The  nervous  system  consists  of  a  brain  or  supraoesophageal 
ganglionic  mass  which  occupies  the  greater  portion  of  the 
head  segment  and  from  which  two  nerve-cords  pass  back- 
wards in  the  lateral  region  of  the  body,  and  in  D.  tceniatus 
possess  ganglionic  enlargements  equal  in  number  to  the  pairs 
of  nephridia  and  the  trunk  segments  and  are  connected  by 
transverse  commissures.  In  other  species,  however,  these 
structures   have   not  been  observed.     Eyes  occur  imbedded 


200 


INVERTEBRATE  MORPUOLOOK. 


iu  the  substance  of  the  supracesophageal  ganglion,  and  tactile 
hairs  occur  at  various  regions  of  the  body. 

Affinities  of  Dinophilus. — The  descriptions  given  of  the  various  known 
species  of  Dinophilus  indicate  a  considerable  variation  in  the  structure  of 
certain  parts,  more  especially  of  the  nervous  system,  which  in  D.  taniatus 
partakes  of  the  metamerism  shown  by  the  nephridia  and  the  bands  of  cilia, 
while  in  other  forms  it  is  apparently  non-metameric.  This  would  indicate 
either  that  the  metamerism  has  been  acquired  within  the  limits  of  the 
genus,  or  else  that  those  forms  lacking  it  are  degraded  in  this  respect  and 
have  descended  from  metameric  ancestors.  Tbere  is  little  justification  to 
be  found,  however,  for  the  calling  in  of  degradation  to  explain  obscure  re- 
lationships unless  there  is  sufficient  collateral  evidence  to  support  such  an 
appeal ;  in  the  present  case  this  seems  to  be  absent,  and  the  marked  simi- 
larity of  the  non-metameric  nervous  system  to  that  of  the  Turbellaria  sug- 
gests an  origin  from  these  forms  and  favors  the  first  hypothesis  as  to  the 
origin  of  the  metamerism.  The  nephridia  also  and  the  character  of  the 
coelom  strengthen  the  probability  of  a  Turbellarian  ancestry. 

A  close  relationship  to  the  Rotifera  has  also  been  suggested  and  is  not 
debarred  by  the  supposition  of  a  descent  from  Turbellarian  forms ;  but  it 
seems  doubtful  if  such  a  relationship  can  be  other  than  a  very  distant  one. 
The  position  of  the  supracesophageal  ganglion  relatively  to  the  cephalic 
cilia  or  prototroch,  and  the  paired  arrangement  of  the  nephridia  as  well  as 
the  occurrence  of  circular  fibres  in  the  subepidermal  musculature,  stand 
in  opposition  to  the  view,  and  the  most  that  can  be  said  is  that  both 
DinophiliLS  and  the  Rotifera  are  to  be  referred  back  to  closely-similar 
ancestors. 

The  afSnities  of  Din'  philus  and  the  Rotifers  to  the  Annelida  will  be 
discussed  in  connection  with  the  latter  group  (p.  217). 


SUBKINGDOM  METAZOA. 

Order  Eehinodera. — Body  cylindrical,  with  11  rings  ;  no  cilia  ;  with  pro- 
boscis ;  minute  forms  ;  marine.     Echinoderes. 

Class  OttfiTOGNATHA. — Marine ;  body  divided  into  three  segments ;  with 
lateral  and  tail  fins ;  mouth  with  chitinous  jaws  composed  of 
series  of  strong  bristles.    Sagitta,  Spadella. 

Class  Rotifera. — Anterior  end  provided  with  a  retractile  crown  of  cilia  ; 
minute  forms  both  aquatic  and  marine.  Floscularia,  Melicerta, 
Ladnularia,  Philodina,  Brachionus,  Asplanchna,  Trocho- 
sphcera,  Pedalion,  HeoMrthra. 

Order  &astrotricha.—l^mvi\/Q  forms  both  marine  and  aquatic ;  ventral  sur- 
face of  body  ciliated ;  no  anterior  crown  of  cilia.  Ichthydium, 
ChcBtonottis. 

Genus  Dinophilus.— Small  marine  forms ;  body  with  5-7  segments,  each 
with  a  ring  of  cilia. 


GENUS  DlNOPHlLUa. 


201 


LITERATURE. 

ECUINODERA. 

^'^^HS!^'-  ^T'^y'''^'^  (Echinoderes).   ihr  aruOominher  Ban   und  ihre 
SteUung  m  System.    Zeitscbr.  fUr  wissensch.  Zoologie,  XLV.  1887 

CttfiTOGNATHA. 

"■  "im *■  "^^  ^**'^'*^^'*-    ^""^  ^<^8Ta^ie'   JenaiflcheZeitschr.  xiv. 

BOTIFERA. 

C.  T.  Hud.on  and  P.  K.  Oo«..    The  Rotifera  <yr  Wheel-animalcule,.    London, 
^'  'xit  mT^'  ''*''  ^""^""^S"^^^^^'  ^'^  Rotatimen.    JenaiscLe  Zeitechr.. 

'•  'let;  ^:uZZ:^t:^:''  ^^^-^^-  ^^^  -^— ^-  ^-logie.  -xv 

GA8TK0TRICHA. 

C.  Zelinka.    m  QoitrotrieJuin.    Zeitschr.  f tlr  wissensch.  Zoologie.  xlix,  1889. 

DIN0PHILU8. 

E.  Komhelt.     ^^erBau  und  Entwicklung  des  DinophUu,  apatris.    Zeitschr 
filr  wissensch.  Zoologie,  xxxvir,  1882.  ^e«8Ciir. 

^xxVn;^86.°°*    ^'^^''^^*^^««-    Q""teriy  Journ.  of  Microscop.  Science. 


202 


ly VERTEBRA  TE  MORPUOLOO  T, 


CHAPTER  X. 


TYPE    ANNELIDA 


The  type   Annelida   includes   a  series   of  forms   among 

which  metamerism  reaches  a  high  grade  of  development.     In 

what  may  be  considered  a  typical  Annelid 

(Fig.  97)  a  number  of  segments  or  meta- 

meres    succeed    one    another    from    the 

head  to  the  tail,  each  one  resembling  its 

predecessor  and  its  successor  in  all  its 

parts  ;  the  nephridia,  reproductive  organs^ 

nerve-ganglia  (n),  and  appendages,  when 

present,  are  repeated  in  each  successive 

segment,  and  each  metamere  is  marked 

oflf  from  its  fellows,  externally  by  a  groove 

surrounding  the  body  and  internally  by 

a    partition    or    dissepiment    extending 

transversely  across  the  coelom  from  the 

body-Avall   to   the  digestive   tube.     This 

latter  structure  and  tlie  blood  vascular 

tubes   cannot   well    from    the  nature   of 

things  be  divided  metamerically,  but  are 

continuous  from  one  end  of  the  body  to 

the  other,  showing,  however,  in  the  meta- 

meric  pouches   and  intermetaraeric  con- 

m      nr*     rk,  „   .„  «™  strlctious  of  the  digestive  tract,  and  in 
Pig.  97. — Di^^giiam  of  ,  "  ' 

General  Plan  of  an  ^^^  metamerically  arranged  lateral  vessels 
Annelid.  of  the  blood  vascular  system  which  encir- 

cle the  digestive  tube,  indications  of  the  di- 
vision which  has  affected  the  other  organs. 
Two  segments,  however,  the  head 
(pr)  and  the  tail,  usually  present  differ- 
ences from  the  rest  in  their  structure ; 
the   head  or  anterior  metamere    bears   sense-organs  when 


a  =  anus. 

ee  =  cerebral  ganglion. 
m  =  mouth. 
n  =  ventral  nerve-cord. 
pr  =  prostomium. 


TYPE  ANNELIDA. 


203 


these  are  developed,  is  destitute  of  uephridia  in  the  adult, 
Hud  contains  priuiurily  the  supruwsopluigeal  guugliou  of  tliu 
nervous  system  (i^'ig.  97,  ce),  the  ganglia  of  the  trunk  meta- 
meres  (w)  lying  ventral ly  to  the  digestive  tube;  while  the  tail 
segment  bears  the  anal  opening  and  usually  presents  other 
characteristics  which  distinguish  it  from  the  preceding  seg- 
ments. It  is  rare,  however,  leaving  aside  this  antero-pos- 
terior  differentiation,  that  a  perfect  metameric  condition  is 
found  in  any  Annelid.  Secondary  changes  may  interfere 
with  the  similarity  of  all  the  metameres ;  a  su})pressiou  of 
parts  usually  present  in  some  '  f  the  segments  may  occur,  as, 
for  instance,  where  the  reproductive  organs  are  confined  to 
one  or  two  metameres,  or  again  there  may  occur  a  differentia- 
tion of  the  anterior  appendages  for  a  special  function  where- 
by a  marked  dissimilarity  between  the  anterior  and  posterior 
metaii teres  is  produced.  Finally,  owing  to  peculiar  habits  of 
life,  the  metamerism  may  be  almost  or  comi)letely  lost,  lioing 
indicated  only,  perhaps,  by  one  set  of  organs,  such  ,ns  the 
nerve-ganglia,  or  else  only  evident  in  the  larval  stages.  Para- 
sitism or  a  fixed  or  tubicolous  habit  of  life  are  among  the 
principal  causes  of  this  degeneration,  examples  of  which  will 
be  seen  later. 

In  consequence  of  this  degeneration  some  Annelids  pre- 
sent u  metamerism  of  a  lower  grade  than  that  found  in  such 
forms  as  the  Nemerteans.  Other  peculiarities  of  structure 
occur,  however,  which  serve,  together  with  the  indications  of 
metamerism,  to  mark  out  the  Annelid  type.  One  of  these 
peculiarities  is  the  occurrence  in  nearly  all  forms  of  a  series 
of  nerve-ganglia  along  the  ventral  nerve-cords  ;  this  feature 
is  of  course  a  part  of  the  metamerism,  but  it  is  not  usually 
marked  in  the  metamerism  of  the  nervous  system  seen  in 
lower  forms.  In  these  scattered  ganglion-cells  occur  all  along 
the  nerve-cords,  which  extend  backwards  from  the  brain, 
while  in  the  Annelidr-  these  scattered  cells  are  associated 
together  to  form  metameric  ganglia.  Another  peculiarity  is 
found  in  the  structure  of  the  nephridia.  These  are  no  longer 
in  all  cases  rows  of  perforated  cells  closed  at  the  inner  end 
by  a  flame-cell,  but  may  consist  of  more  or  less  convoluted 
tubes  lined  by  ciliated  epithelium  and  open  as  a  rule  into 


204 


INVERTEBRATE  MORPHOLOGY. 


the  ccBlom  by  a  wide  funnel-like  extremity.  Provisional 
kidneys  of  the  Turbellarian  type  occur  in  the  larvsB  of  many 
Annelids,  but  the  nephridia  of  the  adult  are,  as  a  rule,  of  the 
character  just  indicated  and  depart  widely  from  the  Turbel- 
larian character.  In  the  third  place  the  reproductive  organs 
are  developed  in  the  peritoneal  lining  of  the  ccelom  and  are 
not  usually  (except  in  the  Hirudinea)  provided  with  special 
ducts.  When  mature  the  ova  or  spermatozoa  are  simply 
shed  into  the  coelomic  cavity  and  make  their  way  to  the 
exterior  through  the  ordinary  nephridia,  or  through  nephridia 
specially  modified  for  the  purpose.  Finally  it  may  be  mem- 
tioned  that  a  blood  vascular  system  is  usually  present. 

I.  Class  Chsetopoda. 

The  ChflBtopoda  are  Annelids  in  which  the  external  seg- 
mentation of  the  body  corresponds  with  the  internal  seg- 
mentation of  the  organs,  and  which  bear  along  the  sides  of  the 
body  two  rows  of  pouches,  the  seta-sacs,  the  cells  lining  which 
secrete  chitinous  spicules  or  setae  of  various  shapes,  which 
serve  for  the  purpose  of  locomotion  or  in  some  cases  consti- 
tute a  defensive  armament. 

The  class  is  conveniently  divisible  into  two  subclasses. 

Subclass  I.  POLTCHXTA. 

The  forms  included  in  this  subclass  are  exclusively  ma- 
rine, and  are  characterized  by  the  presence  on  the  sides  of  a 
greater  or  less  number  of  the  metameres  of  a  pair  of  hollow 
processes  of  the  body-wall  upon  which  the  seta-sacs  occur 
and  which  are  known  as  parapodia.  In  a  few  forms  (Serpida) 
the  parapodia,  and  indeed  the  setee  as  well  (Polygordius),  may 
be  absent,  and  in  others,  such  as  Clymen(Ma,  they  may  be  very 
much  reduced  in  size,  but  as  a  rule  they  possess  a  high  de- 
gree of  development.  In  its  tj'pical  form  a  parapodium 
(Fig.  98)  consists  of  a  dorsal  and  a  ventral  lobe  each  of  which 
bears  seta-sacs  and  setse  (s).  Towards  the  base  of  each  lobe 
there  may  frequently  be  found  a  slender  hollow  process,  the 
dorsal  and  ventral  cirrus  {dc  and  vc),  and  plate-like  or  more 
or  less  dendritic  appendages,  the  brarwhice  {br),  either  modifi- 


TYPE  ANNELIDA. 


206 


im 
jh 
)e 
le 
re 


FlG, 


\  c 


98.— Pauapodium  of 

Nereis  virena. 

br  =  bi'Hiicliia. 
de  =  dorsal  cirrus. 
8  —  setae. 
ve  =  ventral  cirrus. 


cations  of  the  cirri  or  brauclies  arising  from  tliem,  and 
respiratory  in  function,  also  occur.  Muscles  pass  from  the 
body-wall  to  the  parapodia,  which  thus  ^..^..^s-dc 

become  important  organs  of  locomo- 
tion and  in  some  of  the  actively  swim- 
ming species  assume  a  more  or  less 
flattened  plate-like  form. 

The  head  segment  is  generally  well 
diflferentiated  from  those  which  succeed 
it,  being  destitute  of  parapodia  and 
setse,  and  as  a  rule  carrying  a  number 
of  appendages  sensory  in  function,  and 
being  likewise  usually  provided  with 
eyes.  The  cephalic  appendages  may 
be  short  and  rather  stout,  forming 
what  are  termed  palpi  (Fig.  100,  p), 
or  somewhat  longer  and  more  slender, 
forming  the  drri  (c),  or  even  still  more 
slender,  being  then  known  as  tentacles  (t). 

The  body  is  enclosed  in  a  chitinous  covering  secreted  by 
the  subjacent  ectoderm,  here  known  as  the  hypodermis  (Fig. 
99,  hy).  The  musculature  of  the  body-wall  which  lies  below 
the  hypodermis  is  separated  from  this  by  a  basement-mem- 
brane and  consists  of  an  external  layer  of  circular  fibres  {cm) 
and  a  subjacent  layer  of  longitudinal  fibres  {Im)  which  is,  as 
a  rule,  interrupted  in  the  mid-dorsal  and  ventral  lines  and 
also  in  the  region  of  the  two  lobes  of  the  parapodia  so  as  to 
form  four  bundles.  Special  muscles  extend  from  t)ie  body- 
wall  to  the  base  of  the  seta-sacs,  and  furthermore  a  pair  of 
muscle-bauds  cross  the  cavity  of  each  metamere,  in  typical 
cases  passing  from  the  lateral  regions  of  the  dorsal  surface 
downwards  and  inwards  to  be  inserted  into  the  ventral  body- 
wall  on  each  side  of  the  median  line.  The  inner  surface  of 
the  longitudinal  muscle-layers  is  lined  with  a  layer  of  peri- 
toneal cells  which  completely  enclose  the  coelom  (co)  of  each 
metamere,  being  reflected  upon  the  surfaces  of  the  dissepi- 
ments which  form  the  internal  partitions  between  adjacent 
metameres.  The  separation  of  the  coelomic  cavities  of  the 
metameres  is,  however,  rarely  perfect,  openings  occurring  here 


206 


mVEUTEBHA TE  MORPHOLOG  T. 


aud  there  in  the  dissepiments,  and  in  some  forms,  such  as  Capi- 
tellUf  a  number  of  the  dissepiments  may  at  the  breeding  season 
completely  degenerate  so  that  the  cavities  of  the  various 
metameres  concerned  become  perfectly  continuous.  The 
coelom  of  each  metamere  consists  in  reality  of  two  sacs  which 
are  folded  around  the  digestive  tract,  which  they  enclose, 
aud  come  into  contact  Avith  each  other  above  and  below  the 
intestine,  forming  the  dorsal  aud  ventral  mesenteries  (Fig.  99, 
dm  and  um).  That  wall  of  each  sac  which  lines  the  muscula- 
ture of  the  body-wall  is  termed  the  somatic  layer  of  the  peri- 
toneum, while  that  surrounding  the  digestive  tract  is  the 
splanchnic  layer. 

The  blood  vascular  system  consists  of  a  dorsal  vessel 
(Fig.  99,  db)  which  runs  along  the  mid-dorsal  line  of  the  diges- 
tive tract  and  which  is  frequently  contractile  in  portions  of 
its  course,  serving  as  a  heart,  and  a  ventral  vessel  (w6)  lying 
below  the  digestive  tract,  aud  being  connected  with  the  dor- 
sal vessel  by  lateral  trunks,  arranged  metamerically.  From 
these  vessels  branches  are  distributed  to  the  various  regions 
of  the  body.  The  blood  is  frequently  colored,  usually  red, 
and  contains  colorless  corpuscles,  the  coloring-matter  being 
dissolved  in  the  plasma  in  which  the  corpuscles  float.  The 
blood  vascular  system  is  completely  closed  throughout  its 
entire  course,  never  opening  into  sinuses  without  definite 
walls.  In  addition  to  the  blood  which  circulates  within  this 
definite  system  of  tubes  the  coelom  also  contains  a  corpuscu- 
lated  fluid,  frequently  colored  aud  approaching  blood  very 
closely  in  its  characters.  This  hatmolymph  contains  corpus- 
cles, usually  araceboid  in  form,  and  may  circulate  through  the 
body  from  one  metamere  to  another  through  openings  in  the 
dissepiments.  In  a  few  forms,  such  as  Capitella,  it  may  fulfil 
the  functions  of  the  blood,  a  true  blood  vascular  system  being 
wanting,  and  in  this  case  contains,  in  addition  to  the  colorless 
amoeboid  corpuscles,  others  which  are  disk-shaped  and  pig- 
mented. It  seems  probable,  however,  that  the  absence  of  a 
true  blood  vascular  system  is  a  purely  secondary  phenome- 
non, and  accordingly  does  not  indicate  a  primitive  condition. 

The  mouth  is  situated  on  the  ventral  surface  of  the  bodv, 
at  the  junction  of  the  head  metamere  with  the  first  trunk 


TYPE  ANNELIDA. 


207 


metamere,  and  leads  in  many  forms  into  a  strongly  muscular, 
usually  protrusible  pharynx  provided  with  chitiuous  teeth. 
Upon  the  pharynx  follows  the  usually  straight  intestine  which 
opens  to  the  exterior  at  the  posterior  extremity  of  the  body. 
In  Capitella  and  the  allied  genera,  as  well  as  in  certain  mem- 
bers of  the  family  Eunicidse,  an  accessory  intestine  lies  ven- 
trally  to  the  principal  one,  into  Avhich  it  opens  either  both 
anteriorly  and  posteriorly  or  else  anteriorly  alone.     This  ac- 


Fig. 


um      uh  c 

99.— DiAGUAM  OP  Transverse  Section  op  Annelid  (combination  of 
figures  by  Lang  and  Ehlers). 


hr  =  brnnchia. 

c  =  cirrus. 
cm  =  circular  muscles. 

CO  =  cotlom. 

db  =  dorsal  blood-vessel. 
dm  =  dorsal  mesentery. 
hy  =  hypodermis. 


i  =  intestiue. 
Im  =  longitudinal  muscles. 
ne  =  nepbridium. 
ov  =  ovary, 
p  =  parapodium. 
ub  =  ventral  blood-vessel. 
um  =  ventral  mesentery. 


un  =  ventral  nerve-cord. 


cessory  intestine  is  ciliated  and  seems  never  to  contain  food- 
matter  ;  it  has  been  considered  to  be  respiratory  in  function 
and  seems  to  be  a  special  development  of  a  ciliated  groove 
which  runs  along  the  ventral  surface  of  the  intestine  in  cer- 
tain other  forms.  Pouch-like  outgrowths  of  the  intestine  are 
frequently  present  and  may  sometimes  become  essentially 
glandular  in  function.  In  Tlesione  and  in  certain  species  of 
Syllis  pouches  communicating  with  the  anterior  part  of  the 


208 


IN  VERTEBRA TE  MORPHOLOG  T. 


digestive  tract  occur,  which  normally  are  filled  with  air  and 
are  richly  supplied  with  blood-vessels ;  they  may  be  respira- 
tory in  function,  and  have  been  compared  to  the  swim-bladder 
of  fishes. 

The  nervous  system  is  well  developed  in  all  Polychaeta, 
and  consists  of  a  supraoesophageal  ganglionic  mass  situated  in 
the  head  segment,  frequently  presenting  a  division  into  sev- 
eral lobes.     From  it  various  nerves  arise  passing  to  the  an- 
terior segment,  and  in  addition  a  strong  cord  passes  from  it 
ventrally  on  either  side  of  the  oesophagus  to  unite  with  a 
ganglion  lying  below  the  oesophagus  in  the  second  metamere, 
forming  the  circumoesophageal  commissure.     To  the  svboesopha' 
geal    ganglion  of    the    second    metamere    there    succeeds   a 
pair  of  ganglia  in   each  metamere,  each   pair    being  united 
with  the  preceding  and  succeeding  pairs  by  two  longitudinal 
cords  of  nerve-fibres,  the  connectives,  the  wliole  constituting 
the  ventral  nerve-chain,  and  furthermore  the  ganglia  of  each 
pair  are  united  by  a  transverse   commissure.     The  ventral 
nerve-chain   has   therefore   a  distinctly   ladder-like   arrange- 
ment, frequently  somewhat  obscured,  however,  by  the  approxi- 
mation of  the  ganglia  of  each  pair  and  a  consequent  shorten- 
ing of  the  transverse  commissures.     From  the  various  ganglia 
nerves  arise  which  pass  to  the  musculature  of  the  metameres 
and  to  the  hypodermis  and  its  sense-organs.     In  the  major- 
ity of  forms  the   nervous   system  lies  freely  in  the   coelom 
surrounded  by  a  special  sheath,  but  occasionally  in  various 
forms  widely  separated  genetically  from  one  another,  such  as 
Polygordius  and  the  Opheliaceae,  it  presents  a  primitive  char- 
acter in  being    completely  imbedded    in  the    hypodermis, 
recalling  the   condition  in  certain   Nemerteans   and  in  the 
Cnidaria.     Special  nerves  arising  from  the  supraoesophageal 
ganglion  are  supplied  to  the  walls  of  the  digestive  tract,  form- 
ing the  so-called  stomatogastric  nerves. 

Sense-organs  of  various  kinds  are  of  frequent  occurrence 
at  different  portions  of  the  bod}'  of  the  Polychaeta.  In  addi- 
tion to  the  cephalic  and  caudal  cirri  which  are  richly  supplied 
with  nerves  and  are  presumably  tactile  in  function,  eyes  are 
of  very  general  occurrence.  They  are  usually  situated  on  the 
head,  sometimes  in    connection   with  the   hypodermis  and 


TYPE  ANNELIDA. 


209 


sometimes  imbedded  in  the  dorsal  surface  of  the  brain.  For 
the  most  part  they  consist  of  a  cup  of  pigment-cells,  in  which 
numerous  sensory  cells  are  present — a  lens  being  in  some 
instances  developed  above  each  eye.  Occasionally,  however, 
as  in  the  pelagic  genus  Alciope,  the  eyes  reach  a  high  grade 
of  development.  In  some  forms  they  are  not  confined  to  the 
region  of  the  head,  as  for  instance  in  the  genus  Polyophthalmus 
— so  named  from  the  fact  that  pairs  of  eyes  are  found  on  the 
sides  of  a  number  of  the  trunk  metameres ;  in  the  majority 
of  tubicolous  Annelids  eyes  are  found  in  considerable  num- 
bers upon  the  branchial  lobes  of  the  head  segment,  the  genus 
Vermilia  possessing  somewhere  in  the  neighborhood  of  11,000 
separate  ocelli  in  this  region.  These  eyes  are  simply  differ- 
entiations of  the  ectoderm,  and  in  many  cases  are  still  situated 
in  the  hypodermis  ;  they  consist  of  a  number  of  cells  which 
are  prolonged  at  their  inner  ends  into  a  nerve  filament,  while 
peripherally  their  protoplasm  is  converted  into  a  refractive 
substance,  each  of  these  cells  being  separated  from  its  neigh- 
bors by  pigment  deposited  in  its  peripheral  layers,  as  well  as 
by  a  number  of  smaller  pigment-cells.  On  account  of  this 
pigment-sheath  it  is  presumable  that  each  of  these  optic 
elements  or  ommatidia  functions  more  or  less  independently 
of  the  rest,  and  the  eyes  are  to  be  considered  as  compound, 
composed  of  a  number  of  independent  parts  each  of  which  is 
physiologically  an  eye. 

Auditory  organs  or  otocysts  also  occur  in  certain  forms, 
but  cannot  be  considered  as  typical  of  the  Polychseta.  In 
Arenicola  they  consist  of  two  sacs  lying  in  close  proximity  to 
the  circumoesophageal  commissures  and  connected  with  the 
exterior  by  a  narrow  canal,  indicative  of  their  origin  as  invag- 
inations of  the  hypodermis.  The  walls  of  the  sack  are  formed 
by  columnar  cells  terminating  below  in  a  plexus  of  nerve- 
fibrils  and  covered  on  the  surface  turned  towards  the  cavity 
of  the  otocyst  with  a  firm  homogeneous  cuticle,  and  not  pos- 
sessing any  terminal  hairs.  In  the  cavity  a  varying  number 
of  spherical  particles  of  carbonate  of  lime,  the  otoliths,  are 
found.  In  some  forms  a  number  of  such  otocysts  are  present, 
as  in  Aricia,  where  four  or  five  pairs  have  been  found  in  adult 


210 


INVERTEBRA  TE  MORPHOLOG  Y. 


iudividuals ;  but  in  the  majority  of  species  they  do  not  seem 
to  be  developed. 

Ciliated  depressions  which  have  been  supposed  to  be 
olfactory  have  been  described  as  occurring  in  the  anterior 
region  of  the  body  in  various  species,  reaching  a  high  devel- 
opment in  the  Capitellidse,  where  they  form  club-shaped  sacks 
capable  of  being  evaginated.  In  addition  there  are  to  be 
found  scattered  on  the  surface  of  the  body  minute  beaker-shaped 
depressions,  at  the  bottom  of  which  are  cells  bearing  long 
hairs  and  presumably  sensory  in  function  ;  and  furthermore 
in  a  few  forms,  such  as  the  Capitellidse  and  Polyophthahmis,  a 
series  of  sensory  hillocks  occur  along  the  sides  of  the  body — 
a  pair  in  each  metamere,  forming  the  sense-organs  of  the  lateral 
line.  In  the  Capitellidse  these  organs  are  in  the  anterior  met- 
ameres  contained  in  depressions,  but  more  posteriorly  they 
project  slightly  from  the  surface.  The  central  part  of  each 
projection  is  retractile  and  is  formed  of  a  number  of  hair-cells, 
each  of  which  is  in  connection  at  its  inner  end  with  nerve- 
fibril.  No  little  interest  attaches  to  these  organs,  which 
forcibly  recsU,  both  in  their  structure  and  distribution,  the 
lateral  line  organs  of  the  lower  Vertebrates. 

The  nephridia  (Fig.  99,  we),  in  typical  adult  forms,  occur 
as  a  single  pair  in  each  metamere  except  the  two  terminal 
ones.  Each  consists  of  a  usually  contorted  or  coiled  tube 
lined  with  cells  opening  by  a  funnel-shaped  mouth  into  the 
coelom  of  the  metamere,  perforating  the  diasepiment  between 
it  and  the  next  metamere  in  which  the  greater  portion  of  it 
lies  and  in  which  it  opens  to  the  exterior  by  a  small  pore  sit- 
uated on  the  ventral  surface  of  the  body  at  the  base  of  the 
parapodium.  It  is  rare,  however,  that  any  such  metameric 
regularity  of  arrangement  occurs,  and  very  frequently  they 
become  reduced  to  a  small  number,  or  even  to  two  pairs ;  in 
the  tubicolous  forms  a  few  pairs  are  frequently  found  in  the 
anterior  portion  of  the  body  much  larger  than  any  of  the  rest. 
In  addition  to  their  original  excretory  function  they  may  also 
if^rve  as  outlets  for  the  reproductive  elements,  and  in  some 
cdseM  become  specially  modified  for  this  purpose  and  lose 
tlioiv  original  function. 


TYPE  ANNELIDA. 


211 


In  Capitella  only  one  pair,  that  of  the  eighth  metaraere,  becomes  con- 
verted into  a  genital  duct;  and  it  is  interesting  to  note  that  in  this  same 
segment  a  true  excretory  nephridium  is  also  present.  Whether  tliis  indi- 
cates or  not  the  occurrence  originally  of  more  than  one  pair  of  nephridia  in 
each  metamere  remains  to  be  seen,  but  it  is  interesting  in  connection  with 
what  occurs  in  the  Oligochaeta  (see  p.  323). 

The  reproductive  organs  consist  of  local  thickenings  of  the 
peritoneal  epithelium  (Fig.  99,  ov)  in  more  or  fewer  of  the 
segments.  The  ova  or  spermatozoa  fall  when  ripe  into  the 
ccelomic  cavity  and  pass  to  the  exterior  by  the  nephridia. 
With  very  few  exceptions  the  Polychreta  are  bisexual. 

The  classification  into  smaller  groups  is  to  a  certain  ex- 
tent artificial  at  present,  and  does  not  profess  to  have  any 
phylogeuetic  significance.    Three  orders  may  be  recognized. 

1.  Order  Archiannelida. 

This  order  includes  a  few  forms  which  are  supposed  to 
present  more  primitive  structural  characteristics  than  the 
remaining  Polychsets.  They  show  as  a  rule  but  indistinct 
traces  of  an  external  segmentation,  and  are  entirely  devoid  of 
either  parapodia  or  setsB.  Tentacles  occur  at  the  anterior 
extremity  of  the  head  metamere ;  but  no  other  appendages, 
such  as  cirri  or  branchiae,  occur.  The  nervous  system  is  im- 
bedded in  the  hypodermis,  and  the  nephridia  are  short  tubes, 
a  single  pair  occurring  in  nearly  every  segment.  To  this 
group  belong  the  genera  Polygordius  and  Protodrilus. 

2.  Order  Errantia. 

In  this  order  are  placed  the  free-swimming  or  creep- 
ing  Polychfieta,  in  which  a  considerable  similarity  of  the 
various  trunk  metameres  occurs.  The  parapodia  are  as  a 
rule  well  developed,  and  occasionally  are  broad  and  plate-like 
in  adaptation  to  a  free-swimming  existence.  Branchifo  are 
usually  found  on  the  dorsal  lobes  of  a  considerable  number 
of  parapodia;  the  head  is  distinctly  marked  off  from  the 
trunk  and  may  bear  eyes ;  while  the  anterior  portion  of 
the  digestive  tract  is  converted  into  a  protrusible  pharynx, 
usually  armed  with  chitinous  teeth.  To  this  order  belong 
the  genera  Nereis  (Fig.  100),  usually  found  lurking  beneath 


212 


INVERTEBRATE  MORPHOLOGY. 


stoues  during  the  day-time,  but  becoming,  in  some  species  at 
least,  free-swimming  at  night ;  Zepidonottis,  characterized  by 
the  possession  of  elytra  arranged  in  overlapping  series  on  the 
dorsal  surface  ;  Diopatra,  which  forms  tubes  for  itself  by  glu- 
ing together  particles  of  foreign  matter;  and  Autolytus  and 
Syllis,  which  are  peculiarly  pelagic  in  habit,  as  is  also  Alciope, 
characterized  by  its  large  highly-organized  eyes. 

3.  Order  Sedentaria. 
This  order  includes  a  number  of  iorws  which  manufacture 
for  themselves  tubes  of  various  substances — some  being 
merely  composed  of  particles  of  sand  glued  together  by  an 
adhesive  secretion,  while  others  consist  of  a  chitinous  sub- 
stance, to  which  foreign  bodies  may  be  added,  or  even  of  car- 


FiG.   100. — Anterior 
End  of  Nereis  virenB. 
e  =  cirrus. 
p  =  parapodium. 

P^  =  P^^I^-  Pig.     101.— Amp?iitrite    ornata    (after 

t  =  tentacle.  Vbrrill). 

bonate  of  lime.  Within  these  tubes  the  animals  permanently 
reside,  and  in  conformity  with  this  mode  of  life  numerous 
adaptations  of  structure  are  found.  The  head  is  usually  pro- 
vided with  a  number  of  long  cirri  and  the  branchiae  are  for  the 
most  part  confined  to  the  head  region.     In  some  forms,  such 


TYPE  ANNELIDA. 


213 


as  Serpvla  and  Sabella,  plume-like  brauchisB  supported  by  au 
axial  cartilage-like  skeleton  occur  upon  the  sides  of  the  head, 
and  numerous  eyes  may  be  found  in  the  hypodermis  of  these 
structures.  Parapodia  are  as  a  rule  but  slightly  developed, 
sometimes  being  entirely  wanting  though  the  setse  persist, 
those  of  the  lower  parapodial  Icbe  being  usually  hook-like. 
The  protrusible  pharynx  with  chitinous  teeth  does  not  for  the 
most  part  occur.  Amphitrite  (Fig.  101)  lives  in  tubes  in  sand, 
while  TerebeUa  composes  tubes  by  gluing  together  particles 
of  sand.  In  Sabella  the  tubes  are  membranous  in  character, 
while  Serpula  manufactures  more  or  less  contorted  tubes  of 
carbonate  of  lime. 

Development  of  the  Polychceta.  —  An  important  feature  in 
the  development  of  the  Folychseta  is  the  occurrence  of  the 
Trochophore  larva.  A  typical  example  of  this  larva  is  to  be 
found  in  the  development  of  Polygordiua  \  it  is  a  transparent 
organism,  having  the  form  of  two  low  cones  united  by  their 
bases  (Fig.  102).  Just  below  the  junction  of  the  two  cones  is 
the  mouth  (i!/),  leading  by  a  short  stomodceum  or  oesophagus 
into  a  retort-shaped  stomach,  the  intestine  opening  at  the 
apex  of  the  lower  cone.  Above  the  mouth,  along  thf  line 
where  the  two  cones  are  united,  lies  a  band  of  strong  cilia 
arranged  in  two  rows  and  forming  an  almost  complete  girdle 
for  the  body,  being  wanting,  however,  in  the  mid-dorsal  region. 
This  is  the  prototroch  (pro)  or  prseoral  band  of  cilia,  and  par- 
allel to  it  is  a  second  weaker  band  which  passes  behind  the 
mouth — the  paratroch  {po)  or  postoral  band.  The  slight 
groove  between  the  two  bands  is  lined  by  fine  cilia,  the  adoral 
cilia,  and  in  some  Trochophores  a  band  of  fine  cilia  extends 
backwards  along  the  ventral  surface  of  the  body  towards  the 
apex  of  the  lower  cone. 

At  the  apex  of  the  upper  cone  is  a  strong  thickening  of 
the  ectoderm,  the  apical  plate  {ap),  which  is  nervous  in  function 
and  bears  a  number  of  strong  cilia  and  may  also  have  imbedded 
in  it  pigment-spots  which  function  as  light-percipient  organs. 
From  the  apical  thickening  four  nerve-cords  (w)  extend  back- 
wards, one  being  dorsal,  the  other  ventral,  and  the  remaining 
two,  stronger  than  the  others,  lateral.  A  series  of  fine  nerve- 
rings  arranged  concentrically  about  the  apical  thickening  unite 


214 


INVERTEBRATE  MORPHOLOGY. 


these  cords  at  regular  intervals,  the  lower  ring  being  con- 
nected with  the  cells  which  bear  the  protrochal  cilia  and 
forming  the  prototroch  nerve. 

At  the  apex  of  the  lower  cone  and  ventral  to  the  intestine 
lie  two  cells,  or  two  masses  of  small  cells,  which  constitute  the 
mesoblasts  and  give  rise  to  two  longitudinal  mesoblast  bauds 
{mb).    A  few  scattered  cells  are  also  found  between  the  ecto- 


pro> 


Fig.  102  — Tkochophore  op  Poi.ygokdius  (after  Hatschkk). 

A  =  anus.  mb  =  inesoblnat-baud. 
up  =  apical  plate.  »«  =  uephridium. 

M  =  mouth.  pro  =  proeoral  band  of  cilia. 
m,  m',  m"  =  muscles.  po  =  postorul  band  of  cilia. 

derm  and  the  digestive  tract,  some  of  which  elongate  and  be- 
come muscle-fibres  (»w),  and  which  have  been  thrown  off  from 
the  mesoblast  bands.  In  some  forms  a  band  of  muscle-fibres 
underlies  the  prototroch  cells.  In  the  neighborhood  also  of 
the  mesoblast  bauds  in  the  posterior  cone  there  occurs  on 
either  side  of  the  digestive  tract  a  small,  sometimes  branched, 
tubular  body,  the  head-kidney  (we).  Each  kidney  consists  of 
a  row  of  perforated  cells,  terminating  in  a  funnel-shaped 
structure  closed  at  its  mouth  by  a  cell,  the  whole  structure 
thus  agreeing  closely  with  the  nephridia  of  the  Platyhelminths. 
From  such  a  larva  the  adult  condition  is  derived  by  the 
gradual  elongation  of  the  posterior  part  of  the  body — an 
elongation  with  which  the  mesoblast  bands  keep  pace,  the 
mesoblasts  retaining  their  position  at  the  posterior  extremity. 


TYPE  ANNELIDA. 


215 


and  coutiuually  addiug  to  the  bauds  by  the  formation  of  new 
cells.  The  bands  as  growth  proceeds  break  up  into  a  number 
of  masses,  the  mesoblastic  somites,  in  tlie  interior  of  which 
cavities  appear,  and  adjacent  pairs  of  musses  growing  dorsally 
and  veutrally  finally  come  into  contact  above  and  below  the 
digestive  tract,  the  dorsal  and  ventral  mesenteries  of  the 
intestine  being  thus  formed,  and  later  a  metamerization  of  the 
body-wall,  corresponding  with  that  of  the  mesoderm,  also  takes 
place.  The  anterior  cone  of  the  larva,  which  at  first  sur- 
passed in  size  the  posterior  one,  gradually  becomes  smaller, 
and  the  prototrochal  cilia,  and  in  some  cases  the  cells  also,  are 
thrown  ofl*.  The  apical  plate  takes  part  in  the  formation  of 
the  supraoesophageal  ganglion  of  the  adult,  and  the  lateral 
nerve-cords  arising  from  it  form  the  circumoesophageal  com- 
missure, becoming  connected  with  thickenings  of  the  ventral 
hypodermis  arranged  metamerically  and  representing  the  ven- 
tral chain  of  ganglia.  The  head-kidneys  gradually  disappear, 
being  merely  provisional  larval  structures,  and  new  nephridia 
of  the  Annelid  type  develop  from  the  mesoderm  of  the  trunk- 
metameres. 

Although  the  Trochophore  larva  occurs  in  the  life-history  of  many  of 
the  Annelids,  as  well  as  in  other  groups  as  will  be  seen  later,  yet  never- 
theless it  is  not  invariably  present.  In  some  forms  a  single  band  of  large 
cilia  runs  around  the  middle  of  the  body,  which  is  elsewhere  uniformly 
ciliated,  while  in  others  the  cylindrical  larva  is  surrounded  by  several 
bands  of  cilia  succeeding  one  another  at  definite  intervals.  In  certain 
species  the  larva  is  provided  with  very  long  setaB  which  are  thrown  off 
during  larval  life,  and  are  interesting  on  account  of  similar  set®  having 
been  found  in  fossil  forms,  though  absent  in  recent  adult  species. 

It  is  also  worthy  of  note  that  in  some  forms  the  Trochophore  larva  is 
succeeded  by  a  well-marked  stage  in  which,  in  addition  to  the  head  seg- 
nent,  three  trunk  segments  are  developed.  It  is  possible  that  this  may 
represent  an  ancestral  form  from  which  certain  other  groups  have  taken 
their  origin. 

In  what  may  be  considered  exceptional  cases  a  non-sexual  repro- 
duction by  budding  also  occurs.  In  the  genus  Protula  a  zone  of  growth 
occurs  in  the  sixteenth  segment,  and  at  this  point  later  separation  takes 
place,  a  new  head  developing  for  the  posterior  individual  from  the  original 
seventeenth  segment.  In  one  species  of  Syllis  the  new  individuals 
arise  not  only  in  a  linear  series  but  also  as  lateral  buds,  so  that  a 
branching  colony  is  produced  ramifying  through  the  canal  system  of  the 
Hyalosponge  in  which  the  form  lives.    The   buds  eventually  separate  as 


216 


IN  VEUTEUUA  TE  MOUPIIOLOG  Y. 


sexually  mature  male  and  female  individuals  which,  since  they  differ  from 
the  parent  form  in  posses^in^  more  highly  developed  eyes  as  well  as  a 
more  perfect  adaptation  of  tlie  parapodia  for  swimming,  probably  leave  tjie 
sponge  and  swim  about  freely  in  the  ocean  distributing  their  sexual  elements. 
If  this  be  the  case,  this  species  presents  both  colony  formation  and  alterna- 
tion of  generations,  the  latter  phenomenon  being  also  manifested  by  other 
species  of  Syllis  and  by  Autolf/tiis,  in  which  buds  are  produced  linearly, 
differing  from  the  parent  in  the  structure  of  tlie  parapodia  and  separating 
to  lead  a  free  existence  as  male  and  female  indivicluals.  A  modification  of 
this  process  is  found  in  certain  species  of  Nereis,  in  which  at  the  time  of 
sexual  maturity  the  posterior  segments  of  the  body  develop  setro  more 
perfectly  adapted  for  active  locomotion  by  swimming  than  were  those  of 
the  immature  form  ;  those  sexually  mature  forms  were  at  one  time 
referred  to  a  separate  genus,  Heteronereis. 

The  Phylogeny  of  the  Polyehata. — The  origin  of  the  PolychaBta  has  been 
within  recent  years  the  subject  of  considerable  discussion.  The  discovery 
of  the  wide  distribution  of  the  Trochophore  larva  led  to  the  supposition 
that  it  was  an  ancestral  form,  from  which  the  Polychaeta  had  been  de- 
veloped by  a  process  of  linear  budding,  each  metamere  of  the  Polychaet 
body  being  equivalent  to  the  original  Trochophora  and  the  adult  organism 
being  a  co-ordinated  succession  of  Trochophore  individuals.  Other  authors, 
however,  who  do  not  see  in  metamerism  the  result  of  a  budding  of  the 
individual,  but  rather  the  multiplication  of  its  subordinate  parts,  are  in- 
clined to  refer  the  Annelids  to  a  Nemertean-like  ancestor  and  to  consider 
the  Trochophore  larva  a  purely  secondary  adaptation.  Between  these  two 
views  it  is  difficult  to  decide,  and  it  is  possible  that  in  their  plain  statement 
neither  is  quite  correct,  though  each  may  contain  certain  elements  of 
truth. 

It  seems  exceedingly  probable  that  a  larval  form  which  is  met  with  in 
the  life-history  of  the  Annelids  and  Mollusca,  as  well  as  in  a  modified  form 
in  other  groups,  has  some  ancestral  significance.  It  is  difficult  on  any 
other  hypothesis  to  explain  its  occurrence  in  widely  different  groups,  since 
it  seems  hardly  probable  that  it  could  have  arisen  independently  in  several 
instances.  Convergent  evolution  could  hardly  be  carried  to  such  an  ex- 
tent as  to  produce  in  the  Mollusca,  quite  independently  of  any  genetic 
relationships,  a  larva  resembling  in  all  Its  t^iructure  that  of  an  Annelid. 
If  the  Trochophore  occurred  only  in  the  Annelids,  it  might  be  quite  possible 
that  it  had  made  its  appearance  in  the  life-history  of  some  primitive 
Annelid  as  a  secondary  modification  of  a  more  primitive  lauva,  and  had 
reappeared  subsequently  in  the  life-history  of  all  forms  descended  from  this 
Annelid  ancestor,  but  this  would  not  explain  its  occurrence  also  in  the 
Mollusca,  unless  it  be  supposed  that  the  members  of  this  group  have  been 
derived  from  the  primitive  segmented  Annelid,  a  view  that  has  little  to 
recommend  it.  The  working  of  the  biogenetic  law  (see  p.  143)  is  interfered 
with  in  innumerable  instances,  and  the  distinguishing  betNjreen  examples  of 
its  action  and  secondary  modifications  is  the  most  difficult  task  of  the 


TYPE  ANNELIDA. 


217 


3tlC 

lid. 
tble 
live 
lad 
Ibis 
the 
^eu 

to 
l-ed 

of 
the 


f  lubi-yologist.  Tbo  evidence  at  present  available  seems,  however,  to  point, 
in  the  case  of  tlie  Trocliophore,  to  its  being  an  example  of  the  law,  and  to 
this  extent  the  tirsi  of  the  two  views  stated  above  is  probably  correct. 

But,  on  the  other  hand,  this  may  not  be  the  case  with  the  second  part 
of  the  theory.  If  the  view  as  to  the  origin  of  metamerisiu  which  is  advo- 
cated in  this  work  be  correct,  then  the  Annelid  cauuui  be  regarded  as 
having  arisen  directly  by  u  process  of  reproduction  by  budding  of  tlic 
Trochophore.  It  is  not  a  colony  of  Trochophore  individuals,  but  a  single 
elongated  Trochophore  whose  organs  have  undergone  repetition,  producing 
a  high  grade  of  metamerism.  To  this  extent  the  second  of  the  views  may 
be  correct,  but  this  does  not  necessarily  imply  that  the  Annelids  are  to  bo 
derived  from  a  Ncmertean-like  form  in  which  the  metamerism  is  not  quite 
so  perfect.  Metamerism,  as  here  explained,  is  simply  the  following  out 
into  the  higher  individualities  of  the  phenomenon  of  discontinuous  growth 
or  reproduction  by  division  which  characterizes  the  cell,  and  it  is  quite 
possible  that  there  may  be  no  more  genetic  connection  between  the  meta- 
merism of  the  Nemertean  and  that  of  the  Annelid  than  there  is  between 
that  of  the  Cestode  and  that  of  the  Nemertean.  It  may  have  arisen  quite 
independently  in  the  two  forms,  and  in  fact  when  the  details  of 
metamerism  are  examined  in  the  two  groups  considerable  differences  are 
to  be  seen. 

The  view  here  advocated  in  regard  to  the  origin  of  the  Polychaeta  may 
be  briefly  expressed  as  follows  :  The  Polychaeta — and  with  them  the 
Annelida  in  general— have  had  for  their  ancestor  a  non-metameric  form 
of  which  the  Trochophore  is  the  larval  representative,  and  this  in  the 
course  of  its  development  elongated,  the  elongation  being  accompanied  by 
the  repetition  by  a  budding  process  of  certain  organs,  a  high  grade  of 
metamerization  being  thus  produced. 

The  relationships  of  the  Trochophore  seem  to  be  with  the  Turbellaria. 
The  nervous  system,  consisting  of  the  apical  thickening  and  lateral  nerve- 
cords,  is  very  similar  to  that  of  Turbellaria,  and  also  it  is  interesting  to 
notice  the  similarity  of  structure  of  the  head-kidney  with  the  Turbellarian 
nephridium.  The  exceedingly  small  development  of  the  parenchyma  is 
probably  a  secondary  condition,  and  the  presence  of  an  anus  is  an  im- 
portant advance  upon  the  Turbellaria.  An  undoubted  similarity  in  many 
respects  exists  between  the  Rotifera  and  the  Trochophore,  and  the  former 
have  been  regarded  as  persistent  Trochophores  or  else  as  forms  descended 
from  the  Trochophore.  This  latter  view  in  one  of  its  phases  has  already 
been  considered  (p.  195),  and  an  important  difference  in  the  relation  of  the 
supraoesophageal  ganglion  to  the  prototroch  mentioned.  Reasons  have  also 
been  given  for  the  belief  that  the  Rotifera  are  descended  from  Turbellarian 
ancestors,  and  it  seems  probable  that  the  line  of  descent  of  the  Rotifers 
was  identical  for  a  time  with  that  followed  by  the  Trochophore,  the 
former  group  branching  off  from  it  shortly  before  the  Trochophore  ancestor 
made  its  appearance  on  the  scene.     In  this  respect  the  Rotifers  and  the 


218 


INVERTEBltATE  MOKPIIOLOG  Y. 


Trochophore  are  related  to  each  other,  but  lianlly  with  so  close  an  affinity 
as  would  be  implied  by  a  stateiueiit  that  tliey  are  persistent  Trochophores. 


11  '  I 


'ZEZl 


II.  Subclass  Olioochjeta. 

The  Oligoclueta  are  with  few  exceptions  fresh-water  or  ter- 
restrial Chietopods,  aud  preseut  a  much  simpler  body  form 
pr  thau  do  the  Polychsets.     The  first 

segmeut  or  prontomium  (Fig.  103, 
pr)  is  devoid  of  teutacles  or  cirri, 
aud  ouly  iu  a  few  forms  are  eyes 
preseut  upou  it.  The  body  is  divided 
into  well-marked  segments,  but 
parapodia  are  lacking,  though  iu 
the  majority  of  forms  seta\  arranged 
iu  a  more  dorsal  («')  and  a  more 
ventral  {s)  group,  occur  ou  the  sides 
of  each  metamere  ;  in  a  few  forms, 
however,  a  single  series  of  groups 
only  is  present,  while  iu  PericluvUt, 
the  setse  are  arranged  in  a  ring 
around  each  metamere,  and  in  Ana- 
cha'ta  they  are  wanting,  their  place 
being  indicated  only  by  the  sacks 
in  which  in  other  forms  they  are 
Fig.  103.-ANTKinon  End  OF  aeveh)ped  and   which    project   into 

the  ccelora  as  large  hypoilermal 
glands.  As  a  rule,  too,  no  branchijo 
are  present,  the  blood  being  aerated 
through  the  walls  of  the  body,  mi- 
nute branches  of  the    blood-vessels 

,     .  penetrating  into  the  hypodermis  in 

8  =  liiteml  si'taj.  *  •      <.  T 

^■Hl  =  opeuiug  of  viis  deferens,    the  terrestrial  forms  ;  a  few  aberrant 

forms,  however,  possess  either  dorsal 
or  ventral  {Cha'tohranclmn)  appendages,  which  are  probably 
respiratory  in  fuuctiou,  on  many  of  the  segmeuts,  while  iu 
the  genus  Dero  the  linger-like  processes  of  the  terminal 
metamere  are  probably  brauchiir. 

The  exterior  of  the  body  is  covered  by  a  well-marked  cuti- 
cle, aud  beneath  it  lies  the  ectoderm  or  hypodermis,  usually 


0  =  clltdluin. 
gl  =  glands. 
m  =  nioulh. 

Oil  =  opening  of  oviduct. 
pr  =  prostoinium. 

s  =  v«!ntral  setie. 


TYPE  ANNELIDA. 


219 


nial 


111 
ant 
•sal 
bly 

iu 
ual 

liti- 

illy 


rich   iu   glancl-eells   auil  also  contaiuiug  uuiiieroiis  seusuiy 
cells  connected  at  their  inner  ends  with  slender  fibrils  which 
pass  centrally  and  unite  with  others  to  form  nerve-cords  pass- 
injij  to  the  ventral  nerve-cord.     In  some  of  the  sej^ments  near 
the  anterior  ])ortioii  of  the  body  the  hypodermis  is  usually 
thicker  than  elsewhere  and  richer  in  gland-cells,  forming  a 
structure  known  as  the  ditellum  (Fig.  103,  <').    Within  the  hyi)o- 
deriuis  is  a  usually  thick  layer  of  circular  muscles,  and,  internal 
to  tills,  longitudinal  muscles  whose  continuity  as  a  layer  is  in- 
terrupted at  the  dorsal  and  ventral  mid-lines  as  well  as  at  the 
sides.    Muscular  dissepiments  divide  the  ca'loni  into  compart- 
ments  corresponding   to  the    external    segmentation   of   the 
body,  and  th(»  various  compartments  are  lined  by  a  layer  of 
peritoneal  cells  which  along  the  dorsal  and  ventral  lines  is 
reflected  towards  the  digestive  tract,  which  it  surrounds,  form- 
ing a  dorsal  and  ventral  mesentery.     The  former  of  these  fre- 
quently disappears,  as  may  .also  the  ventral  one.    The  dissepi- 
ments are  rarely  })erfect,  being  usually  perforated  so  that  tho 
various  cieloniic  compartments  are  })laced  in  commui.i<*ati(m 
with  each  other,  and  in  rare  cases  a  number  of  the  dissepi- 
ments may  be  wanting,  as  in  JEolosottHt,  where  but  a  single 
one  se])aratiiig  the  head  from  the  trunk  ccelomic  spaces  oc- 
curs.    On  the  median  dorsal  line  of  more  or  fewer  metaraeres 
towards  their  posterior  edge  a  small  opening  usually  occurs, 
the   dorsoJ  jxyrc,   and    similar    })ort^s    are  found  in  the    head 
segment  of  some  forms.     They  phu^e  the  cd'lomic  cavity  iu 
communication  with  the  exttnior,  but  do  not  seem  under  ordi- 
nary circumstaiH'es  to  be  the  means  of  any  extensive  inter- 
change beiweeii  the  external  water  and  the  co'lomic  luemo- 
lyniph,   thoiiu'h  occasionally  this  latter  fluid  may  Hud  exit 
through  tlieui.     Among  the  peritoneal  cells  are  usually  to  be 
found  some  which  onch)se  greenish-brown  particles  and  may 
detach  themselves  from  the  peritoneal  layer  and  float  about 
in  the  hicmolymph,  eventually  dying  and  d"  integrating.     Oc- 
casionally these  cldoni(/()(fm'  veils  are  specially  aggregated  in  a 
furrow  which  runs  nlong  the  dorsal  suriace  of  the  intestine, 
and   immediately   surround    the  dorsal    bluod-vessel.     They 
seem  to  l)o  excretory  in  functi(»n,  ^^erforiiiing  perhaps  to  a  cer- 
tain extent  the  part  of  the  liver-(  :>lls  of  the  Vertebrata. 


220 


INVERTEBRATE  MORPHOLOGY. 


A  circulatory  system  is  always  preseut,  and  consists  of  a 
dorsal  longitudinal  vessel  lying  on  the  dorsal  surface  of  the 
digestive  tract  and  a  ventral  one  lying  below  it,  the  two 
being  united  in  more  or  fewer  of  the  metameres  by  one  or 
two  lateral  vessels  on  each  side.  The  dorsal  vessel  is  con- 
tractile, the  blood  in  it  flowing  towards  tlir  interior  extremity, 
and  the  lateral  vessels  are  also  usually  contractile.  Branches 
are  given  oft'  to  the  muscles  and  to  the  various  organs  in  reg- 
ular metameric  succession,  and  additional  longitudinal  vessels 
are  also  found  accompanying  the  ventral  nerve-cord,  two 
lateral  and  one  ventral.  In  the  terrestrial  forms  tine  branches 
penetrate  into  the  hypoderrais,  the  aeration  of  the  blood  being 
thus  eft'ected  in  the  absence  of  special  branchiae.  The  blood 
in  the  majority  of  forms  is  red  from  the  presence  of  haemo- 
globin dissolved  in  the  plasma  and  contains  colorless  cor- 
puscles. As  in  the  Polychajta  the  ccelom  contains  a  hsemo- 
lymph  in  which  corpuscles  float. 

The  digestive  tract  forms  a  straight  tube,  extending  from 
the  mouth,  situated  on  the  ventral  surface  at  the  junction  of 
the  prostomium  and  tirst  trunk  metamere,  to  the  terminal 
anus.  The  mouth  opens  into  a  short  mouth-cavity,  anrl  this 
into  a  more  or  less  muscular  pharynx,  which  in  most  cases 
can  be  protruded  from  the  mouth  and  is  slung  to  the  body- 
wall  by  numerous  radiating  muscular  bands.  To  it  succeeds 
a  smaller  oesophagus,  which  communicates  posteriorly  in  ter- 
restrial forms,  after  in  some  cases  dilating  to  form  a  sack-like 
thin-walled  crop,  with  a  muscular  gizzard.  The  intestine 
which  succeeds  this  is  usually  somewhat  pouched,  being  con- 
stricted in  the  region  of  the  dissepiments  and  bulging  oit 
into  the  intervening  coelomic  cavities.  In  the  terrestrial  Oli- 
gochsets  its  absorptive  surface  is  increased  by  the  projection 
into  it  along  the  dorsal  surface  of  a  longitudinal  fold,  the 
typhlosoley  the  chloragogue  cells  lying  in  the  furrow  produced 
by  the  fold.  Various  glands  open  into  the  digestive  tract  at 
diflferent  regions,  as,  for  instance,  salivary  glands  which  open 
into  the  anterior  part  of  the  oesophagus  in  some  forms,  and 
calciferous  glands  (Morren's  glands)  which  contain  particles 
of  carbonate  of  lime  and  are  found  opening  into  the  oesopha- 
gus in  terrestrial  forms. 


TYPE  ANNELIDA. 


221 


The  nervous  system  consists  of  a  supraoesopliageal  gan- 
glion (Fig.  104,  ce)  of  a  somewhat  complicated  structure  lying 
in  the  anterior  portion  of  the  body.  In  JEolosoma  it  is  ctm- 
nected  with  the  hypodermis  (i.e.,  the  ectoderm),  but  in  most 
forms  it  lies  upon  the  anterior  part  of  the  digestive  tract,  quite 
separate  from  the  hypodermis.  It  differs  in  position  from 
the  corresponding  ganglion  of  the  Polychaeta  in  that  it  is  not 
usually  situated  in  the  anterior  metamere  or  prostomium,  but 
has  passed  farther  back  and  may  lie  in  the  second,  third,  or 
fourth  metamere  or  even  more  posteriorly.  It  sends  off  nerves 
to  the  sensitive  prostomium  and  gives  rise  to  two  commis- 
sures which  pass  backwards  and  downwards  on  either  side  of 
the  pharj'nx  to  unite  with  the  suboesophageal  ganglion  (so), 
which,  like  the  brain,  is  formed  of  two  more  or  less  fused  lat- 
eral masses,  each  of  which  in  many  forms  shows  indications  of 
being  compound  and  formed  bj^  the  fusion  of  two  or  more 
ganglia  (Lmnbricus).  To  this  there  succeeds  in  each  meta- 
mere a  pair  of  ganglia  each  of  which  is  united  to  its  prede- 
cessor and  successor  by  a  pair  of  connecting  cords,  the  wliole 
ventral  cord  (n)  so  produced  having  a  characteristic  ladder- 
like arrangement.  Usually  the  connecting  cords  are  closely 
approximated,  and  the  same  may  be  the  case  with  the  gan- 
glion pairs,  the  whole  being  ensheathed  in  connective  tissue 
so  that  the  cord  seems  to  be  single.  From  each  pair  of  gan- 
glia in  Lumhricus  three  nerves  pass  out  on  each  side,  the  two 
posterior  ones  being  closely  related  so  as  to  appear  to  be  one. 
>^erves  especially  connected  with  the  digestive  tract,  the 
iomatogastric  nerves^  seem  to  be  present,  but  their  distribu- 
tion and  connections  have  not  yet  been  thoroughly  studied. 
In  some  aquatic  forms  a  lateral  nerve  imbedded  in  the  hypo- 
dermis and  united  anteriorly  with  the  supranesoplmgeal 
ganglion  runs  along  the  lateral  line  between  the  two  rows  of 
setjc,  recalling  the  iateral-line  nerve  of  the  CapitelUda'  among 
the  Polychseta. 

Sense-organs  of  various  kinds  are  present.  Tentacles  are 
absent  throughout  the  group,  and  in  only  a  few  forms  (Nals) 
do  eyes,  consisting  of  pigment-spots  imbedded  in  the  hypo- 
dermis, occur.  Ciliated  depressions  at  the  side  of  the  pro- 
Btomial  segment  occur  in  ^olosoma  and  a  few  other  genera, 


222 


IN  VERTEBRATE  MORPHOLOGY. 


while  tactile  setso  or  papilltf  are  scattered  over  the  body.  In 
those  forms  which  possess  a  lateral  nerve  sense-organs  re- 
sembling those  of  Capitella  occur  metamerically  along  it,  and 
in  the  genus  Slavina  are  increased  in  number  so  as  to  form  a 
circle  of  from  fifteen  to  twenty  papillae  surrounding  each 
metamere  and  innervated  by  a  branch  from  the  lateral  nerve. 
Cup-shaped  organs,  supposed  to  be  gustatory,  occur  especially 
abundantly  on  the  prostomial  metamere. 

The  excretory  system  has  usually  a  typically  metameric 
arrangement — a  single  pair  of  coiled  tubules  lying  in  each 
metamere.  Each  tube  opens  by  a  ciliated  funnel  into  one 
coelomic  compartment  and  then  passes  backwards,  perforating 
the  dissepiment,  into  the  next  succeeding  compartment  in 
which  the  coiled  portion  lies,  and  opens  to  the  exterior  in 
this  metamere  betwet  *^  -^  dorsal  and  ventral  rows  of  setae. 
The  lumen  of  the  coiled  ;  Jon  of  the  tubule  is  intracellular, 
the  tubule  consisting  in  this*  region  of  a  series  of  perforated 
cells  recalling  the  coudition  found  in  the  Platyhelminths.  la 
a  certain  number  of  anterior  metameres  the  nephridia  may 
be  wanting  in  the  adult  coudition,  though  in  younger  stages 
provisional  nephridia  are  to  be  found  in  these  metameres 
later  disappearing. 


Considerable  variation  is  to  be  found  in  the  nephridial  system  of  the 
OligochsBta,  some  of  the  variations  suggesting  important  theoretical  con- 
siderations. A  head-kidney  similar  to  that  described  as  occurring  in  the 
Trochophore  larva  persists  in  the  adult  stage  in  some  Oligochaeta,  and  in 
Ctenudnlus  appears  to  be  the  only  nephridium  which  exists.  In  other 
forms,  such  as  Chcetogaster,  the  entire  nephridial  system  is  composed  of 
tubules  having  a  decided  similarity  to  the  head-kidney  in  the  intracellular 
character  of  the  lumen,  and  in  the  absence  of  any  ciliated  funnel,  the 
inner  end  of  the  tubule  being  closed.  Furthermore  the  cells  enclosing  the 
canal  are  in  addition  perforated  by  nuinerous  minute  branching  canals 
which  open  into  the  central  lumen.  This  fact  suggests  the  complete 
homology  of  the  nephridial  system  throughout  the  entire  body  notwith- 
standing the  usual  marked  histological  distinction  between  the  head- 
kidneys  and  the  nephridia  of  the  trunk  metameres.  There  seems  little 
reason  to  doubt  that  the  Oligochieta  have  been  derived  from  the  Poly- 
chaeta,  and  the  nephridial  system  in  the  two  forms  is,  therefore,  homol- 
ogous. It  must,  therefore,  be  possible  for  a  nephridium  with  an  intra- 
cellular canal  to  be  transformed  into  one  in  which  the  canal  is  intercellular. 
The  nephridia  of  Chcelogaster  are  unquestionably  homologous  with  those 


i^^j^miisimitmamm^S^^i^^' 


TYPE  ANNELIDA. 


223 


of  Liimbtictis,  for  instance,  which  possess  a  terminal  funnel,  and  are  like- 
wise similar  in  structure  to  and  may  be  regarded  as  repetitions  of  the 
head-lcidney,  thus  establishing  the  liomology  between  the  two  forms  of 
uephridium. 

In  carrying  the  homology  of  the  Annelid  nephridium  back  to  that  of 
the  Platyhelminth  the  question  arises  whether  it  is  equivalent  to  the  whole 
branching  system  of  the  Turbellarian  or  only  to  a  part  of  it.  Whichever 
view  of  metamerism  be  taken  the  various  nephridia  of  the  Annelids  are  to 
be  regarded  as  bud-products,  and  each,  therefore,  equivalent  to  a  branching 
Turbellarian  nephridium.  it  has  been  suggested  that  the  Annelid  nephrid- 
ial  system  has  been  produced  by  the  fragmentation  of  an  originally  con- 
tinuous system,  but  for  this  there  is  no  embryological  evidence.  Each 
nephridium  being  a  bud  from  an  undifferentiated  nephridial  blastema  is 
just  as  much  an  organ-individual  as  is  the  branched  nephridium  of  a  Tur- 
bellarian—just  as  much  an  individual,  though  of  a  lower  grade,  as  is  the 
bud  of  a  Polyzoon  developed  from  an  undifferentiated  blastema.  It 
might  be  supposed,  then,  that  the  Annelid  nephridium  might  show  a 
branched  structure  in  certain  primitive  forms,  and  indeed  a  branched 
head-kidney  occurs  in  Polychaet  Trochophores.  A  branched  condition  is, 
however,  rare  in  the  trunk  nephridia,  though  it  does  occur  in  certain  ter- 
restrial Oligochajta  in  which,  however,  it  must  be  regarded  as  a  purely 
secondary  phenomenon  without  any  phylogenetic  significance,  since  in  the 
development  of  such  nephridia  a  single  tube  is  first  formed  which  later  on 
becomes  solid  and  then  gives  off  the  branches,  the  various  nephridial 
branches  of  successive  segments  becoming  sometimes  united.  This 
branched  condition  passes  into  one  in  which  the  various  branches  separate 
and  acquire  independent  openings  ;  several  pairs  of  nephridia,  four  in  the 
anterior  segments  of  a  species  of  Perichccta  and  a  greater  number  in  other 
forms,  occurring  in  a  single  segment.  This  branching  and  multiplication 
of  nephridia  is  confined  to  terrestrial  forms  which  in  their  conditions  of 
existence  are  farthest  removed  from  the  primitive  state,  and  it  is  not 
improbable  that  the  multiplication  bears  some  relation  to  the  assumption 
of  a  terrestrial  mode  of  life.  In  the  genus  Lumbricus,  in  which  the 
nephridia  are  simple  coiled  tubes,  a  duplication  of  the  nephridia  in  some 
segments  is  to  be  found.  The  reproductive  ducts  are  probably  modified 
nephridia,  and  in  all  aquatic  forms  other  nephridia  are  absent  in  the 
metameres  in  which  they  occur.  In  Lnmhrims,  however,  in,  for  instance, 
the  metamere  which  contains  the  oviducts,  two  pairs  nre  present,  one  of 
which  retains  its  original  excretory  function,  while  the  other  has  been 
modified  to  form  a  duct  for  the  reproductive  elements. 

The  reproductive  organs  have  a  very  different  arraugemeut 
from  what  is  found  in  tlie  Polychfeta,  being  limited  to  a 
comparatively  few  metameres ;  and  furthermore  the  Oli- 
gochffita  are  throughout  hernmphroditic,  the  male  and  female 


224 


INVEUrEBRATE  MORPHOLOQ  Y. 


orgaus  both  lying  iu  the  anterior  portion  of  the  body,  usually 
between  the  ninth  and  fourteenth  metameres,  or  sometimes 

even  farther  forward.  There  are 
either  one  or  two  pairs  of  testes 
(Fig.  104,  t),  which  arise  like  the 
ovaries  from  the  peritoneal  epithe- 
lium and  early  break  down  to  a 
greater  or  less  extent,  their  cells 
passing  into  seminal  vesicles  (v-s), 
where  they  undergo  further  de- 
velopment into  spermatozoa.  The 
vasa  deferentia  {vd)  are  modified 
nephridia,  and  are  either  two  or 
four  in  number  according  as  there 
are  one  or  two  pairs  of  testes. 
When  four  are  present  they  may 
open  separately,  or  may  unite  in 
pairs  on  either  side  in  a  common 
atrium,  through  which  they  open 

to  the  exterior,  or  finally  those  of 
Fig.    104.  —  Neuvoxts   System   ,,  . ,  -^  i      i. 

AND  Reproductive  Okgans  ^^]^  ^^"^^  ^i^Ie  may  .unite   a  short 

distance  below  the  funnels,  forming 
for  the  greater  part  of  their  course 
a  single  tube.  There  is  only  a 
single  pair  of  ovaries  (ov),  to  which 
in  some  forms  ovarian  receptacles 
similar  to  the  seminal  vesicles  are 
added ;  and  in  all  but  some  of  the 
lower  forms  oviducts  {od),  which 
are  modified  nephridia,  are  pres- 
ent. In  front  of  the  metameres  which  bear  the  testes  one, 
two,  or  occasionally  three  pairs  of  invaginations  of  the  body- 
wall  occur,  producing  pouches  projecting  into  the  body-cavity 
— the  seminal  receptacles  (rs) — which  receive  the  seminal 
fluid  during  the  mutual  interchange  of  it  which  takes  place  on 
copulation. 

A  satisfactory  subdivision  of  the  OligochaBta  into  orders  has  not  yet 
been  possible ;  indeed  the  various  families  are  so  related  to  one  another 
that  such  a  subdivision  seems  unnecessary.     Formerly  it  was  the  custom 


OP  Luvibricus. 

ce  =  supraoesophageal  gauglion 

n  =  ventral  nerve-cord. 
od  =  oviduct. 
ov  =  ovary. 

rs  =  receptaculum  semiuis. 

80  =  subcesophageal  gauglion. 

t  =  testis. 
vd  =  vas  deferens. 
vs  =  vesicula  seminalis. 


TYPE  ANNELIDA. 


225 


to  recognize  two  orders,  Limicolce  and  Ten-icohr,  aquatic  forms  being 
referred  to  tlie  former,  and  terrestrial  ones  to  the  latter — a  division,  how- 
ever, which  is  decidedly  artificial.  Less  so,  but  still  unsatisfactory,  is  a 
division  into  Naidonwrpha,  reproducing  nou-sexually,  and  Lumbrico- 
morp/ia,  reproducing  by  tlie  sexual  method  only.  It  seems  on  the  whole 
better  to  omit  a  subdivision  into  larger  groups,  and  recoguize  one  into 
families  only. 

Development  of  the  Oligochceta. — lu  the  development  of  the 
OligochsBta  there  is  practically  no  larval  stage,  but  a  sufficient 
amount  of  nutrition  is  supplied  to  the  embryo,  either  in  the 
form  of  yolk  in  the  egg  itself  or  as  an  albuminous  substance 
stored  up  in  the  interior  of  a  cocoon  in  which  the  ova  are 
contained,  to  enable  it  to  pass  through  all  its  early  stages 
while  still  within  the  egg-shell  or  cocoon,  and  to  assume  a 
free  life  only  when  it  has  reached  the  form  of  the  adult.  The 
Trochophore  larva  under  such  conditions  is  useless,  and  is 
suppressed  in  the  ontogeny,  the  development  becoming  thus 
dh'ect  or  of  the  fcetal  type.  This  mode  of  develojjment  has 
been  acquired  as  an  adaptation  to  the  aquatic  or  terrestrial 
life,  in  which,  for  obvious  reasons,  the  occurrence  of  a  free- 
swimming  larva  would  be  an  inconvenience  rather  than  an 
advantage. 

In  the  Polychseta  it  was  stated  that  usually  at  a  very  early 
stage  of  development  one  cell,  later  dividing  into  two,  differ- 
entiates from  the  rest  as  the  primary  mesoblast,  and  gives 
rise  to  all  the  mesodermal  tissues  of  the  adult  worm.  This  is 
an  example  of  a  precocious  segregation  of  the  mesodermal 
material  into  a  single  cell.  It  is  to  be  presumed  that  in  more 
primitive  forms  the  mesoderm  separated  off  from  the  endo- 
derm  only  at  a  relatively  late  period  of  development ;  the 
tendency,  however,  for  the  appearance  of  an  important  struc- 
ture to  be  thrown  farther  and  farther  back  in  the  individual 
development,  to  appear  at  successively  earlier  stages  in  the 
development,  has  asserted  itself  to  such  an  extent  that  the 
mesoderm  in  the  Polychseta  makes  its  appearance  winle  the 
embryo  is  still  composed  of  but  a  few  cells,  becoming  there- 
fore segregated  in  a  single  cell.  Such  a  process  has  further- 
more the  advantage  of  permitting  a  rapid  growth,  the  original 
embryonic  mesoblasts  retaining  their  position  at  the  posterior 


226 


INVERTEBRATE  MORPHOLOGY. 


end  of  the  body  and  giving  rise  by  division  in  a  transverse 
plane  to  rows  of  cells,  the  mesoblast  bands.  Such  a  pre- 
cocious segregation  of  the  mesoderm  also  occurs  in  the  Oli- 
gochieta.  lu  a  Lumbricus  embryo  there  may  be  seen  near 
the  posterior  extremity  of  the  body  the  two  mesoblasts  (Fig. 

105,  m)t  lying  one  on  each  side 
of  the  middle  line,  with  the  raeso- 
blast-bands  {mb)  extending  for- 
wards from  them.  A  little  in 
front  of  them  and  on  either  side 
may  be  seen  another  cell  {nh), 
giving  rise  to  a  band  extending 
anteriorly,  which  later  on  will 
become  differentiated  into  the 
ventral  nerve-cord,  the  cells  which 
give  rise  to  it  being  neuroblasts  ^ 

^n»  while  a  little  behind  and   exter- 

FiG.  1U5.— SuHKACE  View  OF  Pos-       „     ,      .i  -.i  •  i      - 

OF  Lumbricus  oifrer  K.  B.  Wilson),  other  cells  (weand  x)  occur,  giving 

rise  likewise  to  germ-bands,  whose 
further  fate  is  undecided,  though 
it  seems  probable  that  the  inner  of 
the  two  bands  gives  rise  to  the 
nephridia,  the  cells  being  nephro- 
blasts.  Thus  from  a  small  number 
of  cells  the  entire  nervous  system,  with  the  exception  of  the 
supraoesophageal  ganglion  (which  arises  as  a  local  thickening 
of  the  ectoderm,  comparable  to  the  apical  thickening  of  the  Tro- 
chophore),  the  nephridia  and  all  the  other  mesodermal  tissues 
arise,  the  precocious  segregation  of  these  organs  being  carried 
to  an  extent  only  equalled  in  the  Hirudiuea.  Indications  of 
it,  however,  are  found  in  the  Polychseta,  not  only  in  the  meso- 
blasts but  also  in  a  layer  of  cells  occupying  the  ventral 
surface  of  the  embryo,  and  forming  the  so-called  ventral  plate, 
from  which  the  ventral  nerve-cord,  the  nephridia,  and  some  of 
the  musculature  seem  to  arise.  A  reduction  of  the  number 
of  cells  constituting  this  ventral  plate  to  the  smallest  number 
consistent  with  a  bilateral  symmetry,  that  isj  to  two  for  each 


6C  =  ectoderm. 
VI  =  mesoblast. 
mb  —  niesoblast-band. 
nb  =  neuroblast. 
ne  =  uepbroblast. 
X  =  lateral  teloblust. 


^^'^■^ii^mmivm 


TYPE  ANNELIDA. 


227 


e 


sec  of  structures  formed  from  it,  would  give  rise  to  a  condition 
such  as  is  found  in  the  Oligochreta. 

A  number  of  the  simpler  Oligochtets,  in  addition  to  repro- 
ducing in  a  sexual  manner,  also  reproduce  by  division,  and  in 
some  forms  it  plays  a  much  more  important  part  than  the 
sexual  method,  which  in  jEolo/^oma  is  not  yet  known  to  occur. 
In  the  simplest  form  of  this  method  of  reproduction  the 
animal  simply  divides  at  the  middle,  each  portion  after  sepa- 
rating regenerating  the  parts  which  are  wanting.  In  one  spe- 
cies of  Ctenodrihts  each  metamere  except  the  anterior  one 
may  separate  and  become  l  new  individual ;  a  phenomenon 
which  might  be  regarded  as  iliustiating  the  bud  theory  of 
metamerism,  but  which  seems  more  properly  to  be  a  case  in 
which  the  gradual  integration  of  the  multiplied  organs  has 
reached  its  highest  development — the  case  standing  as  the 
culmination  of  the  process  of  metamerization  rather  than  as 
an  example  of  its  mode  of  origin.  In  Nais  a  division  of  the 
new  individuals  may  begin  before  they  have  separated,  and 
chains  may  thus  be  produced  composed  of  individuals  vary- 
ing in  the  stage  of  regeneration  which  they  have  reached, 
but  which  eventually  separate  and  may  later  become  sexually 
mature. 

As  might  be  expected  from  the  occurrence  of  this  mode  of 
reproduction,  the  power  of  regeneration  of  lost  parts  is  pos- 
sessed in  a  high  degree  by  the  Oligoclujeta ;  and  not  only  in 
those  forms  which  habitually  reproduce  by  division,  but  also 
in  forms  like  Lumhricus,  in  which  under  normal  conditions 
this  method  of  reproduction  is  unknown. 

Affinities  of  the  OUgochcEta. -IhavG  is  little  reason  to  doubt  that  the 
Oligochaeta  have  been  derived  from  the  Polychteta,  and  represent  members 
of  that  subclass  which  have  become  specially  adapted  to  aquatic  or  terrestrial 
modes  of  life.  A  few  01igocha?ts,  such  as  Hahxlrilus,  are  marine,  living 
below  stones  between  tides;  but  they  are  undoubtedly  derived  from  aquatic 
forms,  and  cannot  be  regarded  as  having  any  ancestral  significance.  As  re- 
gards the  more  definite  affinities  of  the  group  little  can  at  present  be  stated 
with  certainty.  They  have  l^en  referred  to  forms  like  the  Capitellida), 
in  some  of  which  the  paripodia  are  very  much  reduced,  as  is  likewise 
the  distinctness  of  the  head,  while,  as  in  the  Naids,  lateral-line  sense-organs 
are  present.  A  more  remot  3  relationship  through  the  Archiannelida  has 
also  been  suggested,  but  at  present  no  definite  evidence  is  forthcoming  as 
to  which  view  is  to  be  preferred. 


2^8 


IN  VERTEBRA  TE  MORPHOLOQ  T. 


II.  Class  Uirudinea. 

The  Hirudinea  diflfer  from  the  Choetopoda  in  their  external 
form,  being  destitute  either  of  parapodia  or  setae,  and  possess- 
ing at  the  anterior  end  of  the  body  a  muscular  sucker  at  the 
bottom  of  which  the  mouth  is  situated,  while  a  second  larger 
sucker  used  for  attachment  occurs  at  the  posterior  extremity 
of  the  body.  The  outer  surface  of  the  body  is  distinctly 
ringed,  but  a  comparison  of  the  rings  with  the  internal  organs 
shows  that  they  have  not  a  metameric  value,  but  that  a  num- 
ber of  them,  varying  in  different  forms,  are  included  in  each 
true  segment  of  the  body.  In  Brarwhellion  and  Clepsine  three 
such  rings  correspond  to  a  metamere,  in  IchthyobdeUa  and 
Pontobddla  six,  in  Piscicola  twelve,  and  in  all  the  group  of 
the  GnathobdellidsB  five.  Towards  the  anterior  and  posterior 
ends  of  the  body  a  reduction  of  the  number  of  rings  corre- 
sponding to  a  metamere  is  found,  as  for  instance  in  the  genus 
MacrobdiUa  (Fig.  106),  which  has  in  the  middle  region  five  rings 
to  a  segment.  The  first  two  metameres  consist  of  but  one  ring 
each,  the  third  of  two  rings,  the  fourth,  fifth,  and  sixth  each 
of  three  rings.  At  the  posterior  end  of  the  body  the  twenty- 
third  metamere  consists  of  four  rings,  the  twenty-fourth, 
twenty-fifth,  and  twenty- sixth  of  two  rings  each ;  while  prob- 
ably no  less  than  seven  metameres  whose  rings  are  not 
readily  distinguishable  are  represented  in  the  posterior  sucker. 
The  entire  animal  consists,  therefore,  of  thirty-three  metameres, 
and  this  number  is  characteristic  for  all  the  Hirudinea  ~a 
definiteness  of  number  which  contrasts  strongly  with  the  wide 
variations  found  in  the  Chsetopoda.  This  number  does  not 
include  a  small  lobe  in  front  of  the  most  anterior  metamere, 
which  may  be  equivalent  to  the  prostomial  lobe  of  the  Oligo- 
chseta,  and  may  represent  another  metamere. 

As  in  the  Oligochseta  the  gland-cells  of  the  hypodermis  at 
about  the  time  of  reproduction  become  enlarged  and  more 
abundant  in  a  definite  region  of  the  body,  forming  a  clitellum 
which  is  usufiUy  in  the  neighborhood  of  the  tenth,  eleventh, 
and  twelfth  metameres.  As  a  rule  no  branchiae  occur,  though 
an  exception  is  found  in  the    marine  genus  Brarwhellion,  in 


,  k..<**W«*-fc' 


TYPE  ANNELIDA. 


229 


which  each  ring  of  the  middle  region  of  the  body  bears  an- 
appendage  which  functions  as  a  gill.   In  another  marine  form, 
Pontohdella,  large  warts  occur  on  certain  rings,  which  prob' 


Fig.  107.  —  Diagkams  to  show  Ar- 
rangement OF  Blood-sinuses  of  {A) 
Hmido,  (B)  Clepaine,  and  (C)  Nephelis 
(after  Bourne). 

al  =  digestive  tract, 
c  =  coelora. 
ds  =  dorsu!  sinus. 
Is  and  Iv  =  lateral  sinus  or  vessel. 
n  and  ne  =  ventral  uerve-cord. 
ne  =  nepbridium. 
OV  =  ov;;ry. 

««  =  ventral  sinus. 

ably  are  mainly  respiratory  in  function,  beir^;  richly  supplied 
with  bJood-vessels. 

The  exterior  of  the  body  is  covered  by  a  cuticle,  beneath 
which  lies  the  hypodermis.  The  muscular  tissue  which  un- 
derhes  the  hypodermis  consists,  as  in  other  Annelida,  of  layers 


Fig.   106.— Antbuior  and  Pos- 
terior Extremities  of  Ma- 
crobdella  aestertia  (after  Whitman). 
an  =  anus. 

fo  =  opening  of  oviduct. 
gc  =  copulatory  glands. 
mo  =  opening  of  vasa  deferentia 
oc  =  eyes. 

p  =  nephiidial  pores. 
sp  =  sense-papilla. 

1-100  =  annuli. 

i-xxv  =  melumeres. 


230 


INVERTEBRATE  MORPHOLOGY. 


.  of  lougitmliual  auci  circular  iibres ;  aud  iu  additiou,  between 
these,  u  layer  composed  of  fibres  which  cross  one  another 
obliquely  is  usually  jj resent.  A  marked  distinction  from  what 
occurs  in  the  Cha3topoda  is  found  in  the  ccelom,  which  in  the 
Hirudinea  is  traversed  by  a  parenchyma,  recalling  that  of  the 
Platyhelminths,  so  that  the  actual  cavity  is  to  a  great  extent 
obliterated,  and  the  dissepiments  only  to  be  distinguished  with 
dilUculty.  Those  portitms  of  the  ccelom  which  persist  (Fig. 
107,  Cc)  are  occupied  by  a  red  or  colorless  fluid  containing 
corpuscles  and  identical  aud  continuous  with  that  found  iu  the 
blood-vessels.  The  ccelom  is  in  fact  represented  by  a  number 
of  blood-sinuses,  which  in  some  forms  are  lined  by  an  epithe- 
lium, while  iu  others  such  a  lining  is  wanting.  On  account  of 
the  manner  in  which  the  blood-vessels  anastomose  with  the 
sinuses  it  is  exceedingly  difficult  to  distinguish  which  spaces 
should  be  considered  as  belonging  to  the  circulatory  system 
proper  and  which  to  the  coelom — if,  indeed,  the  two  are  to  be 
considered  fundamentally  distinct.  As  a  rule  four  main  lon- 
gitudinal vessels  or  sinuses  are  to  be  found — viz.,  one  dorsal 
(Fig.  107,  ds),  which  may  be  wanting  (Nephelis,  Fig.  107,  C)  aud 
which  probably  corresponds  to  the  dorsal  vessel  of  the  Chae- 
topoda ;  one  ventral  (vs),  sinus-like  in  character  and  frequent- 
ly^ destitute  of  an  epithelial  lining,  which  surrounds  the  ventral 
nerve-cord  ;  and  two  lateral  vessels  [Iv  and  Is)  unrepresented 
in  the  Chretopods,  and  perhaps  also  to  be  regarded  as  rem- 
nants of  the  coelomic  cavit}'.  Communications  between  these 
longitudinal  vessels  occur  through  the  medium  of  smaller 
vessels ;  and  iu  some  forms,  such  as  Nephelis,  the  connection 
between  the  lateral  and  ventral  vessels  takes  place  through 
ampullae,  globular  vesicles  arranged  in  two  pairs  on  each 
side  of  a  number  of  metameres  aud  leceiving  blood-vessels 
from  the  ventral  sinus,  while  other  vessels  passing  to  the 
main  lateral  vessels  arise  from  them.  In  many  forms,  espe- 
cially among  the  GnathobdellidsD,  a  rich  plexus  of  capillary 
vessels  penetrates  the  hypodermis. 

The  union  of  the  blood  vascular  system  with  sinuses  which  most  prob- 
ably represent  portions  of  the  coelomic  cavity  suggests  an  intimate  relation, 
so  far  as  its  origin  is  concerned,  of  the  vascular  system  with  the  ccelom  ; 
and  this  view  is  borne  out  by  what  has  already  been  seen  to  occur  in  the 


>  •a 


TYPE  ANNELIDA, 


231 


Nemerteans  {\t.  16i5),  tho  lowest  forms  that  possess  a  distinct  blood  vascular 
system.  In  this  group  the  cu'lom,  so  far  as  it  exists,  consists  of  small  sjjaces 
without  any  definite  walls  scattered  through  the  parenchyma.  In  soinr 
forms  the  blood  vascular  system  communicates  with  these  spaces  tiin>ii<,'h 
which  the  blood  circulates,  it  being  only  in  the  most  highly  dilVerentialed 
Nemerteans  that  the  vascular  system  is  closed.  It  migiit  be  supposed  from 
this  that  the  bloo<l- vessels  were  simply  ca'lomic  spaces  which  had  ac(iuired 
defmite  walls;  and  it  seems  probable  that  such  has  been  their  origin.  In  the 
Annelida  a  somewliat  different  state  of  affairs  occurs.  Here,  as  a  rule,  there 
is  a  definite  ccelom  lined  with  i)eritoneum  and  completely  separated  from  the 
cavity  of  the  blood-vessels,  which  seem  to  represent  rather  the  remains 
of  an  original  cavity,  the  so-called  blastoc(jel  (see  p.  52),  which  has  been 
almost  obliterated  l>y  the  growth  of  the  mesodermal  segments  by  the  hollow- 
ing out  of  which  the  ccelomic  cavities  have  been  formed  (see  p.  56).  It 
seems  certain  that  the  ca3lomic  spaces  of  the  Nemerteans  are  likewise  the 
remains  of  a  primitive  blastoccol,  so  that  to  this  extent  the  homology  of  the 
blood-vessels  holds  in  the  two  groups. 

In  the  Ilirudinca,  however,  tlio  blood-sinuses,  if  they  are  ccelomic,  cor- 
respond with  the  coelom  of  the  Polyclneta  ;  and  furthermore,  in  the  Oligo- 
chtBts  and  Polychujts,  as  well  as  in  the  Gephyrea,  as  will  be  seen  later,  the 
haemolymph  contained  in  the  coelom  is  very  nearly  if  not  quite  identical  in 
composition  with  the  blood  contained  in  the  blood-vessels.  These  facts 
would  seem  to  indicate  a  close  relationship  between  the  Annelid  coelom  and 
the  more  primitive  blastocoel;  or,  in  other  words,  would  lead  us  to  suppose 
that  the  ccelom  of  the  Annelids  lined  with  peritoneum  is  not  something 
apart  and  distinct  from  tiie  blastoccol  cavity,  as  has  usually  been  supposed. 
The  view  which  maintains  the  distinctness  of  the  two  forms  of  ccelom  has 
its  origin  in  the  fact  that  in  some  forms,  such  as  Sagitta,  a  ccolom  lined  with 
peritoneum  is  formed  as  an  outgrowth  from  the  primitive  digestive  tract; 
and  it  was  supposed  that  all  ca'lomic  cavities  with  definite  walls  were  i)ri- 
marily  of  a  similar  origin,  and  hence  were  termed  enterocmls  m  contradis- 
tinction to  the  scldzocmls  or  simple  spaces  in  the  mesoderm  without  defi- 
nite walls,  which  are  in  reality  remnants  of  the  blastocoel.  The  significance 
of  true  enterocoels  will  be  discussed  later.  In  the  mean  time  it  may  be 
pointed  out  that  there  is  no  embryological  evidence  in  favor  of  the  Annelid 
ccelom  having  arisen  as  a  series  of  pouch-like  outgrowths  from  the  primi- 
tive digestive  tract.  It  is  rather  to  be  regarded  as  a  schizocoel  whose  char- 
acter has  been  altered  by  metamerization,  and  by  the  manner  of  its  forma- 
tion from  mesoblasts.  On  this  view  the  union  of  the  cavity  of  the  blood- 
vessels with  the  coelom  in  the  leeches,  and  the  similarity  of  the  hsemolymph 
to  the  blood  in  other  forms,  cease  to  be  morphological  puzzles. 

The  mouth  lies  at  the  bottom  of  the  anterior  sucker  and 
opens  into  a  muscular  pharynx,  which  in  some  forms  (e.g.  Clep- 
aine)  is  folded  similarly  to  that  of  some  Turbellaria  (see  p.  134) 
so  as  to  form  a  protrusible  tube,  while  in  others  (e.g.  Hirvdo, 


\m 


1^  VKUTKltliA  TK  MOIU'I/OLOd  V. 


Pig.    108.— Diagram   of 

THK  EXOUKTOKY  Uk- 
rKOUl'CTlVK  AM)  NkK- 
VOIS  SySI"KM8  ov  Ilii'udo 
^ufler  Hoi'KNK). 

<v  =  ctMvbnil  juuiirlii)!!. 
«7)  =  cpididyinus. 

gl  =^  oviilucnl  ulauil. 

If  =  latt  ral  l>K)od-ve!«el. 

«  =  ncphridia. 

ot  =  t>var\ . 

pe  =  penis. 

k  =  testis. 

masses.     Six  of  these 


MiwroMi'lUt)   it   is    tlirown    into    tliioo 
loii^itiuliiuil    iiMisciilHr    rid^i^s     whoso 
etlg(*s     inay     hecoinn     coiiverttul    into 
oliitin,  thus  ftniuiii^'  t«M>th.      Sulivuiy 
ghinils  optui  into  tlu>  phurynx  in  sonio 
forms.     Th*^  hiig«<  stomach  into  whioii 
the  pharynx  opens  heiiin«l  gives  otf  a 
nuniher    of    lateral     pouehes    (elt^vtui 
j)airs  in    IIini<f<>,    seven    in     ClcfMiiH-), 
sonu^times  hranehed  and  increasing  in 
size  from  befort*  backwards,  the   niost 
posterior  pair  being  usually  quite  long 
and  dinu'ted  backwards  })arallel  to  the 
straight  narrow  intestine  wiiich  i)pens 
to  the  exterior  on  the  dor.sal  surface  of 
the  body,  just  anterior  to  the  posterior 
sucker.     Occasioually  only  the  poste- 
rior pair  of  pouches  is  present,  and  in 
a  few  forms  they  are  entirely  wanting. 
The  nervous  system  (Fig.    108)  is 
constructed    on    the    tyj)ical    Annelid 
plan.     It  consists  of  a  circumo'sopha- 
geal  ring    and   a    ventral    nerve-cord 
composed  of  fibres  which   have  their 
origin   in    ganglion-cells    grouped  to- 
gether  at  definite    intervals  into  gan- 
glionic masses.     Several  of  these  gan- 
glionic   masses   correspond    to    single 
segments,   but    at    the    anterior    and 
posterior  extremities    a    considerable 
amount   of    fusion   of   the    metameric 
groups  of  ganglia   has  occurred.     In 
Clepsine  plana  the  j)ortion  of  the  ner- 
vous   system    which    lies    above    the 
oesophagus    consists  of    a    transverse 
band  of  fibres   passing  hiterally  into 
the    circumcMSophageal     commissures 
and     of     a     number     of     ganglionic 
latter  lie  in  front  of  the  band  of  fibres 


ijikM»*Ayj*aKj*** 


TYVK  ANNhlSDA. 


a:w 


Htul  corroMpoiid  to  tlu;  nuttuiiKHo  I'orniud  by  tho  prosloinial 
li>l>(>.  iioliiiid  tint  ti'Hiisvui'so  1)1111(1  Hit!  tour  lulditional  ^<'lll^li- 
onic  inuHsos,  it|i|)iii'oiiliv  i'oiiiiiii^  witii  tli<>  olluir  six  tliu  HupDi- 
(I'Hopliiij^oal  j^Hiij^lioii,  but  m  reality  fonidiij^  together  with  two 
udditioiuil  iiiHSHi'rt  on  the,  vcntnil  sid(!  of  tho  uuivo-cord  bnlow 
th(^  <iNSo}>hu^UK  till)  ^uu}j;lioii  ol*  the  Hucoiid  Hoiiiito.  Iiiiiu(!di- 
a(<'ly  posti'iior  to  tho  two  vi'iitial  masses  is  a  eliaiu  of  oij^dit 
ganglia  lying  ont^  luOiiiid  the  other  on  tho  niid-vttntral  lino  of 
tlu!  coi'tl,  ami  coiTtiSponding  to  tlmso  thoro  ociuir  on  ea(di  side 
along  tlu'  dorsal  surfaeo  of  the  eord  other  eight  masses, 
between  eaeh  successive  pair  of  which  a  norvo  passes  out. 
There  ur*)  therefore  f«)ur  nietanieric  ganglia  ropr«fs«!nte<l  in 
this  complex  structure,  eatrh  consisting  of  six  ganglionic 
masses  and  each  giving  rise  to  a  pair  of  ncirves.  The  sub- 
(eso})hageal  ganglion  accordingly  consists  of  tho  ganglia  of 
four  nietameros,  to  which  must  be  addtul  tho  two  ventral 
masses  of  a  fifth  niotanuiro,  the  supra-  and  sulxesophagoal 
ganglia  representing  together  six  inetamoric  ganglia,  liehind 
the  subu'sophageal  ganglionic  aggn'gate  there  lie  twenty-one 
ganglia  sj'paratetl  at  some  distance  from  each  other,  especially 
antiirioily,  each  one  niprosonting  a  metamero  ;  and  finally  at 
the  posterior  (Mid  of  tho  body  is  anothiir  ganglionic  aggregate, 
repn^senting,  to  judge  from  the  number  of  nerves  arising  from 
it,  s^^von  metamoric  ganglia.  Thus  there  are  in  all  thirty- 
throe,  or,  counting  the  ganglion  which  innervates  th«!  jirosto- 
niiuni,  thirty-four,  nietameric  ganglia — numbers  exactly  corre- 
sponding with  those  obtained  by  (Counting  the  rings. 

The  sense-organs  of  tln^  Hirudinoa  have  cispoirial  interest 
as  showing  an  atlaptation  of  what  may  bo  considered  tacttile 
sense-organs  to  a  ditlerent  ]»ur[iose.  On  (!ach  nietani  do  of 
tho  body  in  all  Hirudinoa,  with  a  few  possible  excepti()ns, 
small  sensory  papilla*  (l*'ig.  lOG,  np)  are  to  be  soon,  arranged 
in  definite  linos.  Tli(*y  occur  in  the  majority  of  forms  on  thcj 
first  ring  of  t^ach  segment,  though  in  some  species  of  Nrp/tiltH 
they  occur  on  all  tho  rings.  On  the  dorsal  surface  of  each 
sensory  ring  there  are  throe  papiihe  on  each  side  of  tho 
middle  line,  and  the  same  arrangement  occurs  ou  the  ventral 
surface,  and  iu  aiblition  a  single  {)apilla  is  found  at  the 
margin  of  the  ring  on  each  side.     There  are  thus  fourt«>»'n 


234 


INVERTEBRATE  MORPHOLOGY. 


longitudinal  rows  of  papillas,  six  on  the  dorsal  surface,  six  on 
the  ventral  surface,  and  two  marginal.  In  the  anterior  and 
posterior  segments  whose  width  is  reduced  the  marginal  pa- 
pillsB  may  be  wanting,  but  throughout  the  rest  of  the  body 
the  number  of  rows  is  constant.  In  structure  these  papillae 
are  somewhat  complicated,  consisting  of  an  axial  bunch  of 
elongated  sensory  cells  bearing  fine  cilia  at  their  outer  ends, 
and  lying  in  the  connective  tissue  in  their  immediate  vicinity 
is  a  varying  number  of  large  cells,  each  containing  a  large 
watery  vacuole  in  the  interior,  the  nucleus,  in  consequence, 
being  pushed  to  one  side.  A  strong  nerve  runs  to  each  pa- 
pilla and  is  supplied  to  the  large  vacuolated  cells  as  as 
to  the  axial  sensory  cells. 

Slight  differences  are  to  be  found  in  various  forms  in  the  structure  of 
these  organs.  In  Clepsine  there  is  an  axial  bunch  of  hair-bearing  cells  to 
which  the  terminal  fibres  of  the  nerve  run,  and  posteriorly  and  below  the 
nerve  are  found  the  large  vacuolated  cells.  In  Hirudo  and  Nephelis  no 
hair- bearing  cells  occur,  the  nerve  occupying  the  axis  of  the  organ  and  the 
vacuolated  cells  being  arranged  symmetrically  around  it. 

It  is  probable,  in  view  of  the  two  kinds  of  constituent  elements  in  Clep- 
sine, that  in  this  and  similar  genei'a  a  double  function  is  possessed  by  the 
sensory  papillae,  the  hair-bearing  cells  having  perhaps  a  tactile  function, 
while  the  vacuolated  cells  are  visual.  It  seems  probable  also  that  primarily 
the  papillae  were  similar  in  structure  and  function  to  the  organs  of  the 
lateral  line  of  certain  Polychaeta,  such  as  the  Capitellidae,  or  perhaps  it  would 
be  better  to  compare  them  with  the  tactile  papillae  of  certain  aquatic 
Oligochaeta,  which  in  the  genus  JSlacina  have  an  arrangement  on  each  meta- 
mere  recalling  tliat  found  in  the  llirudinea. 

Towards  the  anterior  extremity  of  most  of  the  Hirudinea 
a  varying  number  of  eyes  are  found.  In  some  species  of 
Clepsine  but  two  such  organs  occur,  while  in  others  there  are 
six,  and  in  Iliruclo,  Maxirobdella  (Fig.  lOG,  oc)^  and  allied  forms 
there  are  always  ten.  In  the  latter  forms  tlie  eyes  are  always 
arranged  in  a  definite  manner:  one  pair  is  situated  on  the 
anterior  ring  (when  more  than  one  ring  occurs)  of  each  of  the 
five  metameres  immediately  following  the  prostomial  lobe, 
and  if  their  position  be  determined  it  will  be  found  that  they 
occupy  the  place  of  one  of  the  dorsal  sense-papillre,  the  eyes 
being  serially  homologous  with  the  sense-papillre  of  one  of 
the  dorsal  rows.     This  conclusion  is  verified  by  their  struc- 


TYPE  ANNELIDA. 


235 


ture,  since  they  differ  from  the  sensory  papillae  only  in  the 
greater  number  of  the  large  vacuolated  cells  and  in  the  pres- 
ence of  a  quantity  of  black  pigment  in  the  surrounding  tissues. 

Other  sense-organs  somewhat  beaker-shaped  in  character 
are  found  upon  the  prostomium  and  have  been  regarded  as 
gustatory  in  function. 

Nephridia  occur  in  a  number  of  the  metameres  of  the 
middle  portion  of  the  body,  there  being  in  Hirvdo  (Fig.  108,  n) 
and  its  allies  seventeen  pairs.  Each  nephridium  has  a  ter- 
minal funnel,  which  in  Clepsine  has  the  typical  Annelidan 
structure,  but  in  Hirudo  has  been  modified  so  that  the  inner 
extremity  of  each  nephridium  is  constituted  by  a  lobed  spongy 
ciliated  mass  without  any  definite  central  lumen.  The  funnel 
lies  in  a  blood-sinus,  either  the  ventral  one  as  in  Clepsine 
(Fig.  107,  B)  or  the  dorsal  as  in  Fontobd^a,  or  in  a  sinus 
which  surrounds  the  testes  as  in  Hirudo  (Fig.  107,  A),  or  in 
a  special  sinus  which  is  to  be  regarded  as  a  ccelomic  space  as 
in  Nephelis  (Fig.  107,  G).  The  canal  which  traverses  each 
nephridium  is  intracellular  as  in  the  Oligochseta,  and  in  some 
forms  minute  canals  traverse  the  substance  of  each  cell,  open- 
ing into  the  central  lumen.  As  a  rule  the  various  nephridia 
are  quite  separate  and  distinct  from  each  other,  but  in  Pontoh- 
della  and  one  or  two  other  genera  they  unite  to  form  a  net- 
work of  intracellular  canals  traversing  several  metameres. 
Immediately  before  their  exit  to  the  exterior  the  canals 
enlarge  in  some  forms  to  bladder-like  vesicles,  from  which 
a  short  tube  leads  to  the  exterior,  the  opening  being  situated 
either  upon  the  anterior  {Clepsine)  or  the  posterior  {Hirudo) 
ring  of  the  metamere  to  which  the  nephridia  belong. 

The  reproductive  organs  differ  from  those  of  the  Chaeto- 
poda  in  possessing  ducts  which  do  not  seem  to  be  modified 
nephridia  and  which  are  continuous  with  the  walls  of  the 
ovaries  or  testes.  All  the  Hirudiuea  are  hermaphroditic. 
The  ovaries  constitute  in  Clepsine  two  elongated  organs  which 
lie  in  the  middle  region  of  the  body,  extending  through  several 
metameres,  but  in  Hiriido  (Fig.  108,  od)  they  are  small  oval 
or  spherical  bodies ;  their  ducts  dilate  to  form  a  uterus  and 
finally  unite  to  open  on  the  mid-ventral  line  usually  in  the 
eleventh  metamere  (Fig.   106,  fo).     The  testes  (Fig.  108,  te) 


0 


236 


INVERTEDHA  TE  MORPIIOLOG  T. 


consist  of  a  number  of  pairs,  varying  from  twelve  or  more  to 
six  (Clepsine),  of  spherical  bodies  lying  in  the  same  region  of 
the  body  as  the  ovaries.  Each  testis  has  its  own  duct,  which 
opens  into  a  longitudinal  vas  deferens  common  to  all  the 
testes  of  the  same  side  of  the  body.  Anteriorly  the  two  vasa 
deferentia  unite  to  open  in  the  mid-ventral  line  of  usually  the 
tenth  metamere  (Pig.  106,  mo),  frequently  through  a  strong 
muscular  penis  (Fig.  108,  pe).  In  many  forms  special  glan- 
dular thickenings,  supposed  to  be  useful  in  copulation,  occur 
on  the  ventral  surface  of  one  of  the  metaraeres  behind  that 
bearing  the  opening  of  the  oviduct  (Fig.  106,  gc). 

The  Hirudinea  are  at  present  usually  divided  into  two 
orders,  though  it  seems  probable  that  further  division  of  one 
of  them  will  be  necessary  later. 

1.  Order  Gnathobdellidse. 

In  this  order  are  included  the  leeches  which  are  provided 
with  chitinous  jaws  in  the  walls  of  the  muscular  pharynx. 
In  addition  to  this  all  the  members  of  the  order  are  charac- 
terized by  possessing  five  rings  to  each  fully  developed  meta- 
mere. To  this  order  belong  the  Hirudinidse,  characterized  by 
possessing  ten  eyes  arranged  in  pairs  on  the  five  anterior 
metameres  behind  the  prostomium,  and  including  Hir^ido,  the 
medicinal  leech,  a  native  of  Europe,  instead  of  which  Macro- 
Id^a  is  sometimes  used  in  America.  The  Nephelidse,  with 
the  genus  Nephelis,  diflter  in  possessing  fewer  eyes  (four  pairs), 
and  in  having  distinct  segmental  sense-organs  either  wanting 
or  occurring  on  all  the  rings  of  each  segment. 

2.  Order  Rhynchobdellidee. 

The  Rhynchobdellidee  are  characterized  by  possessing  a 
protrusible  pharynx,  as  well  as  by  possessing  three,  six,  or 
twelve  rings  to  a  metamere.  In  the  Ichthyobdellidse,  or  fish- 
leeches,  the  larger  numbers  are  found,  the  number  six  being 
characteristic  of  Pontohdella,  while  twelve  occurs  in  Ptscicola. 
In  the  Clepsinidffi  but  three  rings  are  found  to  each  meta- 
mere, and  the  eyes  are  either  two  or  six  in  number.  To  this 
family  belongs  the  genus  Clepsine,  a  common  fresh-water 
form,  as  well  as  the  tropical  land-leech,  Hfementeria. 


TYPE  ANNELIDA. 


237 


Development  of  the  Hirudinea.  —The  Gnathobclellidie  deposit  their  eggs 
in  chitinous  cocoous,  as  do  the  Oiigochaeta,  and  the  development  is  of  the 
foetal  type,  iu  coutiadistinctioD  to  the  larval,  the  ova  containing  as  a  rule  a 
considerable  amouui  ot  yolk.  The  mode  of  oviposition  of  the  majority  of 
the  Rhynchobdellidat!  is  unknown  ;  but  in  the  genus  Clepsine  the  eggs  are 
fastened  to  the  ventral  surface  of  the  body  of  the  parent,  where  they  un- 
dergo development.  This  resembles  closely  the  development  of  Lumbrictis, 
allowing  for  the  greater  amount  of  yolk  which  is  usually  present.  The 
same  precocious  segregation  of  mesoderm,  nervous  system,  and  nephridia 
in  special  budding  ceils,  ihe  uiesoblasts,  neuroblasts,  and  nephroblasts,  is 
likewise  found,  aud  in  later  siages  the  mesoblast  is  distinctly  segmented 
and  coelomic  cavities  are  present,  which  later  become  to  a  great  extent 
obliterated. 

The  Affinities  of  the  Hirudinea.— It  is  exceedingly  probable  that  the 
ancestors  of  the  Hirudinea  were  to  be  found  in  the  Oiigochaeta,  the  two 
groups  having  not  a  few  structural  features  in  common.  The  embryologi- 
cal  peculiarities  found  in  the  two  groups  are  strikingly  similar ;  and  fur- 
thermore the  aquatic  or  terrestrial  liabits  are  not  a  little  suggestive,  for 
although  some  leeches  are  marine,  nevertheless  the  majority  are  aquatic 
and  a  few  terrestrial.  The  complete  disappearance  of  parapodia  may  be 
considered  a  further  development  of  the  tendency  towards  their  oblitera- 
tion in  the  Oiigochaeta,  where  only  the  setae  are  present,  these  even  having 
disappeared  in  the  Hirudinea  in  consequence  of  the  development  of  the 
suckers  and  a  new  mode  of  locomotion.  The  suggestive  arrangement  ef 
the  sense-papillae  of  the  Oligochaete  Slavina  has  already  been  mentioned. 

It  must  not  be  forgotten,  however,  that  the  differences  between  the  two 
groups  arc  many  and  important.  Such  are,  for  instance,  the  disappearance 
of  the  original  coelomic  spaces,  the  communication  of  the  blood  vascular 
system  with  sinuses,  and  the  occurrence  of  special  ducts  for  the  reproduc- 
tive organs.  These  differences  have,  however,  equal  or  even  greater  im- 
portance when  the  attempt  is  made  to  trace  the  Hirudinea  directly  to  the 
Polychaeta,  and  it  seems  more  satisfactory  at  present  to  refer  them  back  to 
the  Oiigochaeta. 


III.  Class  Oephyrea. 

The  Gephyreans  constitute  a  group  of  marine  worms  which 
differ  from  the  Chtietopoda  principally  in  the  more  or  less 
complete  absence  of  raetamerization.  All  trace  of  it  is  ab- 
sent upon  the  outside  of  the  bod}'';  for  although  the  thick 
cuticle  may  be  marked  by  distinct  rings,  these  bear  no  relation 
to  the  internal  parts  aud  are,  as  in  the  Nematoda,  due  simply 
to  the  thickness  of  the  cuticle.  All  traces  of  parapodia  are 
lacking  iu  many  forms,  while  in  others  they  are  represented 


238 


INVERTEBRATE  MORPHOLOGY. 


only  by  a  pair  of  setae  situated  ou  the  ventral  surface  of  the 
body,  nearer  the  anterior  than  the  posterior  end.  The  body- 
wall  presents  a  close  similarity  in  its  structure  to  that  of  the 
Ghsetopods — difl'ering,  however,  in  the  occurrence  of  a  more  or 
less  pronounced  layer  of  fibres  having  an  oblique  direction. 
Tha  coelom  is  lined  by  a  layer  of  Hat  peritoneal  cells,  but 
shows  no  division  into  more  or  less  distinct  compartments,  no 
trace  of  metamerism,  but,  as  in  the  Ch»etopods,  the  peritoneal 
lining  is  reflected  upon  the  walls  of  the  digestive  tract,  form- 
ing mesenteries  suspending  the  intestine.  As  a  rule  the  dor- 
sal mesentery  disappears,  and  in  some  cases  the  ventral  one 
is  almost  wanting,  the  intestine  being  slung  only  by  a 
number  of  irregular  strands  of  connective  tissue  extending 
from  it  to  the  body-wall.  In  some  forms  {Sipunculm)  the  sur- 
face of  the  peritoneum,  especially  that  covering  the  intestine, 
is  dotted  with  numerous  irregularly  scattered  minute  depres- 
sions, whose  openings  are  guarded  each  by  a  peculiar  ciliated 
ceil,  and  which  contain  cells  comparable  in  function  to  the 
chloragogue  cells  of  the  Chsetopoda.  The  coelomic  cavity  is 
occupied  by  a  haemolymph,  which  in  some  cases  is  colored, 
and  contains  numerous  cell-elements,  some  of  which  may  be 
circular  in  outline  and  colored  by  ha3moglobin,  while  others 
are  amceboid  and  colorless. 

A  blood  vascular  system,  principally  developed  in  the  an- 
terior portion  of  the  body,  is  present  and  appears  to  be  com- 
pletely closed,  though  connections  with  the  coelom  are  said  to 
exist  in  some  forms.  In  Sipttnculus,  for  instance,  the  system 
consists  of  a  collar  surrounding  the  cesophagus,  sending 
branches  into  the  tentacles  which  surround  the  mouth,  and 
dorsally  dilating  into  a  wide  sinus  lying  just  below  the  brain ; 
and  from  this  sinus  a  dorsal  vessel  (Fig.  109,  Bs)  passes 
backwards  along  the  digestive  tract  for  a  short  distance,  end- 
ing blindly  where  the  oesophagus  joins  the  stomach.  In  Echiu- 
rm  a  ventral  vessel  runs  the  entire  length  of  the  body  just 
above  the  nerve-cord,  and  it  is  united  with  the  dorsal  vessel 
by  lateral  vessels  at  its  anterior  and  posterior  extremities. 

The  digestive  tract  may  be  either  straight  (Priapulus)  or 
considerably  convoluted  {EcMurits  and  Sipunculus,  Fig.  109, 
Int),  and  the  anus  is  in  some  forms  terminal  (Echiurus),  while 


TYPE  ANNELIDA. 


239 


in  others  the  intestine  bends  upon  itself  and  passes  forward  to 
open  on  the  dorsal  surface  near  the  anterior  end  of  the  body 
(Fig.  109,  A).  Throughout  the  greater  extent  of  the  intestine 
there  ruus  along  its  ventral  surface 
a  ciliated  groove  which  is  no  doubt 
homologous  with  the  accessory  in- 
testine of  certain  Polychseta  (see 
p.  207). 

The  nervous  system  partakes  of 
the  absence  of  distinct  metamerism 
which  characterizes  the  other  parts. 
It  consists  of  a  brain  lying  in  the 
anterior  portion  of  the  body  above 
the  oesophagus  and  sending  a  com- 
missure downwards  and  backwards 
on  each  side  to  form  the  circum- 
oesophageal  collar.  These  two  com- 
missures unite  to  form  a  single 
nerve-cord  (Fig.  109,  li)  extending 
the  entire  length  of  the  body  in  the 
ventral  median  line,  diflteriug  from 
the  ventral  cord  of  the  Chaetopoda 
in  the  absence  of  ganglia.  Nerve- 
cells  are  scattered  along  the  entire 
length  of  the  cord  and  are  not 
aggregated  into  special  ganglia, 
though  slight  indications  of  such 
an  aggregation  are  found  in  Priapu- 
lus.     Nerves  are  given  off  at   more 


Fig.  109.— Stuuctuue  of  Si- 
punculus    Oouldii  (after  an- 
drewr). 
A  =  nnus. 
Bs  =  blood  vessel. 
,  ,         .  ,  .  ,         rfR  =  dorsiil  retmctormuscle. 

or  less  regular   intervals   on  either  J^^  ^  i„(estitie. 

side,    a    somewhat    metaiueric    ap-    n  =  nerve- conl. 
pearance  being  thus  produced,  but    ^  =  nephridium. 
the    corresponding    nerves   of    op-    Oe  =  ai,ox>\mgus. 

Ov  =  ovfti'v 
posite  sides  do  not  invariably  arise  ^^  ^  ^^.„j,,;,  ,etractor  muscle. 

from  the  cord  opposite  each  other. 

One,  two  or  three  pairs  of  nephridia  (Fig.  109,  ne)  are  as 
a  rule  present  and  form  conspicuous  brown  tubes,  which  com- 
municate by  a  funnel  with  the  body-cavity  at  one  extremity 
and  with  the  exterior  of  the  body  at  the  other.    They  are 


240 


INVERTEBRATE  MORPUOLOOT. 


undoubtedly  homologous  with  the  nephiidia  of  the  Chsetop- 
oda,  possessing  the  same  relations.  In  a  few  forms  {BonelUa, 
Phascolion)  a  single  nephridium  only  is  present.  In  addition 
to  these  in  Echiurus,  Thalassema,  and  allied  genera  there  is  a 
usually  much-branched  organ  on  either  side  lying  in  the  body- 
cavity  and  opening  into  the  terminal  portion  of  the  intestine. 
Numerous  ciliated  funnels  occur  upon  the  branches  placing 
the  organ  in  communication  with  the  body-cavity.  This  so- 
called  "  respiratory  tree  "  (so  named  from  a  supposed  homol- 
ogy with  the  similarly  named  organs  of  the  Holothuria  (q.  v.) 
are  probably  nephridia,  though  whether  or  not  they  per- 
form excretory  functions  is  not  quite  clear.  In  Priapnlus 
these  organs  are  rc^)resented  by  branched  tubes,  the  branches 
of  which  terminate  blindly  in  flame-cells,  resembling  thus  the 
excretory  organs  of  the  Platj^helminths,  and  in  Sipunculus 
rudiments  of  these  organs  have  been  described  as  short  tubes. 

The  Gephyrea  are  bisexual,  the  reproductive  organs  {oc) 
forming  small  digitate,  elongate,  or  ovoid  processes  arising 
from  the  peritoneal  lining  of  the  body-cavity ;  but  in  some 
forms  {Sipunculus)  their  products  early  escape  into  the  cce- 
lomic  cavity,  in  which  they  float.  The  exact  manner  in  which 
the  ova  and  spermatozoa  escape  to  the  exterior  has  not  been 
definitely  ascertained  for  the  majority  of  forms,  but  it  seems 
probable  that  the  nephridia  serve  as  the  generative  ducts. 
In  Priapuhis  the  "  respiratory  trees  "  are  said  to  give  rise  to 
the  reproductive  organs,  and  also  to  serve  as  the  reproductive 
ducts — a  behavior  which  would  render  exceedingly  probable 
the  supposition  that  they  are  modified  nephridia. 

Two  orders  are  recognizable  in  the  Gephyrea. 


1.  Order  Echinreee. 

The  EchiuresB,  sometimes  known  as  the  Gephyrea  armata, 
are  characterized  by  the  presence  on  the  ventral  surface  of 
the  body,  in  front  of  the  openings  of  the  nephridia,  of  a  pair 
of  seteB — the  genus  Echiurus  possessing,  in  addition  to  these, 
two  circles  of  setae  at  the  posterior  extremity  of  the  body. 
The  anus  is  terminal  in  all  the  known  species,  and  the  ter- 
minal portion  of  the  intestine  has  opening  into  it  the 
branched  respiratory  trees.     The  anterior  end  of  the  body  is 


TYPE  ANNELIDA. 


211 


prolonged  into  a  prostcmium  o£  considerable  size  overlying 
the  mouth ;  it  may  be  short  and  broad  as  in  Echiurv^t  more 
elongated  and  slender  as  in  Thalassema,  or  deeply  bifurcated 
at  the  extremity  as  in  £oneUia. 

A 


Pig.  110.— Bonellia  viridia  A,  Adult  Female  opened  so  as  to  snow  the 
PRINCIPAL  Oroanb  ;  B,  malti  luucli  eularged  in  proportion  to  the  female 
(trom  Hkbtwio). 

c  =  cloaca  m  =  muscles. 

d  —  ludimentary  intestine.  s  =  proboscis. 

g  -  respiratory  trees.  s  (in  Fig.  B)  =  spermatozoa. 

i  =  iutustiue.  vd  =  vas  deferens. 

u  =  single  uephr'dium  which  serves  also  as  the  oviduct. 

The  last-named  genus  is  interesting  as  affording  an  exam- 
ple of  sexual  dimorphism,  the  males  being  small  Turbellarian- 
like  organisms  which  live  parasitically  in  the  anterior  portion 
of  the  digestive  tract  of  the  female,  only  coming  to  the  exterior 
for  the  purpose  of  copulation. 

3.  Order  Sipunculacea. 

The  Sipunculacea,  to  which  the  term  Oephyrea  inermes  is 
also  applied,  is  an  order  including  forms  which  lack  all  traces 


242 


INVEKTEBRATE  MORPHOLOGY. 


of  set».  In  Priapidus  the  iutestiue  is  almost  straight  and  the 
anus  terminal ;  but  in  Sipuncultts  ami  the  allied  geuera,  such  as 
Phascolosoma  and  Fhascolion,  the  digestive  tract  is  convoluted 
and  bent  back  upon  itself,  so  that  the  anus  lies  on  the  dorsal 
surface  near  the  anterior  extremity  of  the  body.  A  "  respira- 
tory tree  "  is  absent  or  rudimentary  as  a  rule  except  in  Pria- 
pubis  and  allied  genera,  and  the  large  prostomial  lobe  char- 
acteristic of  the  Echiurea)  is  lacking.  The  anterior  por- 
tion of  the  body,  however,  is  capable  of  being  invaginated  by 
means  of  stroug  retractor  muscles  (Fig.  109,  dB  and  vli)  into 
the  fore  part  of  the  digestive  tract,  forming  the  so-called  in- 
trovert. The  extremity  of  this  is  provided  with  a  circle  of 
liuger-like  or  branched  tentacles  in  the  centre  of  which  lies 
the  mouth,  and  which  are  supposed  to  have  a  respiratory 
fuuctiou,  being  richly  supplied  with  blood.  In  Priapuhts 
these  are  absent,  but  at  the  posterior  end  of  the  body  there 
is  a  prolongation  which  bears  papilla-like  processes  which 
probably  function  r^a  respiratory  organs. 

Development  and  Affinities  of  the  Gephyrea.~The  early  development 
of  the  Gephyrea  resembles  closely  that  of  the  Polycliaota,  more  especially  in 
the  Echiureae.  In  this  order  a  Trochophore  larva  is  formed  resembling  very 
closely  the  typical  Polygordius  trochophore,  the  similarity  extending  even 
to  a  segmentation  of  the  primitive  mesoderm  bands.  In  later  stages  this 
metamerism  of  the  mesoderm  disnopears,  no  trace«of  it  being  found  in  the 
adult  forms.  In  llie  isipunculacea  the  larva  differs  from  the  Trochophore 
m  lacking  the  typical  prworal  band  of  cilia,  thougli  this  may  be  weakly 
developed  in  some  forms,  such  as  Phascolosoma.  The  postoral  cilia  are,  on 
the  other  hand,  strong.  A  further  difference  is  found  in  the  absence  of 
metamerization  of  the  mesoderm,  which  at  a  very  early  stage  of  develop- 
ment forms  a  layer  lining  the  interior  surface  of  the  body-wall,  and  also 
covering  the  digestive  tract  and  enclosing  a  cuilomic  cavity  continuous 
through  the  entire  body. 

Notwitlistanding  these  important  differences  there  seems  little  room  for 
doubt  but  that  the  Sipuncuhis  larva  has  arisen  as  an  adaptation  of  the 
typical  Annelidan  Trochopliore  still  represented  in  the  development  of  the 
Echiurete.  By  these  forms  a  close  relationship  is  shown  to  the  Polychaeta  ; 
and  the  Gephyrea  are  to  be  regarded  as  Polychaeta  which  have  secondarily 
lost  a  metamerization  originally  present  in  the  adult  ancestors  and  still 
represented  in  the  Echiurus  larva,  but  lost  even  in  the  larval  stages  of  the 
Sipunculacea, 

Since  the  discovery  of  the  larval  forms  of  certain  Echiurid  and  Sipun- 
culid  forms  there  has  been  a  tendency  to  regard  these  two  orders  as  being 


,j,„^4,JU)#«Uiibit»:^ 


TYPE  ANNELIDA. 


243 


much  less  closely  related  than  they  are  here  supposed  to  bo.  The  EchiureoB 
urt'  still  held  to  have  Auuelidan  affinities,  while  the  Sipuuculacea  are  as- 
signed to  tlie  next  type  to  be  described.  This  teudeacy  has  its  origin  in 
tile  attachment  of  too  great  importance  to  the  metamerism  which  is  indi- 
cated in  the  Echiurid  trochophore  but  lacking  in  the  Sipuuculid  larva, 
Tiiero  seems  no  good  ground  for  supposing  that  its  absence  in  the  latter 
group  may  not  be  sufficiently  explained  by  the  assumption  that  it  repre- 
sents the  final  stage  of  the  reduction  of  metamerism  of  which  the  transient 
segmentation  of  the  Echiurid  is  a  stage.  In  their  anatomical  character- 
istics the  adult  forms  of  the  two  groups  are  too  much  alike  to  bo  assigned 
to  different  types  and  the  similarities  of  detail  too  numerous  to  warrant  tiie 
belief  that  they  have  been  independently  acquired.  It  seems  much  more 
probable  that  both  orders  have  descended  from  segmented  ancestors— the 
degeneration,  if  degeneration  it  can  be  called,  having  been  carried  to  a 
greater  extent  in  the  Sipunculacea  than  in  the  Echiurcse,  and  having  in 
consequence  been  thrown  back  upon  the  larval  stages  and  so  obscuring  the 
developmental  evidences  of  the  phylogeny. 

A  connecting  link  between  the  Echiurew  and  the  PolychsBta  has  been 
traced  by  some  authors  in  the  genus  Stemaspis,  at  one  time  associated  with 
the  Gephyrea  but  now  universally  assigned  to  the  Polychaeta.  In  this 
genus  the  metamerization,  though  to  a  certain  extent  reduced,  is  still  pro- 
nounced, 8.  arcuata  consisting  of  from  twenty  to  twenty-two  metameres, 
of  which  the  anterior  seven,  together  with  the  head-lobes,  may  be  invagi- 
nated— the  introvert  of  the  Sipunculacea  being  thus  recalled.  On  the  ven- 
tral surface  near  the  posterior  extremity  of  the  body  are  two  shield  like 
plates  armed  with  setae,  and  at  the  posterior  extremity,  as  in  Priapulus, 
are  a  number  of  filamentous  appendages  which  are  regarded  as  branchiae. 
Setae  are  present  on  all  the  metameres  except  the  fifth,  sixth,  and  seventh  ; 
those  of  the  eighth  to  the  sixteenth  metameres  being,  however,  concealed 
beneath  the  hypodermis.  The  digestive  tract  is  somewhat  convoluted,  but 
opens  terminally  ;  the  ventral  nerve-cord  shows  traces  of  ganglionic  swell- 
ings, and  at  the  posterior  end  of  the  body  possesses  a  marked  enlargement ; 
and  only  two  nephridia  are  present.  The  musculature  and  the  vascular 
system  resemble  those  of  the  Polychaeta  rather  than  those  of  the  Gephyrea, 
while  the  reproductive  organs  are  peculiar  in  possessing  special  ducts, 
which,  it  has  been  held,  show  no  indications  of  being  modified  nephridia. 

In  many  respects,,  accordingly,  Steniaspis  does  hold  a  position  interme- 
diate between  the  Echiureae  and  the  Polychaeta,  and  it  seems  not  improb- 
able that  it  may  represent  an  offshoot  from  near  the  base  of  the  line  along 
which  the  Gephyrea  have  been  differentiated.  Whether  this  be  the  case  or 
not,  it  is  exceedingly  probable  that  the  Gephyrea  have  been  derived  from 
the  Polychaeta,  the  Echinrea'  preserving  more  numerous  traces  of  their  an- 
cestry than  do  the  SipimculacesB. 


244 


INVERTEBRATE  MORPUOLOOT. 


IV.  Class  Myzostomew. 

The  Myzostometo  constitute  a  group  of  Annelids  which  pre- 
sent but  few  traces  of  a  typical  metameric  form,  being  much 
nioditied  by  their  parasitic  habit.  All  the  known  forms  are 
parasitic  upon  Orinoids,  some  producing  malformations  of  the 
pinnules  of  their  host  in  the  form  of  cysts  in  the  interior  of 
which  they  live.  The  body  of  Myzostomum  (Fig.  Ill)  is 
flattened  and  oval,  a  number  of  flnger-like  processes  or  cirri 
((')  projecting  around  the  margin.  There  is  no  trace  of  external 
segmentation,  although  five  pairs  of  parapodia  (  />),  each  with 
an  axial  supporting  chitinous  rod  and  a  single  hooked  seta, 
occur  on  the  ventral  surface.  On  the  same  surface  too,  near 
the  margin,  are  to  be  found  in  most  species  three  or  four 
sucker-like  depre^tsions  {au)  on  each  side,  which  have  been 
supposed  to  represent  highly-modified  nephridia. 

The  body  is  covered  by  a  thick  cuticle  beneath  which  lie 
the  hypodermis  and  the  musculature  of  the  body-wall,  which 
has  the  characteristic  Annelidan  arrangement.  A  body-cavity 
can  hardly  bo  said  to  exist  (unless  it  be  indicated  by  the  space 
occupied  by  the  ova),  the  interior  of  the  body  being  completely 
filled  up  by  the  internal  organs  and  by  numerous  muscle- 
bands  passing  both  dorso-ventrally  and  from  side  to  side, 
these  latter  in  some  forms  being  arranged  in  such  a  way  as 
to  represent  incomplete  dissepiments.  There  is  no  blood 
vascular  system. 

The  mouth  is  situated  near  the  anterior  end  of  the  body  on 
the  ventral  surface  and  opens  into  the  proboscis-sheath, 
within  which  lies  the  proboscis  {ph\  constructed  upon  Mie 
same  plan  as  that  of  the  Rhynchobdellid  Hirudinea.  Around 
the  extremity  of  the  proboscis  are  arranged  a  number  o^ 
short  tentacles,  and  its  walls  are  very  muscular ;  beli  i  1  u 
opens  through  a  short  oesophagus  into  the  wide  inte  ^) 

from  which  three  (or  two)  branched  pouches  project  o.      i  ther 
side  towards  the  margin  of  the  body.     The  short  and  reluLi .  el} 
narrow  rectum  (r)  opens  near  the  posterior  end  of  the  body, 
uniting  shortly  before  its  termination  with  the  oviduct. 

The  nervous  system  consists  of  a  circumoesophageal  com- 
missural ring  upon  which  lie  numerous  scattered  ganglion  cells 


I 


TYPE  ANNELIDA. 


245 


likewise  surroniidiug  the  cosopliagus  and  apparently  represent- 
ing the  supratesophageal  ganglion.  Numerous  longitudinal 
nerves  pass  forward  from  the  ring  to  unite  with  another  ring 
around  the  base  of  the  proboscis  from  which  nerves  pafis  to 
the  tentacles.  Below  the  intestine  lies  a  large  ganglionic  mass 
with  which  the  circuuKKSophageal  commissures  unite  and 
which  gives  off  a  number  of  peripheral  nerves.     This  mass  is 


mo 


ov 


Fig.  in.  —  MyzoHtomum  (after  von  Ohaff). 

r  =  rectum. 
s  =  stomach. 
8U  =  sucker. 
t  =  testes. 
u  =  uterus. 


e  =  cirrus. 
elo  =  cloacal  opening. 
fo  =  opening  of  uterus  into  clo>.''n. 

0  =  opening  of  male  reproductive  organs, 

p  =  purapodiuni. 


ph  =  proboscis. 

composed  of  several  (probably  6)  united  gaug!!*«.  and  represents 
the  ventral  nerve-cord  of  other  Annelids.  Nerves  p^.ss  pre- 
sumably from  the  supraoesophageal  ganglion-cells  Mong  the 
dorsal  wall  of  the  intestine  and  seem  to  constitute  a  sympa- 
thetic system.  The  only  structures  which  can  be  considered 
sense-organs  are  the  marginal  cirri  and  the  tentacles  of  the 
proboscis,  which  probably  have  &,  tactile  function.  No  traces 
of  eyes  have  yet  been  observed. 

Nephridia,  unless  they  be  represented  by  the  sucker-like 
depressions  and  the  oviducts,  are  wanting.    The  MyzostomesB 


246 


INVKltTEDllATE  MORPIIOLOQY. 


are  as  a  rule  lierraaplirodite.  It  seems  doubtful  if  the  ovaries 
Lave  actually  been  made  out,  the  large  masses  of  ova  {pv) 
lyiug  between  the  branches  of  tlie  intestinal  ])ouches,  which 
have  been  considered  ovaries,  being  more  probably  original 
ccelomic  spaces  which  have  become  tilled  with  ova  set  free 
from  the  ovaries;  while  the  so-called  uterus  («/),  lying  im- 
mediately above  the  intestine,  and  which  in  mature  animals 
is  closely  packed  with  ova,  is  probably'  of  the  same  nature. 
Three  oviducts,  one  dors.il  and  two  lateral,  pass  from  the  uterus 
to  open  ifo)  into  the  rectum  near  its  termination,  though  the 
dorsal  one  in  some  forms  may  open  directly  to  the  exterior 
near  the  anus. 


If  the  uterus  is  correctly  identified  as  a  coolomic  space,  then  it  seems 
liot  improbable  that  tlio  oviducts  may  represent  modified  nepbridia. 
Tiieir  opening  into  tlie  rectum  is  a  secondary  condition  and  does  not 
necessarily  stand  in  opposition  to  their  uephridial  character,  since  practi- 
cally the  same  conditions  obtain  in  some  Kotifera. 

The  testes  {t)  are  branched  organs  lying  for  the  most  part 
between  the  intestine  and  the  nervous  system,  though  isolated 
masses  occur  in  some  forms  near  the  margin  of  the  body.  On 
each  side  two  vasa  deferentia,  one  anterior  and  one  posterior, 
convey  the  spermatozoa  to  a  muscular  sperm-vesicle  opening 
to  the  exterior  at  the  margin  nearly  opposite  the  centre  of 
the  body  {mo). 

In  some  species,  notwithstanding  their  hermaphroditism,  "  comple- 
menlal  males,"  small  individuals  which  possess  ripe  spermatozoa  wiiile 
lacking  ova,  have  been  described  as  ojcurring.  Further  observations  have 
not,  however,  tended  to  eontiriii  this  idea  in  its  original  sense,  since  tiiese 
small  individuals  have  i)een  found  to  be,  like  the  larger  ones,  hermaphro- 
<lites,  being  secondary  adaptations  from  the  prevailing  hermaphroditic 
condition,  and  not  having,  therefore,  the  same  sign iticanee  jus  the  "eom- 
plemental  males"  of  the  Cirrhipedia  ((j.  v). 

Then^  can  be  lit  tie  room  for  doubt  but  that  the  MyzostomoiU  are  Annelida 
degenerated  by  parasitism,  and  that  they  are  most  closely  related  to  the 
Polyclueta.  It  is  interesting  to  note  in  this  connection  the  etTect  their 
])<irasitic  and  sessile  mode  of  life  under  ecpmble  external  conditions  has  biul 
in  producing  indications  of  a  radial  symmetry. 


^,«^,iJto»*!tSt**^ '•*''•' 


TYrK  ANNELIDA. 


247 


APPENDIX  TU  THE  TYPE  ANNELIDA. 

Class   Fhobonioje. 

The  class  Phoroiiidio  iuclmles  ji  siiij^le  geuns,  l^horojiis,  of 
which  but  a  few  species  are  kuowu.  Tlioy  are  all  marine 
forms  of  comparatively  small  size,  reachiiij^  iu  some  cases  a 
leutith  of  50  mm.  Each  individual  is  contained  within  a 
chitiuous  tube  to  which  particles  of  sand  are  in  some  cases 
agj^hitiuated,  and  is  worm-like  and  cylindrical  in  form,  the 
anterior  xtremity  of  the  boily  beinjj;  provided  with  a  horse- 
shoe-shaped fold,  termed  the  lophophore  (Fig.  112,  <0,  bearing 
a  number  of  tentacles  arranged  around  its  margins.  Between 
the  two  circles  of  tentacles  is  situated  the  moutli  (/>),  over 
which  hangs  a  fold  known  as  the  ephsfonw,  representing  the 
prostomium  or  pneoral  lobe  of  the  larva.  Outsitle  the  area 
enclosed  by  the  tentacles  is  the  anus,  on  either  side  of  which 
a  pore,  tlie  opening  of  a  nephridiuni,  is  found. 

The  ectoderm  of  the  body-wall  is  separated  by  a  distinct 
basement-membrane  from  a  layer  of  circular  muscles,  within 
which  is  a  second  layer  of  longitudinal  muscles  (/) — an  arrange- 
ment resembling  that  founil  in  the  body-wall  of  the  Annelids, 
luternally  the  longitudinal  muscle-layer  is  lined  by  a  layer 
of  peritoneal  cells  enclosing  a  spacious  ctelom.  Near  the 
anterior  end  of  the  bod}'  theio  is  a  transverse  septum  sepa- 
rating oil",  more  or  less  i)erfectly,  an  anterior  chamber,  with 
which  the  cavit}'  of  the  epistome  and  of  the  lophophore  com- 
municates, from  a  larger  posterior  chamber  iu  which  lie  the 
intestine  and  reproductive  oigans,  and  which  is  divided  lon- 
gitudinally by  three  mesenteries  extending  from  the  intestine 
to  the  body-wall.  One  of  these  mesenteries  accompanies  the 
intestine  throughout  its  entire  extent,  while  the  other  two 
lateral  mesenteries  are  iu  connection  ouly  with  the  sides  of 
the  desceudiug  limb  of  the  intestine. 

The  teutacles  are  processes  of  the  body-wall,  with  a 
ciliated  ectoderm,  and  contain  a  chitinous  axial  supporting 
tissue. 

A  completely  closed  blood  vascular  system  is  preseut, 
consisting  below  the  transverse  se]>tunj  of  two  longitudinal 


248 


mVERTEBHATE  MOJiPUOLOOT. 


vessels  {h  and  /)-  One  of  these  (/)  divides  (g)  near  the  an- 
terior extremity  of  the  body,  the  two  branches  passing  into  a 
circular  vessel  lying  at  the  bases  of  the  tentacles  and  sending 

branches  up  into  them.  The 
vessels  which  return  the 
blood  from  the  tentacles  open 
into  a  second  ring  external 
to  the  first,  and  from  it  two 
vessels  pass  backwards  and 
unite  to  form  the  second 
longitudinal  triink  from  which 
numerous  csecal  pouches 
arise.  All  the  vessels  have 
contractile  walls,  and  the 
blood  which  they  contain 
possesses  nucleated  red  cor- 
puscles. 

The  digestive  tract  is  bent 
Pig.    113.— Lateral  view  of    An-  upon    itself    {d    and    e),  the 
TBitiuR  Region  of  Plunoim  (after  mouth  and  anus,  as  already 

BouBNK).  described,  lying  in  close  prox- 

a  =  lopbopborc. 

b  =  mouth,  surrounded  by  epitome. 
e  =  lopbopboral  disk. 
d  =  oeaopbagus. 
r  =  inlt'siine. 
J  -  vt'utral  blood-vessel. 
g  =  bmuch  off. 
h  =  doi-siil  blood-vessel. 


■1 
? 

■  'h- 

I 


imity  at  the  anterior  extrem- 
ity of  the  body.  Several 
regions,  such  as  oesophagus, 
first  stomach,  second  stom- 
ach, and  intestine,  are  to  be 
distinguished,  and  along  one 
i  =  longitudinal  muscle  of  body-wall,  surface  of  the  oesophagus  and 
k  =  iuteiteutaculur  membrane.  ^j,^^^     stomach    runs     in     P. 

arcMtecta  a  ridge,  becoming  a  groove  in  the  stomach  region, 
of  ciliated  gland-cells,  which  recalls  the  accessory  intestine 
of  the  GephjTea.     There  are  no  special  digestive  glands. 

The  nervous  system  is  completely  imbedded  in  the  ecto- 
derm. It  consists  of  a  nerve-ring,  following  the  outline  of 
the  lophophore  at  the  bases  of  the  tentacles  and  surround- 
ing, therefore,  the  mouth  but  not  the  anus.  From  it  a  nerve 
runs  backward  asymmetrically  upon  one  side  of  the  body 
to  near  the  posterior  extremity.  It  contains  a  large  cle.wr 
rod-like  structure  which  seems  to  be  a  colossal  nerve-fibre. 


1 


I 

•IS 


m 


^;,it.*  .«*  *  • 


TYPE  ANNELIDA. 


249 


tto- 
of 

a- 

rve 
dy 

re. 


The  only  sense-organs  which  have  been  described  are  a  pair 
of  ciliated  depressions  lying  one  on  each  side  in  the  concavity 
of  the  lophophore  ;  no  definite  statement  can  be  made  as  to 
their  function. 

A  single  pair  of  nephridia  is  present,  opening  into  the 
posterior  chamber  of  the  coelom  by  funnel-like  mouths,  and  to 
the  exterior  on  each  side  of  the  anus.  They  serve  not  onl}' 
for  excretion,  but  also  as  ducts  for  the  reproductive  elements. 
The  various  species  of  Phoronis,  with  the  possible  exception 
of  P.  architeda,  are  hermaphrodite,  the  ova  and  spermatozoa 
developing  from  cells  of  the  peritoneum  lying  in  the  vicinity 
of  the  pouched  longitudinal  blood-vessel.  They  are  shed 
from  their  place  of  formation  into  the  coelomic  cavity  and 
thence  pass  to  the  exterior  through  the  nephridia. 


Fio.  118. — Mbtamorphobih  of  Actinotrocha  (after  Mrtscrnikopp  from  BALrocR). 

in  =  itivagiiintion. 

Development  of  the  Phoronidce. — In  their  development  the 
various  species  of  Phoronis  so  far  as  known  undergo  a  very 
remarkable  metamorphosis.  The  larva  which  develops  from 
the  ovum  is  known  as  Actinotrocha  (Fig.  113,  ^)  and  is  a  some- 
what elongated  structure  possessing  at  the  anterior  end  a 
large  hood  which  overhangs  the  mouth,  its  edge  bearing 


250 


INVERTEBRATE  MORPHOLOGY. 


stroug  cilia.  Behind  the  mouth  are  a  number  of  ciliated 
tentacle-like  processes  arranged  in  a  horseshoe-shaped 
curve,  their  cilia,  together  with  those  of  the  edge  of  the 
prostomial  hood,  forming  a  band  encircling  the  mouth.  The 
digestive  tract  opens  to  the  exterior  at  the  posterior  ex- 
tremity of  the  body,  and  the  axis  of  the  body  is  the  ax.s 
passing  through  the  anus  and  the  centre  of  the  prostomial 
lobe.  A  little  later  {B)  an  invagination  {in)  of  the  body-wall 
into  the  coelom  of  the  larva  develops  on  the  ventral  surface 
behind  the  band  of  ciliated  processes  and  becomes  of  a  con- 
siderable size.  At  the  time  of  the  metamorphosis  this  in- 
vagination is  suddenly  everted  (Fig.  113,  G  and  D),  the  intes- 
tine being  carried  with  it  as  a  loop,  and  entirely  new  axial 
relations  are  thus  brought  about.  The  long  axis  of  the  body 
is  now  (D)  almost  at  right  angles  to  what  it  was  in  the 
Actinotrocha,  and  since  the  invagination  originally  formed 
on  the  ventral  surface  of  the  larva,  the  body  of  the  adult 
Phoronis  must  be  regarded  as  formed  by  an  excessive  de. 
velopment  of  the  ventral  surface,  the  dorsal  surface  being 
represented  only  by  the  short  interval  between  the  mouth,  or 
rather  the  epistome,  and  the  anus.  The  epistome  represents 
the  prostomial  lobe  of  the  larva,  and  the  ciliated  processes 
represent  the  lophophoric  region,  though  they  themselves  are 
afterwards  replaced  by  the  permanent  tentacles. 

Tliere  can  of  course  be  no  question  but  that  this  remarkable  metamor- 
phosis is  a  secondary  phcnonionon,  and  it  seems  prol)able  that  its  acquisi- 
tion stands  in  relation  to  tiie  tubicolous  habits  of  the  adult  which  neces- 
sitate tlie  change  of  the  principal  axis  of  the  body.  The  metamorphosis  is 
the  means  of  avoiding  a  slow  and  tedious  change  necessitated  by  the  differ- 
ent habits  of  the  larva  and  the  adult,  just  as  the  occurrence  of  the  chry- 
salis stage  in  the  development  of  the  butterfly  is  required  on  account  of 
the  great  differences  between  the  mouth-parts  of  the  larval  catcniillar  and 
the  adult  butterfly. 

The  affinities  of  Phoronis  cannot  be  considered  to  bo  finally  settled  as 
yet,  though  there  has  been  a  tendency  of  late  years  to  associate  them  with 
the  Polyzoa.  They  also  seem  to  show  affinities  to  the  Gephyroa,  and  by 
some  authors  are  considered  nore  correctly  referable  to  that  group.  The 
tendency  to  develop  the  ventral  surface  of  the  body  at  the  expeuse  of  the 
dorsal  and  so  to  form  a  new  body-axis  is  seen  in  Sipunculus  and  carried 
to  its  culmination  in  Phoronis,  and  further  similarities  between  the  two 
forms  are  to  be  found  in  the  character  of  the  nepbridia  and  in  the  occur- 


Hi 


TYPE  ANNELIDA. 


251 


rence  of  a  closed  blood  vascular  system.  The  lophophore  of  Phoronis,  and 
the  epistome,  are  on  the  other  hand  characteristic  Polyzoan  features,  and  it 
seems  not  at  all  improbable  that  Phoronis  occupies  an  intermediate  posi- 
tion between  the  Gephyrea  and  the  Polyzoa.  There  is  this  at  all  events  to 
be  noted  concerning  the  Prosopygia  (see  following  chapter),  and  that  is 
that  they  are  certainly  closely  related  to  the  Annelida.  If  the  supposition 
advanced  on  p.  243  to  the  effect  that  the  Sipunculacea  are  to  be  regarded 
as  Annelida  which  have  secondarily  lost  their  metamerism  be  correct,  and 
it  Phoronis  really  indicates  a  derivation  of  the  Prosopygia  from  Gephyrean- 
like  ancestors,  then  the  Prosopygia  too  must  be  regarded  as  Annelida  in 
which  all  traces  of  metamerism  have  been  lost.  This  view  seems  preferable 
to  that  which  would  refer  the  Polyzoa,  for  instance,  back  to  unsegmented 
ancestors— back,  that  is  to  say,  to  the  non-segmented  ancestors  of  the 
Annelida. 


SUBKINGDOM  METAZOA. 


by 


TYPE  ANNELIDA. 

I.  Class  CHiETOPODA.  —  Metamerism  usually  well  marked  ;  with  dorsal 
and  ventral  rows  of  setai  along  the  sides  of  the  body. 
I.  Subclass  PoLYCH^TA. — Marine  forms  ;  with  the  seta?  usually  borne 
upon  lateral  lobes  of  the  l)ody  (parapodia). 

1.  Order  Arcfiiannelida. — Without  parapodia.    Pjli/gordins. 

2.  Order  Errantia. — Elongated  swininiing  or  creeping  forms; 

metauieres  more  or  less  similar.  Nereis,  Lepidonotus, 
Diopatra,  Autolytus,  Hesione,  Syllis,  Alciope,  Capitella, 
Polyophthalmus,  Arenicola,  Arieia. 
8.  Order  iSedentaria. — Usually  tubicolous  ;  anterior  motameres 
more  or  less  different  from  the  rest.  Aviphitrife,  Herpula, 
Sabella,  Terebella. 
II.  Subclass  Olkioch.eta. — Aquatic  or  terrestrial  forms;  with  setae, 
but  without  parapodia. 

1.  Order  Naidomnrpha.  —  For  the  most    part    aquatic  ;    fre- 

quently reproducing  non-sexually;  nephridia  serve  as  re- 
productive ducts.  Nais,  Dero,  Chcetobranchus,  ^olo- 
soma,  Chmtogaster,  Ctenodrilus,  Tubifex, 

2.  Order  Lnmbricomorpha.  —  For  the  most    part  terrestrial ; 

not  reproducing  non  sexually  ;  special  reproductive  ducts. 
Ltimbrians,  Perichxtta,  Annrlurta. 
II.  Class  HiRUDiNEA. — Metanieristn   well   marked  ;    without  set!©  ;   with 

anterior  and  posterior  suckers. 
1.  Order  Onathobdellidir. — Mouth  with  three  more  or  less  well 
developed  teeth  ;  pharynx  not  protrusible.    Hirudo,  Ma- 
crobdella,  Nephelis. 


252 


INVERTEBRATE  MORPHOLOGY. 


2.  Order  RhynchohdelUdce.—yi iihoni  teeth  and  with  protrusible 
pharynx.     Clepsine,  Pontobdella,  IHseicola,  BranchelUon. 

III.  Class  Gepuyrea. — Metamerism  indistinct ;  without  parapodia. 

1.  Order  Echiurece. — With  setae.    Echiurus,  ITuUassemaj  Bo- 

nellia. 

2.  Order  SipunculacecE. — Without  setSB.    8ipunculus,  Priapu- 

lus,  Pfiaseolosomay  Phascolion. 

IV.  Class  Myzostome^.— Parasitic  on  Criuoids ;  approximating  a  radial 

symmetry.    Myzostoma. 

APPENDIX. 

Class  PHORONm.£. — Without  metamerism  ;  tubicolous  ;  with  lophophore. 
Phoronis. 

LITERATURE. 

CH^TOPODA. 

A.  de  llnatrefagea.     Histoire  naturelle  de»  Anneles  marina  et  (Teau  douce. 

Paris,  1865. 
E.  Ehlers.    Die  Borstenumrmer.    Leipzig,  1864-68. 
E.  Meyer.    Studien  Uber  den  Korperbau  der  Anneliden.    Mittb.  a.  d.  zoolog. 

Station  zu  Neapel,  vii,  1887  ;  viii.  1888. 
H.  Eiiig.     Die  Capitelliden  dea  Oolfea  von  Neapel.    Fauna  und  Flora  des  Golfes 

von  Neapel.     Monogr.,  xvi,  1887. 
J.  Fraipont.    Le  genre  Polygordiue.    Fauna  und  Flora  des  Qolfes  von  Neapel. 

Monogr.,  xvi,  1887. 

E.  Claparede.    Reclierches  anatomiquee  aur  lee  OligoeMte».     Qenfeve,  1862. 

F.  yejdovsky.     Monographie  der  Enchytrmden.    Prag,  1879. 
System  und  Morphologic  der  Oligoehoeten.     Prag,  1884. 

Numerous  papers  on  Oligochseta  by  G.  E.  Beddard  and  W.  B.  Benham  in 
Quarterly  Joum.  Microscop.  Science. 

HIRUDINEA. 

A.  Xoqoin-Tandon.    Monographie  de  lafamUle  de  Hirudineea.    Paris,  1846. 
C.  0.  Whitman.    The  Leeches  of  Japan.  Quarterly  Journ.  of  Microscop.  Science, 

XXVI.  1886. 
The  Metamerism  of  Clepsine.    Festschr.  zum  siebenzigsten  Geburtstage 

Rudolf  Leuckarts.     Leipzig,  1892. 

A.  0.  Bourne.    Contributions  to  tlie  Anatomy  of  the  Hirudinea.    Quarterly 

Journ.  of  Microscop.  Science,  xxiv,  1884. 

GEPHYREA. 

B.  Oreef.    Die  Echiuren  {Oephyrea  armata).   Nova  Acta  Leopol.  Carol.  Akad. , 

XLi,  1879. 
E.  Selenka.     Die  Sipuneuliden.     Wiesbaden,  1888. 
E.  A.  Andrews.    Notes  on   the  Anatomy  of  Sipunculua  Oouldii,  Pourtalee. 

Studies  from  the  Biolog.  Labor.  Johns  Hopkins  Univ.,  iv,  1890. 


A 


"  1 


^ 


M 


TYPE  ANNELIDA. 


253 


MYZ08T0MIDA, 

L.  von  Oraflf.    Das  Genua  Myzostoma.     Leipzig,  1877. 

Report  on  the  Myzostomida.     Scient.  Results  of  the  Voyage  of  H  M,S 

Challenger.     Zool,  x,  1884. 
J.  Beard.    On  the  ^Life-history  and  Development  of  tfie  Genua  Myzoatoma. 

Mittb.  a.  d.  zool.  Station  Neapel,  v,  1884. 

PHOBONISA. 

E.  B.  WiUon.    The  Origin  and  Significance  of  the  Metamorphosis  of  Aetinotrocha. 

Quarterly  Journ,  Microscop.  Science,  XXI,  1881. 
C.  J.  Cori.     Unterauchungen  iiber  die  Anatomie  und  ffistologie  der  Oattung 

Phoronia.    Zeitscbr.  fttr  wissensch.  Zoologie,  Li,  1891. 


254 


INVEHTEBUATE  MOUi'HOLOQY, 


\\ 


CHAPTER  XI. 


TYPE  PKOSOPYGIA. 


i 


1 


The  members  of  the  type  Piosopygia  are  compact,  soli- 
tary, or  colouial  organisms  destitute  of  a  true  metamerism 
uud  iiaviug  the  digestive  tract  usually  beut  upon  itself,  so 
that  the  auus  lies  iu  more  or  less  close  proximity  to  the 
mouth  and  therefore  near  the  anterior  end  of  the  body.  A 
chitinous  or  more  or  less  calcareous  investment  is  formed 
about  the  exterior  of  the  body,  and  in  some  cases  assumes 
the  form  of  a  calcareous  bivalve  shell,  similar  to  that  of  the 
Pelecypoda  in  its  general  appearance,  although  in  the  rela- 
tions of  the  valves  to  the  body-surfaces  and  iu  other  particu- 
lars (see  p.  327)  there  are  very  decided  differences,  the  simi- 
larity being  simply  an  analogy. 

A  more  characteristic  feature,  however,  is  the  presence  at 
the  anterior  end  of  the  body  of  a  circular  or  horseshoe-shaped 
fold,  or  else  of  two  armlike  lateral  processes,  forming  what 
is  termed  the  lophophore,  upon  which  are  borne  a  number  of 
tentacles  which  play  important  rules  not  only  in  obtaining 
food,  but  also  in  the  process  of  respiration,  no  branchial  or 
other  special  respiratory  organs  being  present. 

A  more  or  less  spacious  coelom  is  usually  present,  trav- 
ersed by  muscle-libres  and  some  specially  developed  muscle- 
bauds,  though  the  muscular  system  is  on  the  whole  poorly 
developed.  The  coelom  contains  a  ha^molymph,  but  a  sepa- 
rate blood  vascular  system  and  heart  is  entirely  wanting. 
The  nervous  system,  in  accordance  with  the  absence  of  met- 
amerism, is  exceedingly  simple,  consisting  either  of  a  single 
ganglion,  lying  between  the  mouth  and  anus  and  sending  «  ff 
nerves  to  the  various  regions  of  the  body,  or  else  of  a  nerve- 
ring  surrounding  the  cesophagus,  with  more  or  less  distinct 
supra- and  sub-cesnphageal  ganglionic  enlargements.  Special 
sense-organs  are  wanting. 


,-^^^^,,,u^4a«^'*-'^ 


TYPE  PliOSOPrOJ.L 


256 


A  pair  of  simply-constructed  uephridia  are  present  in 
some  forms,  but  in  many  a  special  excretory  organ  seems  to 
be  entirely  wanting.  Bisexuality  is  the  usual  arrangement, 
altliougli  in  the  Polyzoa  hermaphroditism  is  of  not  unfre- 
quent  occurrence. 

The  great  majority  of  the  Prosopygia  are  marine  in  habi- 
tat, tho  igh  a  number  of  Polyzoa  are  inhabitants  of  fresh 
water.  The  type  may  be  divided  into  two  well-marked 
classes,  the  Polyzoa  and  the  Brachiopoda. 


Ive- 

[iCt 

lial 


I.  Class  Eoltzoa. 

The  Polyzoa,  a  group  usually  spoken  of  by  German  zool- 
ogists as  the  Bryozoa,  are  almost  without  exception  colonial 
organisms,  forming  encrusting,  massive,  or  more  or  less  den- 
dritic masses  composed  of  a  large  number  of  small  individuals 
or  polypides,  each  of  which  is  enclosed  within  a  chitinous  or 
in  some  cases  partially  calcified  investment,  the  zocecium,  from 
the  mouth  of  which  the  anterior  portion  of  the  body  bearing 
the  lophophore  may  be  protruded.  This  outer  investment  or 
ectocyst  (Figs.  115  and  116,  ec)  is  lined  upon  its  interior  sur- 
face by  a  layer  of  ectoderm-cells,  within  which  is  a  delicate 
peritoneal  lining,  these  two  layers  together  constituting  the 
true  body-wall  or  endocyst  (Fig.  115,  en)  practically  destitute 
of  muscle-tissue,  though  a  sphincter  is  usually  present  at 
the  mouth  of  the  cup,  which  may  thus  be  closed  over  the  re- 
tracted polypide. 

A  more  or  less  spacious  coelom  (Fig.  115,  co)  is  present  in 
the  majority  of  forms,  containing  a  hmraolymph  and  tra- 
versed by  a  number  of  muscle-strands  (m)  which  may  1  ; 
aggregated  into  special  retractor  bands  ;  but  in  one  ord  le 
Endoprocta  (Fig.  114),  these  are  wanting  and  in^  cw  the 
ccelom  is  reduced  to  a  very  small  space  between  the  body- 
wall  and  the  digestive  tract.  This  latter  structure  has  the 
characteristic  U-  or  Y-shaped  form  and  presents  but  little 
differentiation  into  special  parts,  though  an  cesophagus, 
stomach,  lined  with  glandular  so-called  liver-cells,  and  rectum 
may  be  distinguished.  An  anus  (Figs.  115  and  116,  a)  is 
always  present  and  may  be  situated  either  within  or  without 


i\ 


206 


INVEliTEBHATE  MOliPUOLOGY. 


the  urea  ouclosed  by  the  lopliophore.  The  nervous  system 
is  exceediuf^ly  simple,  cousistiug  of  a  siugle  gaugliouic  mass 
(Figs.  114  and  115,  ce)  lyiug  betweeu  the  mouth  aud  auus, 
nerves  ramifying  from  it  to  the  various  parts  of  the  body. 
The  only  sense-organs  that  have  been  detected  are  situated 
upon  the  free  portions  of  the  body,  more  especially  on  the 
lophophoral  tentacles  {t),  aud  are  represented  by  scattered 
ectodermal  cells  each  of  which  bears  a  strong  cilium  aud  is 
in  connection  with  a  nerve-iibre ;  they  have  been  assigned  a 
tactile  function,  though  it  seems  probable  that  they  react  to 
stimuli  of  various  kinds  and  have  a  much  more  generalized 
function. 

The  arrangement  of  the  excretory  and  reprodnctive  organs 
varies  considerably  in  different  forms  and  may  be  more  con- 
veniently  described  in  connection  with  the  various  orders. 

1.  Order  Endoprocta. 

This  order  contains  but  a  small  number  of  forms,  which, 
with  one  exception,  Urnatella,  are  marine  in  habitat.     They 

all  possess  the  power  of  reproducing 
by  budding,  colonies  being  thus  formed, 
as  in  Pedicellinay  Ascopodaria,  and  other 
genera  with  the  exception  of  Loxosoma, 
in  which  the  buds  separate  completely 
from  the  parent  at  a  relatively  early 
stage  of  their  development.  Each  in- 
dividual (Fig.  114)  is  a  cup-shaped 
structure,  prolonged  posteriorly  into 
a  stalk  (d)  and  upon  the  rim  of  the 
cup,  which  represents  the  lophophoral 
fold,  or  slightly  below  it  on  the  inner 
Fig.  lU.-SiNGLK  iNm-  surface,  the  tentacles  (<)  are  arranged 
viDUAL   OF   PedieeUina   in  a  circle  surrounding  a  depression, 

the  vestibule,  into  which  open  both 
the  mouth  and  the  anus,  the  situation 
of  the  latter  opening  within  the  circle 
of  tentacles  having  suggested  the  name 
given  to  the  order.  The  tentacles  can 
be   coiled  in   a  circinate    manner,   so  as  to   lie   completely 


/t  — 


(after  Nitsche). 
ee  =  ganglion. 

I  =  so-ciilled  liver. 

»  =  stomach. 
tt  =  stalk. 

t  =  tentacles. 


TYPE  PROSOPYGIA. 


257 


Ji* 


within  the  vestibule,  and  the  rim  of  the  cup  can  be  closed 
over  them,  owing  to  the  presence  in  it  of  a  circular  baud  of 
muscle-fibres. 

The  entire  body  is  covered  by  a  delicate  cuticular  ectocyst 
similar  to  the  cuticle  of  the  Annelids,  below  which  is  the 
ectoderm  containing  numerous  gland-cells,  as  well  as  scat- 
tered sensory  hair-beariug  cells  which,  however,  have  not  been 
found  to  exist  in  some  genera  {Ascopodariu).  Scattered  mus- 
cular fibres  occur  in  the  body-wall,  but  they  do  not  as  a  rule 
reach  an  extensive  development. 

The  coelom  is  of  very  slight  extent  ind  in  Loxosoma  is  re- 
placed by  a  gelatinous  matrix  enclosing  branching  cells  and 
muscle-fibres  and  recalling  the  parenchyma  of  the  Turbel- 
laria.  Imbedded  in  this  parenchyma  is  the  U-shaped  diges- 
tive tract,  beginning  with  the  mouth  situated  in  the  vestibule 
and  overhung  by  a  well-marked  epistome.  The  mouth  leads 
into  a  narrow  oesophagus  lined  by  ciliated  columnar  cells, 
and  opening  below  into  a  saclike  stomach  (Fig.  114,  a) 
which  forms  the  lower  transverse  portion  of  the  U.  The 
cells  of  its  anterior  (vestibular)  wall  if)  are  large  and  destitute 
of  cilia,  and  contain  numerous  granules,  on  which  account 
they  have  been  termed  "liver-cells."  The  intestine  forms 
the  ascending  limb  of  the  U,  and  like  the  oesophagus  is  cili- 
ated, opening  into  the  vestibule  at  the  summit  uf  a  well- 
marked  papilla. 

The  nerve-ganglion  lies  below  the  floor  of  the  vestibule 
between  the  epistome  and  the  anal  papilla  and  is  a  single 
dumb-bell-shaped  structure  from  which  from  one  to  three 
nerves  arise  on  each  side,  branching  to  be  distributed  to  the 
tentacles  and  muscles  of  the  body. 

A  single  pair  of  nephridia  occurs,  opening  into  the  vesti- 
bule, and  each  is  composed  of  a  number  of  perforated  cells, 
the  lumen  being  ciliated.  It  is  doubtful  whether  a  fiame-cell 
occurs  at  the  inner  extremity  as  in  the  Annelid  head-kidneys, 
which  otherwise  they  resemble.  Most  of  the  Eudoprocta  seem 
to  be  bisexual,  though  Pedicdlina  is  perhaps  hermaphrodite. 
The  reproductive  organs  arise  from  the  mesoderm  of  the  body- 
wall,  forming  masses  projecting  into  the  parenchyma,  and  are 
provided  with  special  ducts  which  either  remain  independent 


258 


IN  VERTEBRA  TE  MORl'UOLOQ  Y, 


of  each  other  (Pedicdlina),  or  unite  together  to  form  a  single 
tube  and  open  into  the  vestibulu,  between  the  epistome  and 
the  ganglion. 


2.  Order  Eotoprocta. 

The  order  Ectoprocta  includes  the  great  majority  of  forms 
which  are  referable  to  tlie  class  Polyzoa.  They  are  without 
exception  colonial  forms  of  small  size  in  which  the  tentacles 
are  arranged  either  in  a  circle  or  in  the  form  of  a  horseshoe 
surrounding  the  mouth,  the  anus  being  invariably  situated, 
contrary  to  the  arrangement  in  the  Endoprocta,  outside  the 
limits  of  the  lophophore.  The  tentacles,  too,  when  retracted 
are  not  flexed  or  coiled  as  in  the  Endoprocta,  but  are  simply 
approximated  to  form  a  bunch,  each  tentacle  being  straight 
and  parallel  to  its  fellows. 

The  most  characteristic  peculiarity  of  the  Ectoprocta, 
however,  is  the  power  Avhich  they  possess  of  withdrawing  or 
retracting  the  anterior  portion  of  the  body  with  its  crown  of 
tentacles  within  the  posterior  part  (Fig.  115).  This  latter 
portion  is  enclosed  in  the  ectocyst  {ec)  to  which  the  body- wall 
is  closely  adherent  and  which  forms  a  chitinous  or,  in  some 
cases,  more  or  less  calcareous  cell,  termed  a  zocecimn.  At  the 
mouth  of  the  cell  the  cuticle  becomes  suddenly  exceedingly 
thin,  so  that  the  anterior  portion  of  the  body  is  quite  mobile, 
and  by  means  of  sjiecial  retractor  muscles  (m)  may  be  with- 
drawn within  the  zocecium.  The  retraction  is  a  process  of  in- 
vagination, similar  to  what  occurs  in  the  withdrawal  of  the 
pharynx  of  the  Annelida,  the  most  anterior  lophophoric  part 
of  the  retracted  portion  not,  however,  sharing  in  the  invagina- 
tion :  the  whole  process  indeed  is  similar  to  what  may  be 
obtained  when  one  finger  of  a  glove  is  caught  from  within 
somewhat  less  than  half  way  from  the  tip  and  drawn  down 
towards  the  palm  ;  half  of  the  lower  portion  will  thus  be  in- 
vaginated  within  the  other  half,  while  the  tip  of  the  finger 
remains  uninvaginated. 

The  portion  of  the  body-wall  enclosed  by  the  ectocyst  is 
thin,  its  longitudinal  muscles  being  for  the  most  part  sepa- 
rated in  the  form  of  bands  traversing  the  coelom  and  function- 


TYl'K  PJtOaorYGIA. 


2r)9 


iug  as  retractoru,  while  the  circular  muscles  are  specially  ilo- 
veloped  as  a  rule  ouly  aruuuil  the  mouth  of  the  cup,  which 
may  by  their  actiou  be  closed  over  the  retracted  teiitaclus. 
A  relatively  spacious  ccelom,  coiitaiuiug  a  colorless  cor[)UHcu- 
lated  heemolymph  is  present,  aud  is  liued  by  fattened  perito- 


en 


Fig.  115.— Diagram  op  the  Stuuctuiie  of  Alcyonidium  albidum  (after 

Provho). 
a  =  anus.  io  =  intiTtciitaculiir  orgau. 

ce  =  giuiglion.  m  =  retriutor  muscle. 

CO  =  ca'loiii.  ov  =  ovary 

ee  =  ectocyst.  t  =  tentacles. 

en  =  emiocyst.  te  =  testis. 

Ileal  cells,  some  of  which  bear  tufts  of  cilia.  The  intestiue 
has  a  chfiracteristic  Y-shape  (Fig.  116),  its  posterior  portion 
being  prolonged  backwards  to  form  a  orcal  pouch,  from  the 
extremity  of  which  a  band  or  plate,  the.  funicnlns  (Fig.  116,/), 
containing  in  some  cases  muscular  fibres,  and  lined  by  perito- 
neal cells,  arises,  and  passes  backwards  to  be  inserted  into 
the  ectocyst  posteriorly.  The  anus  (a),  as  already  stated, 
opens  on  the  anterior  surface  of  the  body,  outside  the  limits 
of  the  lophophore,  and  between  it  and  the  mouth  lies  the 
nerve-ganglion,  which  is  frequently  hollow  and  sends  oflf 
nerves  to  the  various  poitions  of  the  body 


260 


INVEHTEBHA  TB  MOliPUOLOO  Y. 


As  iu  the  Eiidoprocta,  a  heart  aud  a  blood  vascular  system 
is  entirely  wanting.  Special  excretory  organs  seem  to  be 
wautin;^  iu  the  marine  Ectoprocta,  excretion  being  performed 
apparently  by  the  hjeun  lymph-corpuscles  and  other  meso- 
derni-cells,  especially  those  of  the  funiculus,  as  well  as  by 
the  granular  cells  of  the  stomach  and  caecal  pouch.  In  6Vi«- 
t(iti'll((,  a  fresh-water  form,  iiowever,  a  pair  of  ciliated  cauals 
o})euing  into  the  ccelom  by  ciliated  funnels  have  been  de- 
scribed, and  ])resumably  are  excretory  in  function,  the  two 
canals  uniting  together  to  open  to  the  exterior  by  a  single 
pore  situated  between  the  mouth  and  anus.  Up  to  the  pres- 
ent, however,  these  structures  have  not  been  observed  iu 
other  forms  and  apparently  they  do  uot  exist  iu  the  marine 
forms. 

In  some  of  these  latter  (Alcyonidium,  etc.),  however,  a  cili- 
ated tubular  structure,  which  communicates  at  one  end  with 
the  ccelom  and  o])eus  to  the  exterior  between  the  tentacles 
at  the  other,  occurs  and  has  been  termed  the  hdertenUicuhir 
organ  (Fig.  115,  io).  It  suggests  a  nephridium  iu  its  relations, 
but  apparently  does  not  possess  an  excretory  functi<m,  but 
serves  as  an  exit  for  the  reproductive  elements  to  the  exterior. 
In  other  marine  forms  and  in  all  the  fresh-water  genera  such 
special  reproductive  ducts  have  not  been  observed,  and  the 
mode  of  escape  of  the  sexual  products  in  these  forms  is  still 
unknown.  Many  of  the  Ectoprocta  are  hermai>hrodite,  the 
ova  and  sj)ermatozoa  (Fig.  115,  or,  te)  arising  from  the  ])eri- 
toueal  mesoderm,  frequently  from  that  surrounding  the 
funiculus.  AVhether,  however,  hermaphroditism  is  a  charac- 
teristic of  the  order  or  not  is  a  point  as  yet  undecided. 


1.  Suborder  PhyUtctolcrmata. 

The  members  of  this  suborder  are  exclusively  inhabitants 
of  fresh  water  and  are  characterized  by  the  tentacles  being 
arranged  ih  a  horseshoe-shaped  manner  (except  in  the  gcMius 
Frvdvrirdla,  where  they  form  a  circle),  and  by  the  occurrence 
of  a  well-developed  lobe  or  epistome  overlapping  the  mouth. 

The  colonies  assume  various  shapes  in  ditt'erent  genera, 
being  sometimes  dendritic  and  incrusting   stones  or    other 


TYrE  PUOaOPYUlA. 


201 


bodies,  as  iu  Firdvrkrihi,  or  fonuiiij^  compact  masses,  as  in 
AfcyoiieUa  iiud  /j>/>liopns,or  v\mi  iniiu^  ca])ul)lo  of  motion,  as 
iu  Crisfafi'lla.  In  some  forms,  a.^.  /.ophopufi,  the  cctocjst  j)os- 
sesses  a  gelatinous  consistency,  though  usually  it  is  cliitinous, 
auil  the  various  /oivcia  are  in  fnu'  communication  with  one 
another,  not  beinj^  separated  by  transverse  j)artitions. 

In  addition  to  multiplying  by  the  usual  processes  )f  bud- 
ding and  I)y  ova,  the  Phylactohemata  develop  upon  the 
funiculus  special  internal  buds,  termed  s((ifoU(i.sts,  which  are 
enclosed  within  dense  cliitinous  capsules.  These  are  set  free 
by  the  dying  anil  disintegration  of  the  parent  and,  being  pro- 
tected b>'  the  resistant  caps'de,  retain  their  vitality  under 
conditions,  such  as  cold  anti  .Iryness,  that  destroy  the  adult 
individual.  They  are  evidently  a  special  provision  f«)r  tin? 
p<»rpetuation  of  the  species  (bneloped  in  accordance  with  thn 
fresh-water  habitat,  in  which  the  organisms  are  exposed  to 
various  conditions  not  apt  to  be  nu^t  with  in  the  ocean  ;  it  is 
interesting  to  note  in  this  connection  thti  occurrence  of  gem- 
mules  in  the  fresh-water  sponges  which  are  strictly  company 
ble  to  the  statoblasts  and  have  a  similar  significance. 

2.    Sult,)nler  (it/niunl(fnntfft. 

The  fJymnolii'mata  aHMlistinguished  from  the  Phylactohe- 
mata  by  being,  with  the  single  exception  of  tlui  genus  I*nlii(h'- 
crlld,  umriuv  in  habitat,  by  the  tentacles  being  arianged  in 
the  form  of  a  circle,  and  by  the  invariable  absence  of  an  epi- 
stonu'. 

As  in  tiie  Phylactohemata,  the  colonies  vary  ;^..'atly  in 
form,  being  in  some  cases  encrusting,  .Ut'iiihronifhtni,  I'lns- 
fi'ii,  in  others  blanching,  Sri'Kftnctlhn'ia,  or  in  <»thers  again 
massive,  Alci/oiiitlinni  ;  ami  furtlnMiiiore  the  ectocyst  pre- 
sents varying  degn>es  of  consistency,  being  fre<pn'ntly  «'hitin- 
ous,  l)ut  occasionally  sonu'what  gelatinous  or  calcitied  to  a 
greater  or  less  extent.  The  /ocecia  are  nttt  in  fr«'e  communi- 
cation with  eatrh  other,  but  each  i^'  "losed  below  or  posteri- 
orly by  a  transverse  cliitinous  plati'  in  which  pi'rfoiations  are 
said  to  be  jiresent,  though  doubt  has  rect>ntly  been  thrown 
upon  their  existence  as  perforations.  In  shape,  too,  the 
zoiL'cia  like  the  colonies  vary  greatly,  especially  so  far  as  tlieir 


11 


262 


IN  VEHTEBliA  TK  MOIWIIOLOG  Y. 


mouths  are  concerned,  and  it  is  possible  to  divide  the  Gymno- 
hiMiiata  into  tliree  groups  or  tribes,  based  upon  these  differ- 
ences. In  the  tribe  (Ji/clo,stomata  the  zotucia  are  usually 
cylindrical,  and  the  mouth  is  circular  and  destitute  of  any 
aj)pt'ndages  ;  iu  the  ( 'teno,sfomata  the  mouth  is  closed  during 
rt'traction  by  a  series  of  bristles  which  surround  it  {Alcyonul- 
iutn);  while  in  the  Cliihishnnutn,  in  which  the  ectocyst  is 
usually  hrui  anil  frequently  calcihed,  the  mouth  is  closed  by  a 

lid,  the(>/x'/Y7</M///,  luruished  with  spe- 
cial muscles  [HikjuIh,  jMemhranipont). 
In  this  last-niinied  tribe  a  poly- 
morphism of  the  individuals  compos- 
a  colony  is  freipiently  found. 
Scattered  anionp  the  ordinary  indi- 
viduals others,  the  Avirulurm  (Fig. 
11(1,  II  v),  may  be  found  having  Ihe 
ap]>earaiit'(^  of  a  bird's  head,  the 
lower  beak  being  fastened  to  the  head 
by  a  hinge  and   havint;  inserted  into 


av  i"« 


rni 


av 


it  strong  muscles  ;  bumdies  of  sensory 
hairs  are  also  present,  and  when  tlujso 
are  stiniulate<l  the  lower  beak  is 
rapidly  sna])ped  against  the  upper 
arid  the  stimulating  organism  thus 
caught.  There  can  be  little  doubt 
but  that  tlu'S(>  Avicularia  are  specially 
moditied  individuals  whose  head  and 
uppei'  beak  represtMit  the  ordinary 
individual,  while  the  lower  beak  may 
j)ossil>ly  be  the  ecpiivalent  of  the 
opiucnlum  ;  ])hysiol(»gically  they  have 
been  usually  regarch^l  as  spt'ciali/ed 
for  th(^  purpose  of  catching  food  f()r 
the  ordintii'v  individuals,  but  it  is  not 
inipiobabh'  that  their  services  may 
rather  b(»  of  u  t'leansing  nature,  re- 
moving from    the  colony  partid 


Pro.     116.  —  PoUTION     OF     A 

Colony  of  /iuyiilu. 

a  —  anuH. 
av  =  aviculiiiin. 
bb  —  brown  body. 
ee  —  ort(t(Vsi. 
/  r-  funiculus. 
oe  —  oviccll. 
rm  —  reiiacior  muscle. 
t  —  teutiiclt'M. 

f 

dirt  and  the  excreta,  which  by  accumulating  might  inttu-fero 
with  the  proper  function  of  the  tentacles.     Another  polynior- 


es  o 


TYPE  rilOSOPYOIA. 


sea 


l)lii(!  form  is  kuowu  as  tlie  Vihrucuhiy  aud  cousists  of  a  sK'iuler 
lilaiiKMit  niovably  artieuluted  to  a  rouiuled  strui'tnre  coire- 
s[)oudiiJ^  to  the  head  of  the  Avicularia  ;  the  lilanieuts  wave 
contiuually  to  aud  fro  and  are  probably  tactile  iu  fuuction. 
Ill  many  forms,  to«),  in  the  neif^hborhood  of  the  mouths  of  the 
oriliuary  individuals  sac-like  poucdies  occur,  iu  which  the 
ova  underj^o  their  development.  These  structures,  known  as 
ovicells  (Fij;.  IK),  DC),  or  oacia,  have  also  be(>n  considered  motli- 
tied  indiviiluals,  but  seem  rather  to  be  organs  of  the  ordinary 
individuals,  arising  as  a  ])ouchin<^  of  their  walls.  Finally,  not 
infruqueutly    certain    individuals    relinquish    their   nutritive 


Fio.   117. — ..'1,   Lauva  or  I'tdkeUinn  (after  Matkciikki  ;   li,   Ci/p/iouuuten  iiifter 

I'llOL'Hl'). 

uU  -  adiiesive  orgiiii.  «  =  sloumcli. 

ctil  ■■■-  ciiloltu.  «A  =  shell. 

cor  —  curoikti.  pyr  =  pyrilonn  oij^uu. 

function  and  serve  as  root-like  auchors  on  stem-like  supports 
for  the  entire  colony. 

As  r(?fj[ards  the  internal  sti'ucture  of  the  (Tymiiolii'matu  it 
is  unnecessary  to  add  anything  t(t  what  has  alreatly  been 
stated  in  ilescribing  the  general  characteristics  of  the  or<ler 
Ectoprocta. 

Di'vehiptnent  of  the  PoJyzoa. — The  larva  of  JVdinlliuu  (Fig. 
117,  .1  ),  which  may  be  taken  as  a  type  of  the  Endoprocta,  is  a 
somewhat  ilome-shaped  f)rgHnism,  the  summit  of  the  dome 
being  occupied  by  an  apical  thickening  (cal)  bearing  a  tuft  of 
cilia,  while  at  the  margin  then'  is  a  stout  ciliary  band,  the 


H 

m 


264 


IN  VERTEBRA  TE  MORPHOLOO  Y, 


corona  {cor).  The  cavity  of  the  tlome  is  occupied  by  the  U- 
shaped  dij^estive  tiuct  (.v),  the  mouth  aud  anus  both  opening 
within  the  circle  formed  by  the  corona,  a  deep  depression, 
the  vestibule,  lyiu«^  between  the  two.  In  the  coelom  above 
the  floor  of  the  vestibular  dei)ressi()n  are  a  number  of  meso- 
derm-cells,  and  also  a  ciliated  canal  composed  of  a  single  row 
of  perforated  cells  and  probably  excretory  in  function.  Upon 
one  surface  of  the  larva  between  the  marginal  ciliated  band 
and  the  a})ical  thickening  is  a  peculiar  glandular  organ  termed 
the  cement-gland  (pyr),  around  the  mouth  of  which  are  situ- 
ated a  number  of  strong  cilia. 

Tho  devtlopmt' nt  of  this  larva  into  the  adult  form  is  accompanied  by  a 
niiinbcr  of  remarkable  changes,  which  in  their  details  and  significance  are 
not  yet  tiiorotighly  understood.  The  larva  settles  upon  the  ventral  or  oral 
face  and  siiortly  thereafter  one  wall  of  the  vestibule  becomes  pushed  over 
towards  the  other  and  eventually  unites  with  it,  the  original  vestibular 
cavity  becoming  divided  into  two  portions,  one  of  which  remains  in  c<m- 
nection  with  the  surface  of  fixation  and  later  degenerates,  while  the  other 
has  opening  into  it  the  mouth  and  anus,  though  the  former  opening  at 
about  this  period  becomes  closed.  Later  a  renuirkable  rotation  through 
180°  of  the  digestive  tract,  togetln  r  with  the  portion  of  the  vestibule  in 
connection  with  it,  occurs,  the  portion  of  the  body  immediately  above  tho 
point  of  fixation  elongating  to  form  the  stalk  of  tho  adult,  becoming  at  the 
same  time  filled  with  mesodermal  tissue.  The  mouth  opens  again  into  tho 
vestibular  cavity,  the  tentacles  ari.se  from  tho  wall  of  the  cavity  which 
later  opens  to  the  exterior,  the  adult  form  being  thus  a.ssuiiied.  The  fate 
of  the  apical  thickening  and  of  the  cement-gland  is  uncertain  ;  they  have 
been  described  as  degenerating  without  taking  any  part  in  the  formation  of 
the  adult  organs,  though  it  has  been  suggested  that  the  apical  thickening 
may  give  rise  to  the  nervous  .system  of  the  adult. 

The  form  of  the  larvie  in  the  Ecto})rocta  is  subject  to  con- 
siderable modification.  In  Meinhrnnipora  and  some  other 
genera  the  larva  is  known  as  the  Cyphonautes  (Fig.  117,  />), 
having  been  so  designated  before  its  life-history  was  eluci- 
dated. It  has  a  somewhat  triangular  outline  and  is  character- 
ized by  being  enclosed  in  a  bivalved  chitinous  shell  [sh).  At 
the  apex  of  the  triangle  is  the  apical  thickening  (ca?),  with  its 
elongated  cilia,  while  around  the  base  there  is  to  be  found  the 
corona  {cor).  A  well-develo])ed  digestive  tract  is  present,  both 
the  mouth  and  anus  opt'uing  upon  the  basal  surface  of  the  tri- 
angle,  and  therefore  within  the  area  enclosed  by  the  corona. 


TYPE  PR080PYGIA. 


265 


con- 
)tlier 
\  li), 

icter- 
At 

til  its 

the 

)otli 

tri- 

louu. 


This  vestibule  is  a  deep  depression  of  the  oral  surface  of  the 
larva,  diflferiug  from  that  of  the  PedicelHna  larva  ouly  in  hav- 
ing an  arch-like  thickeuing  of  its  walls  (ouly  oue  side  of  the 
arch  is  represeuted  in  the  tigure)  which  imperfectly  separates 
au  oral  portion  of  the  vestibule  from  a  posterior  or  anal 
portion,  a  glandular  depression  situated  in  the  roof  of  this 
latter  portion  constituting  the  adhesive  organ  («(/).  In  front 
of  the  oral  vestibule  is  situated  a  ciliated  depression  from 
which  projects  a  tuft  of  long  cilia  and  which  appears  to 
correspond  to  the  cement-gland  of  the  PedicelHna  larva  and 
to  a  glandular  structure  in  the  mt)re  modified  Ectoprocta 
larvm,  known  as  the  pyriform  organ  (pyr),  by  which  name  it 
may  be  known  here.  The  similarity  of  this  larva  to  that  of 
PedicelHna  is  clear,  the  details  of  organization  of  the  two  forms 
agreeing  part  for  part ;  in  other  Ectoprocta,  however,  great 
differences  are  to  be  found.  In  the  genus  Jhigula,  for  exam- 
ple, the  larva  (Fig.  118)  is  a  barrel-shaped  organism  at  the  one 
extremity  of  which  is  a  thickening, 
the  calotte  {cal),  which  appears  to 
correspond,  in  part  at  any  rate,  to 
the  apical  thickening  or  dorsal  organ, 
as  it  is  sometimes  termed,  of  Pedicel- 
Una  and  Cyphonanlefi.  The  sides  of 
the  barrel  are  formed  by  a  circle 
of  elongated  cells  forming  the  corona 
and  equivalent  to  the  marginal 
corona  of  the  other  larva^ ;  it  does 
not,  however,  form  a  simple  baud  in 
Jimfula,  but  its  cells  are  much 
shorter  on  one  of  the  faces  of  the 
embryo    th.Ji  elsewhere,   producing"; 

a  well-marked  groove  at  the  apex  of  which  lies  the  pyriform 
organ  (/)?/>•)  whose  homologies  in  ('npJuyndutes  havo  already 
beon  pointed  out.  A  peculiarity  of  thih  larva  is  the  entire 
absence  of  a  digestive  tract,  the  lowor  end  of  the  barrel  being 
occupied  by  a  depression,  the  adhesive  organ  {ad). 

Between  such  a  larva  as  that  just  described,  entirely  des- 
titute of  a  digestive  tract,  anil  that  of  ( 'j/phonautes  intermediate 
stages  occur,  as  for  instance  in  the  larva^  of  the  Cyclostomata, 


Fto.  118.  -Lakva  of  liugula 

Jl<(bi'lliil<i  (iifiiT  Hakhoih). 

Letters  us  in  FiJ,^  117. 


266 


INVEHTEBHA  TE  MORPIIOL  00  Y. 


I 
1 

It  i     ! 


in  which  the  (lijj;estive  tract  is  represented  by  a  yolk-laden 
mass  of  tissue,  haviujj;  little  resemblance,  however,  to  the 
differentiated  endodermal  tube  of  Pcdicdlina.  The  occur- 
rence of  such  forms,  however,  shows  that  the  absence  of  the 
tract  in  IJugula,  etc.,  is  the  result  of  progressive  degeneration, 
such  larvie  as  those  of  PedicelUna  and  Cyphonautes  rej)resent- 
ing  the  primitive  condition  more  nearly  than  the  remarkable 
larva  of  JJiujida. 

The  transforination  of  tlie  Ectoproctous  larvae  into  the  adult  is  even 
more  remarkable  than  that  of  PedicelUna.  Fixation  takes  place  by  the 
oral  surface,  the  adhesive  organ  being  evaginated  for  the  purpose,  and  is 
succeeded  by  a  degeneration  of  the  corona  and  pvriform  body.  In  Cijpho- 
nantes  the  digestive  system  conipletely  degenerates  likewise,  a  new  one 
being  formed  later,  the  tissue  in  the  neighborhood  of  the  apical  thickening 
taking  a  prominent  part  in  its  formation.  In  those  larva'  which  are  desti- 
tute of  a  digestive  tract  one,  corresponding  to  the  .second  one  of  Cypho- 
nauU's,  develops  after  hxation,  likewise  from  the  tissue  of  the  apical  thick- 
ening. The  exact  method  of  this  regeneration,  for  so  it  may  be  con- 
sidered, can  hardly  be  descril)i'd  here  without  leading  to  a  recapitulation 
of  details  too  minute  for  the  scope  of  this  work.  It  may  be  remarked, 
however,  that  the  phenomena  do  not  seem  to  merit  the  designation  of  an 
alU'rnalio!!  of  generations,  as  might  at  lirst  sight  be  supposed,  but  are 
rat  her  simjdy  a  metamorphosis  the  signitieance  of  which  is  at  present 
decidedly  ol)seuri'. 

Baddintj  of  the  Polyzoa. — As  already  stated,  colony  formation  by  bud- 
ding is  a  characteristic  feature  of  the  Polyzoa,  //ouuvomrt  alone  not  pre- 
senting tills  method  of  growth,  though  lik«'  other  forms  it  reproduces  by 
budding,  the  l)uds,  liow(!ver,  .separating  at  an  early  stage  from  the  parent. 
In  the  Kndoprocia  a  stolon  arises  from  near  the  point  of  fixation  of  the 
l)iimaty  individual  which  develops  from  the  ovum,  mesoderm  tissue  from 
the  stalk  of  this  individual  migrating  into  the  stolon,  but  there  is  no  pro- 
longation into  it  of  the  i)areMtal  eiidoderm.  At  a  mttreor  less  definite  part 
of  the  stolon  the  ectodermal  cells  thicken  and  later  on  invaginate  towards 
the  centre  of  the  stolon.  This  itivagination  beeonnvs  surrounded  by  meso- 
derm already  present  in  the  stolon,  and  later  ditferentiales  into  two  cavi- 
ties, one  of  which  niaiiis  connection  with  tht;  exterior  and  forms  the 
Vestibular  chanil)er,  from  tlu;  walls  of  wliieh  the  tentacles  develop,  while 
the  other  boeoines  the  digesfivi'  tract,  its  original  connection  with  the  ves- 
tibular cavity  becoming  the  anus,  the  mouth  developing  later  as  a  <lepres- 
siou  of  the  floor  of  the  vestil)ular  cavity  which  joins  the  stomach.  It 
is  interesting  to  note  that  from  th(^  ectodtM-jnal  invagination  the  nervous 
sy.stem  as  well  as  the  digestive  tract  develops. 

In  the  Ectoprocta  i)ractically  the  same  method  obtains  i!i  the  budding, 
though  the  stolon  is  represented  by  the  tip  of  a  branch  or  even  by  the 


,,v*ii*iiiis»>»*** 


TYPE  VliOSOPYGIA. 


207 


tissue  in  \\w  neighborhood  of  the  moutli  of  the  zoa^ciura.  The  eolony 
R'.sulting  from  continued  budding  becomes  accordingly  as  u  rule  much  more 
compJict  than  in  the  Kndoprocta,  eacii  polypide  being  more  or  less  approxi- 
mated to  its  predecessor. 

Closely  related  to  (he  process  of  budding  is  that  of  regeneration,  also 
of  frecpient  occurrence  among  the  Polyzoa.  Among  the  Kndoprocta  Pedi- 
cellina  shows  the  process  in  a  periodical  though  not  simullaiieous  moult- 
ing of  the  poiyj)ides,  new  ones  developing  from  (he  tip  of  the  stalk  which 
b(tre  liie  amputated  poly|)ide.  Here,  as  in  ordinary  l>uddiiig,  the  tissues 
concerned  appear  to  be  ectoderm  and  mesoderm,  the  stalk  containing  no 
prolt»ngation  of  the  original  eiuloderm. 

Ill  (he  Ec(oproc(a,  however,  regeneration  is  carried  to  a  greater  extent. 
In  examining  any  eolony  of  Jini/ida,  for  example,  in  sonu;  df  the  zoo'eia  in 
add'lion  to  the  polypide  a  brown  mass  may  be  seen,  the  so-called  "  Ixown 
body"  (Fig.  11(1.  (ill)  \  ill  others  thc^  ))rown  l)ody  may  be  .seen  without 
any  distinct  polypide.  This  body  is  (Ik;  result  of  the  degeneration  of  the 
digestive  tract  and  otlmr  organs  of  (he  original  poly|)ide,  only  its  Itody- 
wall  or  endocyst  persisting,  from  whicli  new  organs  are  developed  and  the 
polypide'  regenerated.  Tht;  signiiicancu!  of  this  process  is  not  clear,  but  il 
has  been  suggested  that  it  stands  in  relation  lo  I  lie  process  of  excretion, 
the  formation  of  the  brown  body  oe(;urring  in  forms  whi('h  do  n«»t  possess 
any  special  excretory  organs.  It  is  now  known  that  in  the  marine  Keln- 
procta  the  excretory  products  are  taken  up  in  part  by  the  cells  of  the 
stomach  and  r  ical  pouch,  a  fact  which  .seems  to  harmoni/e  with  the  sug- 
gested significance  of  the  brown  body. 

The  formation  of  a  new  polypide  frotn  ectoderm  and  mesoch'rm  appar- 
etitly  is  a  (iillicull  fact  to  explain  on  the  tlieoi y  of  the  genn-layers.  Il  is 
possilile.  however,  to  regai'tl  the  ti.ssiie  fi-oin  wiiieii  Ituds  arise  as  iiiidiffer- 
entiated  enil)ryonic  tissin*  passed  on  fioni  polypide  to  polypide  and  trace- 
able back  to  the  end»ryonic  tissue  of  tiie  ovum.  In  tin  formation  ot  each 
|»oly|(i(le  a  certain  ainonni  of  llit^  tissue  lieeomcs  dilVeretitiaied,  but  sona? 
still  retains  its  emi)ryonic  character,  a  continual  ion  of  ine  budding  pnteess 
being  thus  po.ssible. 

AJffinities  of  the  Ihli/ziut.-  'Www  seems  to  belittle  rooru  tor  doul)i  but 
that  the  Kndoprocta  represent  more  nearly  the  original  EVdyzoa  than  do 
the  Kctoprocta.  TluMr  colony  f<  riuition  is  of  a  more  -imple  t".>rm  iIimii 
that  of  the  other  groiii).  they  po^^^<■ss  nepliridia  which  are  waiiiiii::  in  the 
majority  of  the  Kctoprocta,  ami  th(;ir  developinent  i.s  miieli  -nnpler.  the 
highly   modified  larva  of  the  niaruie  Kcio|iroeta  having  iiiuiitMiii>(eiily  liet^i 


cm>hi 


derivtjd  from  one  approximating  in  siriiclure  that  of  / 
iMitlis  representing  a  stage  in  the  evolmioti. 

Similarities  have  been  traced  between  \\w  l^t'<lh->lliiHi  ar- a  and  the 
AiMielid  Troi'hophore,  and  il  is  not  improhiil)le  that  tlii.s  ntwlnrue  lwt*n  tIh* 
true  derivation  of  the  group,  in  which  ca.se  the  I'olyzoa  jw*'  ti«  h»*  rr^sarded 
Jis  forms  which  have  never  ])osse.ssed  jiny  tnu-es  '4  mftamei-iM^in  but  stand 
in  aiiout  the  same  relationship  to  the  Annelida  as  do  the  iioiut^ra. 


268 


IN  VERTEBRA  TE  MORPIIOLOG  Y. 


Another  view,  however,  which  has  had  ardent  supporters  is  that 
whicli  recognizes  a  relntionsiiip  between  the  Polyzoa  and  Phoronis.  There 
is  a  lophophoro  in  botli,  likewise  a  U-like  bending  of  the  digestive 
tract,  and  tiie  ncphridia  of  I'lturonis  may  be  considered  coini)arable  to 
tlioso  of  tlie  Kndoproeta.  But  here  the  siniihirity  c«'ases,  TIjc  anus  in 
J'/iorotiia  is  outside  tlie  limits  of  tlie  lopliophoro  and  is  comparable  in 
position  with  that  of  the  Ectoprocta,  a -point  wliich  tells  against  this 
phylog«^ny  since  these  forms  have  been  shown  to  be  less  primitive  than  the 
Endoprocta.  If,  however,  this  |)hylogeny  should  jirove  to  be  correct,  it 
will  show  a  (h'scenl  for  the  I'olyzoa  from  metameric;  Annelids,  through  the 
(Jephyrea,  since  it  is  to  this  group  that  Phoronis  seems  to  be  most  nearly 
related. 


III.  Class  Brachiopoda. 

The  Brachiopoda  coustitute  n  very  well-tlefined  group 
wlioHe  present  poverty  in  species  is  iu  striking  contrast  to  its 
great  development  during  Pulujozoic  times.  Like  the  Poly- 
zoa they  possess  a  tentuculate  lophophore  (Fig.  120,  Jp)  which 
usually  takes  the  form  of  two  exceedingl}'  elongated,  some- 
times spirally-coiled,  arm-like  processes  projecting,  one  on 
either  side,  from  the  anterior  portion  of  the  body,  and  fur- 
nished upon  their  outer  or  posterior  border  with  tentacles. 
The  body  is  usually  somewhat  short  and  stout,  and  prolonged 
l)osteriorly  into  a  peduncle  {pe)  or  stalk  which  is  in  some 
cases  at  least  provided  with  adhesive  papilhe  and  serves  as 
an  anchor. 

The  most  characteristic  feature  of  the  Brachiopoda  is, 
however,  the  presence  ff  a  bivalved  shell  (Fig.  119)  similar 
to  that  of  a  bivalve  Moilusk,  with  which  forms  the  Brachio- 
l)()ds  were  until  com})aratively  recently  associated.  From 
near  the  base  of  the  peduncle,  upon  the  dorsal  and  ventral 
surfaces  of  the  body,  a  fold  of  the  body-wall  is  found,  which 
contains  a  cavity  iu  communication  with  and  indeed  in  reality 
a  portion  of  the  coeloinic  cavity.  These  two  folds  are  of  sulH- 
cieut  size  to  enwrap  or  enclose  the  bo<!  and  the  lophophore 
and  are  termed  the  iiKintlc-hihcs  (Fig.  120,  iu),  the  space  between 
them  and  the  body  being  known  as  the  mantle-cavity.  They 
subserve  largely  if  not  eiitin^ly  the  function  of  respiration, 
the  portion  of  the  c<L'loni  which  they  contain  being  more  or 
less  divided  uj)  into  a  system  of  lacuujo  through  which  the 


^ 


# 


TYPE  PliOSOPYUIA. 


209 


liiL'iuolymph  circulates.  Upon  the  outer  surface  of  each 
mantle-lobe,  and  formed  by  it,  is  a  valve  of  the  shell,  composed 
of  a  certain  amount  of  orj^anic  matter,  but  largely  of  carbonate 
of  lime.  Since  the  nmntle-lobes  are  dorsal  and  ventral  in 
position,  so  too  are  the  valves  of  the  shell,  and  consequently 
their  hinge-line  is  posterior  and  their  mouth  anterior.  In 
a  number  of  forms,  which  may  be  grouped  together  as  the 
suborder  Teslicardines,  the  shells  along  the  hinge-line  are 
j)rovided  with  interlocking  teeth,  a  true  hinge  being  present, 
the  peduncle  in  these  cases  jjcrforating  a  backward  prolonga- 
tion or  beak  of  the  lower  valve.  In  a  few  genera,  however, 
forming  the   suborder  Jicardinen,  uo  such  hinge  is   present, 


IS, 

lilar 
ihio- 
'rom 
itral 
nch 
ility 
ulH- 
lore 

een 
hey 
ion, 
or 

the 


Fio.  119.— DoiiSAL  Valve  OF. Spt/'ifer,  showing  Aum  Skelkton  (after  Leunis). 

the  peduncle  passing  out  between  the  two  valves  of  the  shell. 
Special  muscles  are  present  extending  from  one  valve  of  the 
shell  to  the  other  and  are  necessary  both  for  the  opening  and 
the  closing  of  the  shell,  and  furthermore  it  should  be  noted 
that  except  for  a  slight  ditt'erence  in  concavity  both  valves  of 
the  shell  are  similar  and  symmetrical. 

It  will  be  seen  by  conn)aring  the  various  facts  mentioned 
here  with  what  is  said  on  p.  327  regarding  the  shell  of  the 
bivalve  MoUusks  that  the  structures  in  the  two  grou[)s  are 
very  different.  This  difference  is  emphasized  by  the  presence, 
in  the  majority  of  the  Testicardines,  of  a  calcareous  su])port 
for  the  coiled  lophophore  attat^hed  to  the  inner  surface  of  the 
dorsal  valve  (Fig.  119).  It  consists  of  a  pair  of  calcareous 
rods  which  project  downwards  and  forwards,  uniting  to  form 
a  transverse  arch,  and  may  give  rise  on  each  side  to  a  spirally- 
wound  process  upon  which  the  coils  of  the  lophophore  rest. 

The  body-wall  is  composed  of  an  outer  layer  of  ectoderm 


m 


270 


ly  VEHTEmiA  TE  MOliVUOLOU  Y, 


from  which  uuiuerous  piipilliu  or  iu  somo  cases  branchiug 
procGHses  arise,  pr<>jectiu{^  iuto  correspoudiug  cavities  or 
tubes  iu  the  substauce  of  the  shell-valves.  Below  the  ecto- 
(leriu  is  a  uiore  or  less  homogeueous  couuective  tissue  con- 
taiuiu«^  cells  aud  recalliuj^  the  mesoghea  tissue  of  the  C(L'leu- 
terates.  Scattered  luuscle-tibres,  arruuged  transver>i'>ly  aud 
lougitutliually,  occur  iu  the  mautle-lobes  aud  iu  the  bod^-wall, 
but  then;  are  uo  detiuite  uiuscular  layers  such  as  are  fouud  iu 
the  Auuelida,  though  the  h)Ugitudiual  muscles  of  the  peduucle 
are  well  developed.  Special  luuscles,  which  cauuot  be  cou- 
sidered  dillereutiatious  of  the  uiusculature  of  the  body-wall, 
traverse  the  ccelom  froui  oue  valve  of  the  shell  to  the  other, 
oue  pair,  the  divaricators,  beiug  iuserted  iu  such  a  way  as  to 
cause  by  their  coutractiou  a  sejjaratiou  of  the  two  valves, 
while  auother  pair,  the  adductors  (Fig.  120,  (im),  approxiuuite 
tlu'Ui.  Other  muscles  also  occur,  such  as  the  adjustores, 
wiiiirh  produce  lateral  ujoveuuMits  of  the  shell-valves,  aud  pro- 
tractors aud  retractors  (Fig.  120,  rm)  of  the  })eduucle. 

The  Cd'lom  is  liued  by  a  peiitouoal  epitheliuiu  aud  cou- 
taius  a  cor])usculated  liaMuolyuiph  which  is  driveu  about 
through  the  ccelouiitt  s[>aces,  and  the  lacuute  iu  the  uiautle- 
fohls  aud  the  loph()})h()re  which  communicate  with  them,  by 
the  contractions  of  the  body-wall  and  the  musculature,  there 
beiug  uo  distinct  heart  or  blood-vessels.  A  dorso-veutral 
mesentery  which  slings  the  intestine  is  present  and  divides 
the  body-c<elom  more  or  less  completely  into  two  lateral 
chambers,  and  furthermore  two  transverse  partitions  or  dis- 
sepim<Mits  occur  in  several  forms  a. id  divide  the  coelom  iuto 
anterior,  middle,  and  posterior  compartments,  an  arrangemeut 
recalling  the  metamerism  of  sucdi  a  form  as  Sit^jitta  (p.  187). 

The  mouth  ()i)ens  at  the  anterior  end  of  the  body  betweeu 
the  two  lophophoric  arms  and  leads  into  a  short,  somewhat 
muscular  (esophagus,  which  posteriorly  communicates  with  -i 
stomach-like  dilatatiou  (Fig.  120,  i)  into  which  ojjgu  the 
ducts  of  oue  or  more  pairs  of  branching  tubular  glauds — the 
so-called  liver  or  digestive  gbiuds  (h.  Behind  the  stomach  lies 
the  intestine,  which,  in  most  of  the  Ecardines,  such  as  /Jnguhif 
beuds  ui)on  itself  aud  o})eus  iuto  the  mantle-cavity  iu  the 
mid  dorsal  line  near  the  anterior  eud  of  the  body.     Iu  Crania, 


TYPE  PROSOPYUIA. 


271 


howover,  it  opeus  posteriorly,  while  in  TcrebratuHtia,  Argiupe, 
aiitl  Wtddheimia,  in  fuct  iu  all  the  Te. sticarUiueu,  it  euda 
bliiully,  the  UUU8  being  wautiu^,'. 

The  iiervouB  system  cousists  of  uu  (usoph.ijjieal  riiij^'  lying 
iu  the  connective  tissue  substance,  the  lower  portion  being  iu 
connection  v;ith  the  ectoderm  and  slightly  swollen,  represent- 
ing probably  the  suboesophageul  ganglion  of  the  Annelida; 


Fig.  120.— Stuucti'uk  ok  Tciehmtulina  xepteutrionalis. 

am  r=  udductor  muscle.  ne  =  nepliridiuiii. 

»  =  intosliue.  vv  --  ovary. 


I  =  liver-lobes. 
Ip  =  lophophoie. 
m  —  iimntle. 


pe  —  pedtiiick'. 
nn  ~  relractor  muiicle. 
a  -  shell. 


a  similar  swelling  in  the  dorsal  portion  of  the  ring  represents 
the  supracesophageal  ganglion,  and  in  addition  there  are  usu- 
ally two  further  lateral  gangliou-like  swellings.  Nerves  ]>ass 
otr  from  the  upper  ganglion  to  the  lophophore  and  other 
regions,  and  from  the  lower  one  to  the  mantle,  muscles,  etc., 
botli  sets  terminating  in  the  superficial  layers  of  the  lopho- 
])hore-tentacles  or  of  the  mantle  in  a  network  of  ganglion 
cells  and  fibres.  No  trace  of  a  ventral  nerve-cord  in  addition 
to  the  sub(JL'sophageal  ganglion  is  present. 

Sense-organs  are  but  poorly  develoi)ed,  neither  eyes  nor 
auditory  organs  occurring.  The  tentacles  on  the  lophophoric 
arms  are  iu  all  probability  sensory,  as  indicated  by  their  rich 
nerve-supply,  and  the  papilhe  of  th^  mantle-ectoderm  which 


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272 


INVERTBBRA  TE  MORPHOLOQ  Y. 


project  into  the  canals  of  the  shell  have  been  stated  to  be 
sensory,  containing  an  axial  nerve-fibre  terminating  in  a  sen- 
sory cell. 

The  nephridia  (Fig.  120,  ne)  are  represented  by  two  or 
four  [Rhynchoneda)  funnel-shaped  short  tubes  which  open  by 
a  fimbriated  mouth  at  one  extremity  into  the  ccelomic  cavity 
and,  rapidly  narrowing  towards  the  outer  end,  open  by  a  small 
pore  into  the  mantle-cavity.  In  addition  to  their  probable 
excretory  function,  these  structures,  as  in  some  of  the  Anne- 
lida, serve  also  as  ducts  for  the  passage  to  the  exterior  of  the 
reproductive  elements.  These  are  derived  from  the  coelomic 
peritoneum  and  form  branching  masses  (Fig.  120,  ov)  lying  in 
some  cases  in  the  coelomic  spaces  of  the  mantle,  or  in  addition 
extending  into  the  body,  as  in  most  Ecardines,  or,  as  in  Tere- 
bratulina,  confined  to  this  region.  Most  of  the  Brachiopods 
are  bisexual  apparently,  though  it  is  possible  that  Lingida 
and  perhaps  some  other  forms  may  be  hermaphroditic,  the 
male  and  female  elements  maturing  at  different  tin:  ?s. 

Development  and  Affinities  of  the  Brachiopods. — The  Testi- 
cardines  are  characterized  by  the  occurrence  of  a  free  larval 
stage  destitute  of  a  shell.  In  Argiope  (Fig.  121)  it  is  appar- 
ently divided  into  four  segments,  the  most 
anterior  of  which  bears  two  eye-spots  and 
assumes  an  umbrella-like  form,  long  cilia 
projecting  from  the  margin.  The  third 
segment  develops  two  folds  which  enclose 
the  posterior  segment  and  bear  on  their 
margin  bunches  of  setee  inserted  in  seta- 
sacs  and  recalling  the  setsB  of  certain  Annelid 
larveB.  After  swimming  about  for  a  time 
the  larva  settles  down  and  fastens  itself  by 
the  posterior  segment  and  the  mantle-lobes 
turn  forward  to  enclose  the  anterior  seg- 
ments. The  posterior  segment  becomes  the 
peduncle  of  the  adult,  and  the  shell  de- 
velops on  the  surface  of  the  mantle-lobes,  whose  bunches  of 
setae  are  thrown  off.  The  mouth  makes  its  appearance  only 
after  fixation  just  ventral  to  the  eye-spots,  and  around  it  there 
develops  a  ring  of  tentacles  placed  somewhat  obliquely,  and 


Fio.   lai.— Lauva 

«)F   Argiope   (after 

KOWALBWBKI). 


■,.,:i^  iinS&oJi."-*^*-*' 


mmm 


TYPE  PKOSOPTGIA. 


273 


later  elongating  laterally  to  form  tho  coiled  lopbophore  with 
its  numerous  tentacles. 

The  early  stages  of  the  development  of  the  Ecardines  is 
not  known,  but  in  Lingula  the  larva  is  free-swimming  long 
after  the  shell  has  formed,  the  peduncle  being  late  in  develop- 
ing. In  this  form  also  the  lophophore  arises  as  a  circle  of 
tentacles  surrounding  the  mouth  and  subsequently  elongates 
laterally.  . 

The  affinities  of  the  Brachiopods  have  long  been  an  open  question. 
They  were  by  early  writers  regarded  as  Mollusca,  later  as  Annelida  or 
closely  related  to  that  group,  but  are  now  usually  considered  to  be  more 
nearly  related  to  the  Polyzoa  than  to  any  other  forms  and  to  be  most 
properly  associated  with  them,  the  general  likeness  of  a  young  Lingula,  for 
instance,  to  a  Polyzoan  being  very  striking.  The  presence  of  the  manlie- 
lobes  and  the  shell  seem  to  mark  the  Brachiopoda  as  something  far  removed 
from  the  other  members  of  the  type  Prosopygia,  but  it  must  be  remembered 
that  in  the  larval  Ectoproctous  Polyzoa  the  corona  behaves  in  a  manner 
closely  similar  to  the  Brachiopod  mantle  and  it  is  not  impossible  that  the 
two  structures  may  have  something  in  common. 

Another  distinguishing  feature  of  the  Brachiopods  is  the  indication  of 
a  segmentation.  The  presence  of  two  dissepiments  and  in  RhyticJionella  of 
two  pairs  of  nephrldia  certainly  suggests  metamerism,  but  objection  has 
been  raised  to  the  dissepiments  having  any  metameric  significance,  on  the 
ground  that  they  do  not  bear  the  proper  relationships  to  the  body  axis  to 
be  regarded  as  comparable  to  the  dissepiments  of  the  Annelida.  It  has 
been  stated  by  some  authors  as  a  characteristic  of  the  Prosopygia  that 
their  body  axis  is  bent  upon  itself  so  that  the  two  ends  are  approximated 
and  one  surface,  the  dorsal,  is  almost  obliterated,  while  the  other,  the 
ventral,  is  very  much  enlarged,  as  seems  to  be  actually  the  case  in 
Phoronis.  It  must  be  remembered,  however,  that  the  terms  dorsal  and 
ventral  are  not  to  be  defined  by  reference  to  the  digestive  tract  alone,  but 
other  structures  have  also  to  be  taken  into  consideration.  Thus  it  is  quite 
possible  that  in  the  Polyzoa  the  approximation  of  the  mouth  and  anus  indi- 
cates simply  a  bending  of  the  digestive  tract  and  a  migration  forwards  of 
the  anus  and  not  necessarily  a  bending  of  the  body  axis;  and  the  varying 
position  of  the  anus  in  the  Ecardinate  Brachiopods  tends  to  support  this 
idea,  the  body  axis  in  Crania  with  a  terminal  anus  certainly  being  similar 
to  that  of  Lingula,  in  which  the  anus  lies  far  forwards.  In  this  connec- 
tion, too,  the  arrangement  in  Sipuneulus  is  of  interest,  the  nerve-cord  show- 
ing the  usual  relations  to  the  body  axis,  while  the  digestive  tract  is  bent 
upon  itself  and  the  anus  opens  far  in  front  of  the  posterior  extremity.  In 
the  Sipunculacea  there  can  be  no  question  of  a  difference  of  the  body  axis 
in  the  various  forms,  and  it  seems  probable  that  the  supposed  bending  of 
the  body  axis  in  tho  Prosopygia  has  not  really  occurred,  but  that  there 


Mi 


274 


INVERTEBRATE  MORPHOLOGY. 


has  been  simply  a  bending  of  the  digestive  tract  and  a  migration  forwards 
of  the  anus. 

If  this  be  the  correct  way  of  regarding  the  matter,  then  there  is  no 
reason  for  disputing  the  homology  of  the  dissepiments  of  the  Brachiopoda 
with  tliose  of  the  Annelida,  and  the  idea  that  they  represent  a  metamerism 
is  borne  out  by  the  arrangement  of  the  two  pairs  of  nephridia  of  Rhyn- 
ckonella.  The  question  is  then,  Does  the  metamerism  of  the  Brachiopods 
indicate  a  descent  of  the  Prosopygia  from  metameric  ancestors,  i.e.,  from 
Annelids  through  Gephyrean-like  forms,  or  is  it  a  structural  feature  inde- 
pendently acquired  by  the  Brachiopods  ?  The  evidence  at  our  disposal  is 
not  sufficient  for  the  solution  of  this  problem,  and  all  that  can  be  main- 
tained is  that  a  very  close  relationship  exists  between  the  Polyzoa  and  the 
Brachiopoda. 

SUBKINGDOM  METAZOA. 

TYPE  PROSOPYGIA. 

I.  Class  Polyzoa.— Small,  usually  colonial  forms ;  lophophore  circular  or 
horseshoe-shaped ;  no  bivalve  shell ;  no  mantle-lobes. 

1.  Order  Endoprocta. — Mouth  and  anus  both  witiiin  tlie  area  enclosed 

by  the  lophophore.  Loxosotna,  Pedicellina,  Ascopodaria^ 
Urnatella. 

2.  Order  Ectoprocta.  — Anus  outside  the  area  enclosed  by  the  lopho- 

phore. 
1.  Suborder  Plujlactolannata. — Fresh-water  forms  ;  lophophore 
usually  horseshoe  shaped  ;    epistome  present.     Erederi- 
cella,  Alcj/onella,  Lophopus,  Cristatella. 
3.  Suborder  Gi/mnoliemata. — Usually  marine  ;    lophophore  cir- 
cular ;  no  epistome. 
Mouth  of  zooecium  without  bristles  or  operculum  {Cyclo- 

stomata).     Crisia. 
Mouth  of  zooecium  usually  surrounded  by  bristles  which 
close  over  it  {Ctetiostomata).    Paludicella,  Alcyonidium. 
Mouth  of  zooecium  provided  with    an  operculum  (Chilo- 
stomata).    Membranipora.  Btiytda,  Flxstra,  Scrupocel- 
laria. 
II.  Class  Brachiopoda. — Non-colonial  and  of  moderate  size ;  lophophore 
usually  arm-like  and  coiled  into  a  spiral ;  mantle-lobes 
and  bivalve  shell  present. 
1.  Order  Ecardiiies.  —  Shell- valves  not  hinged  ;    peduncle  protrudes 
between  the  valves  ;  anus  present.    Lingida,  Crania. 

3.  Order  Testkardines. — Shell-valves  hinged  ;  peduncle  when  present 

protruding  through  perforation  in  the  ventral  shell ;  anus 
wanting.  Terebratula,  Waldheimia,  Argiope,  Rhyncho- 
nella. 


TYPE  PliOSOPYGlA. 


275 


LITERATURE. 

I'OI.YZOA. 

G  J.  Allman.     A  Monograph  of  the  Fresh- water  Polyzoa.     London,  1856. 

T.  Hincks.     A  History  of  the  British  Marine  Polyzoa.     London,  1880. 

H.  Nitsohe.     Beitrdge  zur  Kenntuiss  der  Bryozoen.     Zeitschr.  fUr  wissenscb. 

Zoologie,  XX,  1869  ;  xxi,  1871  ;  xxv,  Suppl.,  1875. 
A.  Hyatt.     Observations  on  Polyzoa.     Subm-der  Phylactokvmata.     Proceedings 

Essex  Institute,  iv  and  v,  1866-68. 
S.  F.  Harmer.     On  the   Structure  and  Development  of  Loxosoma.     Quarterly 

Journ.  Microscop.  Science,  xxv,  1885. 
C.  B.  Davenport.     Papers  in  Bulletin  of  the  Museum  of  Comp.  Zoology,  xx, 

1890  ;  XXII,  1891  ;  and  xxiv,  1893. 


BHACHIOPODA. 

E.  S.  Morse.     On  the  Sydematic  Position  of  the  Brachiopoda.    Proceed.  Boston 

Soc.  Nat.  History,  xv,  1893. 

W.  K.  Brooks.  The  Development  of  Lingula.  The  Chesapeake  Zoolog.  Labo- 
ratory.    Scient.  Results  of  the  Session  of  1878. 

J.  F.  van  Bemmelen.  UntersucJiungen  ilber  den  anatomiscJten  und  histologischen 
Bau  der  Brachiopoda  testicardinia.    Jenaiscbe  Zeitschr.,  xvi,  1883. 

M.  A.  Sohnlgin.  Argiope  Eowalewskii.  Bin  Beitrag  zur  Kenntniss  der  Brachio- 
poden.     Zeitschr.  flir  wissenscb.  Zoologie,  XLi,  1884. 

H.  G.  Beyer.  A  Study  of  the  Structure  of  Lingida  {Glottidia)  pyramidata. 
Studies  from  the  Biolog.  Labor.  Johns  Hopkins  University,  iii,  1886. 

F.  Blochmann.     Untermchungen  Ober  den  Bau  der  Brachiopoden.    Jena,  1893. 


276 


INVERTEBRATE  MORFHOLOOT. 


CHAPTER  XII. 


TYPE    MOLLUSCA. 


- 


While  the  Annelida  are  cliaracterized  by  an  elongated 
form  of  body,  the  Mollusca  present  the  opposite  condition, 
being  compact,  non-metameric  organisms,  though  at  the  same 
time  primitively  bilateral  in  the  arrangement  of  their  organs. 
Upon  the  external  surface  of  the  body  a  cuticular  secretion 
is  formed  in  which  usually  particles  of  carbonate  of  lime 
are  deposited,  a  calcareous  shell  being  thus  developed,  which 
encloses  more  or  less  perfectly  the  soft  body,  assuming, 
however,  very  different  forms  in  the  various  groups.  It  is 
essentially  a  dorsal  structure  developed  in  the  majority  of 
forms  from  a  depression  on  the  dorsal  surface  of  the  body 
— the  shell-gland  (Fig.  122,  /) — and  in  some  forms  may  be 
entirely  confined  to  this  area.  Usually,  however,  a  circular 
or  bilateral  fold  of  the  body,  the  mantle  (c),  arises  peripheral 
to  the  margins  of  the  shell-gland  and  extends  downwards 
towards  the  ventral  surface,  and  the  growth  of  the  shell  may 
accompany  that  of  the  mantle-fold,  so  that  the  entire  body  is 
enclosed  by  or  may  be  retracted  within  the  greatly-developed 
shell.  Even  in  cases,  however,  in  which  the  shell  is  but 
slightly  developed  the  mantle-folds  retain  their  development, 
forming  a  marked  structural  feature  of  the  Mollusca,  and  en- 
closing a  more  or  less  spacious  cavity,  the  mantle-cavity,  in 
■^vhicli  lie  the  respiratory  organs  and  into  which  the  intestine 
and  uephridia  and  reproductive  ducts  open. 

The  body-wall  is  formed  of  an  external  layer  of  ectoderm, 
below  which  a  more  or  less  thick  layer  of  muscle-tissue  is 
found  whose  fibres  sometimes  ihow  the  arrangement  in  cir- 
cular and  longitudinal  layers  characteristic  of  the  Annelida, 
but  usually  the  simplicity  of  this  arrangement  is  interfered 
with  by  a  development  of  connective  tissue  in  which  irregu- 
larly-arranged muscle-bundles  lie.     Upon  the  ventral  surface 


TYPE  MOLLUSC  A. 


277 


of  the  body  there  is  a  special  thickening  of  the  muscle-tissue 
to  form  a  "  foot "  (Fig.  122,  p\  which  assumes  a  great  variety 
of  forms,  and  special  muscles  are  developed  for  its  protrac- 


Fig.  122.— Diagrams  showing  the  Arrangement  of  the  Organs  in  an 

Ideal  Mollusk  (after  lankestek). 

a  =  tentacle.  i  =  ctenidium. 


b  =  head. 

c  =  niargiu  of  mantle 

d  =  margin  of  shell. 

e  =  edge  of  body. 

/  =  edge  of  shell  depression. 

g  =  shell. 

gc  =  cei-ebral  ganglion. 
gpe  =  pedal  ganglion. 
gpl  =  pleural  ganglion. 
?i  r=  osphradium . 


k  =  reproductive  pore. 

I  =  nephrldial  pore. 
m  =  anus. 
n  and  p  —  foot. 

r  =  coelom. 

a  =  pericardium. 

t  =  testis. 
u  =  nephridium. 

V  =  ventricle  of  heart. 
zl  =  liver. 


tion  or  retraction  when  this  is  necessary,  as  well  as  for  the 
closure  of  the  shell  in  those  forms  (Pelecypoda)  in  which  it  is 
a  bilateral  structure. 

The  coelom  is  in  some  forms  a  relatively  spacious  cavity, 
traversed,  however,  even  in  these  cases  by  thin  bands  of  con- 


i' 
1 

1 

1 

i       ; 

1 

t 
1 

i  '' 

^ 

f 

; 

!    ? 

1 
i 

1 

■i 

!l 


i     'Il 


278 


INVEHTEBRATE  MORPHOLOOY. 


nective  tissue,  but  more  usually  it  is  reduced  to  a  system  of 
lacuuar  spaces  (a  so-called  scliizocoel)  by  the  development  of 
muscle-bundles  traversing  it  in  various  directions.  A  special 
portion  of  it  (the  so-called  enterocoel)  is,  however,  always  en- 
closed in  definite  walls  lined  by  a  peritoneal  layer  of  cells, 
forming  a  cavity,  the  pericardium  (Fig.  122,  s),  which  lies  nor- 
mally near  the  dorsal  surface  of  the  body,  containing  the 
heart  and  having  the  inner  ends  of  the  nephridia  (w)  opening 
into  it.  The  blood  vascular  system  consists  of  a  primitively 
three-chambered  heart  (Fig.  122,  f)  enclosed  within  the 
pericardium  and  composed  of  a  tubular  muscular  ventricle 
and  two  wing-like  auricles  which  open  into  the  ventricle, 
their  openings  being  guarded  by  valves  which  prevent  regur- 
gitation. From  the  anterior  and  posterior  extremities  of  the 
ventricle  aortte  arise,  which,  however,  as  a  rule  soon  lose 
themselves  in  the  coelomic  lacunse.  There  is  thus  no  distinc- 
tion between  the  blood  and  pseud-haemal  fluids  in  the  Mol- 
lusca,  since  the  blood  vascular  system  is  not  closed.  The 
blood  is  a  colorless  fluid  in  which  numerous  amoeboid  cells 
float  and  which  holds  in  solution  a  substance,  haemocyauin, 
which  subserves  a  respiratory  function  in  a  manner  similar 
to  the  haemoglobin  of  the  Vertebrata. 

The  heart  is  a  systemic  heart,  as  is  usual  in  the  Invertebrata,  and  con- 
tains only  aerated  blood,  whicli  it  propels  through  the  lacunae  of  the  body. 
Returning  from  these,  the  blood  passes  either  directly  to  the  respiratory 
organs  or  branchiae,  or  else  a  greater  or  less  portion  of  it  traverses  first  the 
walls  of  the  nephridia  and  then  passes  to  the  branchiae.  From  these,  in 
which  it  is  aerated,  it  is  received  into  the  auricles,  and  on  their  contraction 
is  forced  into  the  ventricle. 

In  some  Mollusca  respiration  is  carried  on  by  the  general 
surface  of  the  body,  but  such  an  arrangement  must  be  re- 
garded as  the  exception.  As  a  rule  special  respiratory  or- 
gans are  present  in  the  form  of  one  or  more  pairs  of  plume- 
like processes  {denidia)  of  the  body-wall  (Fig.  122,  i)  lying 
free  in  the  mantle-cavity.  They  have  various  forms  in  the 
different  groups,  but  consist  essentially  of  a  central  axis  con- 
taining an  afferent  and  efferent  canal  for  the  blood  and  bear- 
ing a  single  or  double  series  of  filaments  whose  walls  are  thin 
and  whose  ectoderm  is  ciliated,  an  interchange  of  the  gases 


TYPE  M0LLU8CA. 


279 


of  the  blood  with  those  of  the  water  contained  in  the  mantle- 
cavity  being  thus  readily  effected,  a  renewal  of  the  water  con- 
stantly taking  place  in  consequence  of  the  action  of  the  ecto- 
dermal cilia.  The  tliiu-walled  mantle-fold  is,  however,  a  very 
efficient  adjunct  to  the  branchias  in  respiration,  the  spaces 
within  the  fold  being  portions  of  the  lacunar  coelora  and  con- 
sequently containing  blood ;  indeed  in  some  cases  the  mantle 
assumes  completely  the  respiratory  function,  the  ctenidia 
becoming  rudimentary. 

The  digestive  tract  is  a  usually  more  or  less  coiled  or  con 
voluted  tube  in  which  various  regions  may  be  distinguished. 
In  a  few  forms,  characterized  either  by  the  slight  develop- 
ment of  the  mantle  or  its  development  as  two  lateral  folds, 
the  anus  is  terminal  in  position,  but  when  an  extensive  cir- 
cular mantle-fold  is  developed  the  intestine  bends  upon  itself 
and  opens  upon  the  side  of  the  body,  more  or  less  anteriorly, 
into  the  mantle-cavity.  Immediately  behind  the  mouth  chi- 
tinous  teeth  (Fig.  123,  lij)  are  usually  developed  in  the  wall  of 

8t 


Fig.  133. — Buccal  Mass  A>rD  Radula  op  Helix  (after  Howes). 

ce  =  cerebral  gnnglion.  re  =  nidular  cartilage. 

hj  =  boroy  jaw.  rd  =  radula. 

im  =  intrinsic  muscles.  rd'  =  radular  sac. 
pgl  =  pedal  gland.  st  =  opening  of  salivary  gland. 

the  pharynx,  and  behind  these  a  large  muscular  thickening 
generally  occurs,  the  buccal  mass,  in  connection  with  which 
is  developed  a  characteristic  Molluscan  structure,  the  lingual 
ribbon  or  radula  {rd).  The  sides  and  floor  of  the  pharynx  in 
this  region  are  largely  thickened  by  the  development  in  them 
of  muscular  tissue  {im).  The  thickening  of  the  floor  is  usually 
so  extensive  as  to  project  into  the  pharyngeal  cavity,  forming 


280 


IN  VERTEBRA  TE  MORPIIOLOO  7. 


•I   i 


the  so-called  tongue,  and  iu  addition  to  the  muscular  tissue 
two  or  more  pieces  of  cartilage,  the  radular  cartilages  (I'c),  are 
frequently  found  iu  it.  Covering  the  tongue  is  a  stout  chi- 
tiuous  membrane,  the  basal  membrane,  which  bears  upon 
its  surface  a  usually  enormous  number  of  chitiuous  teeth 
arranged  in  transverse  rows,  so  that  the  basal  membrane 
and  the  teeth,  together  constituting  the  radula  {rd),  recall 
somewhat  the  appearance  of  a  flat  tile.  Behind  the  tongue 
the  floor  of  the  pharynx  is  produced  downwards  and  back- 
wards into  a  pouch,  the  radula-sac  (rd'),  sometimes  of  con- 
siderable length,  into  which  the  radula  is  continued  on 
the  ventral  wall,  the  cells  (odontoblasts)  which  form  the  teeth 
as  a  cuticular  secretion,  lying  at  the  bottom  of  the  sac.  The 
tongue,  with  its  radula,  can  be  protruded  to  a  greater  or  less 
extent  from  the  mouth  bj'  special  protractor  muscles,  and  its 
intrinsic  muscles  serve  to  give  it  a  slow  licking  movement, 
whereby  the  radula  acts  as  in  the  manner  of  a  file  or  rasp 
upon  the  object  with  which  it  is  in  contact.  Owing  to  this 
action  the  radula  is  continually  being  worn  away  at  its  anterior 
end,  but  is  also  continually  being  pushed  forward  upon  the 
tongue  by  the  addition  of  new  teeth  to  its  posterior  portion 
at  the  base  of  the  radula-sac. 

In  connection  with  the  digestive  tract  various  glands  are 
usually  present,  of  which  the  most  constant  are  the  salivary 
glands  and  the  "  digestive  glands."  The  former  open  into 
the  pharynx  and  in  some  cases  reach  extensive  devolopment ; 
their  function  for  the  most  part  is  but  little  understood,  but 
in  some  predaceous  Gasteropods  their  secretion  has  been 
found  to  contain  a  considerable  amount  of  free  sulphuric  acid 
which  probably  serves  to  soften  the  calcareous  shell  of 
Echinoderms  and  other  Mollusca  which  serve  these  forms  as 
food.  The  "  digestive  glands  "  open  into  a  dilated  portion  of 
the  intestine,  usually  termed  the  stomach,  and  are  usually 
paired,  voluminous,  much-branched  tubular  glands  whose 
function  is  indicated  by  the  name  applied  to  them.  They 
seem  to  be  the  physiological  representatives  of  the  pancreas 
of  the  Vertebrata,  and  to  secrete  digestive  ferments  which 
are  brought  into  contact  with  the  food  in  the  stomach. 

The  nervous  system  of  the  Mollusca  (Fig.  124),  in  accord- 


etwu«i''«*^ 


TYPE  MOLLUSCA. 


281 


lorms  as 


auc'3  with  the  absence  of  metamerisiu ,  lacks  the  ladder-like 

arrangement  which  characterizes  the  Annelida.     Nevertheless 

there  are  two  ganglionic  masses,  each  in  typical  cases  com- 

posed  of  two  ganglia  which  may  he  honiologizud  with  the  su- 

praoesophageal  and  the  most  anterior  sulxKsophageal  ganglia 

of  the  metameric  forms,  and  are  known  respectively  as  the 

cerebral  (Fig.  124,  ce)  and  pedal  (pe)  ganglia.     The  former  lies 

above  the  tL^sophagus  behind  the  buccal 

mass    and  is  connected  by    nerve-cords 

termed  connectives,  surrounding  the  ceso- 

phagus,  with   the   pedal   ganglion.     The 

cerebral  ganglion  gives  off  nerves  which 

pass  to  the  eyes  and  otocysts  (of)  and  to 

the   tentacular   structures   of   the  head, 

while   the   pedal    ganglion    receives   its 

name  from  the  fact  that  it  sends  nerves 

to  the  muscular  mass  forming  the  foot. 

In  addition  to  this  system  of  nerves  and 

ganglia  there   is   another  system  highly  p^^  i24.-Diaoram  of 

developed  in  the  Mollusca  which  would 

seem  to  correspond  to  the  visceral  system 

found  in  some  other  forms.     It  consists  bu  =  buccal  ganglia. 

typically  of  a  pair  of  pleural  ganglia  (pi),    '^  =  ««'"'-''^''*'  ganglion. 

-,•11.  .,1  . 1       «  ,,        ot  =  otocyst. 

one  of  which  lies  upon  either  side  of  the  ^^  ^  p^,.^^^,  g^^gij^„ 

pharynx,    being   united    by    connectives   pg  =  pedal  ganglion. 
with  both  the  cerebral  and  pedal  ganglia,    pi  =  pleural  gaugliou. 
From  each  pleural  ganglion  a  nerve-cord    «»  =  ^i^^eral  ganglion, 
passes  backwards  to   unite  with  one  or 
more  visceral  ganglia  (vi),  situated  below  the  intestine  near  its 
posterior  termination,  and  on  each  of  these  visceral  cords  a 
ganglion  occurs,  the  parietal  gun^liou  {pa),  from  which  nerves 
pass  to  the  gills,  or  rather  to  the  sense-organ  which  is  in  con- 
nection with  them.     The  pleural  ganglia  innervate  especially 
the  mantle  and  the  body-wall  behind  the  head,  the  visceral 
ganglia  send  branches  principally  to  the  various  viscera,  while 
the  parietal  ganglia,  in  addition  to  the  branches  which  go  to 
the  gills  and  their  sense-organs,  also  assist  in  the  innervation 
of  the  mantle. 

Besides  these  principal  ganglia,  however,  others  connected 


Nervous  Bystkm  of 

MOLLUSK. 


1^ 


282 


INVEHTEBRATE  MORPIIOLOQ Y. 


Uw 


with  either  the  cerebro-pedal  or  pleuro-visceral  system  may 
be  developed,  the  most  coustaut  of  which  are  the  buccal  guu- 
glia  (pu)  which  lie  at  the  sides  of  or  more  usually  below  the 
buccal  mass  which  they  iuuervate  aud  are  united  by  commis- 
sures with  the  cerebral  ganglia.  Two  nerve-rings  in  such 
cases  surround  the  (esophagus,  i.e.,  that  formed  by  the  cere- 
bro-pedal and  that  of  the  cerebro-buccal  connectives. 

This  description  has  reference  only  to  what  may  be  con- 
sidered a  typical  condition,  and  it  must  be  remembered  that 
frequent  modifications  of  it  may  occur.  In  the  Gasteropods, 
for  example,  in  Avhich,  in  accordance  with  the  development  of 
a  circular  mantle-fold,  the  anus  comes  to  lie  on  the  anterior 
portion  of  the  body-wall,  a  peculiar  crossing  of  the  pleuro- 
visceral  commissi"*°is  occurs  in  some  cases,  and  as  a  result 
what  was  originally  the  right  parietal  ganglion  comes  to  lie 
upon  the  left  side  of  the  body  and  the  original  left  ganglion 
upon  the  right  side.  Further  consideration  of  this  arrange- 
ment may,  however,  be  postponed  until  the  Gasteropods  are 
under  discussion.  Mention  should,  however,  be  made  here 
of  another  not  unfrequent  modification  of  the  typical  arrange- 
ment of  the  nervous  system,  which  consists  in  the  concentra- 
tion of  the  ganglia  and  the  shortening  of  the  various  connec- 
tives. This  may  affect  only  the  cerebral,  pedal,  and  pleural 
ganglia,  bringing  them  into  close  approximation,  or,  as  in  some 
Cephalopods,  the  visceral  ganglia  may  also  be  carried  forward 
so  that  all  the  principal  ganglia  are  united  into  a  single  lobed 
mass  closely  surrounding  the  oesophagus  behind  the  pharynx. 
This  condition  constitutes  of  course  the  culmination  of  the 
concentration  process,  but  variou:,  gradations  of  it  are  to  be 
found  in  the  different  groups. 

Sense-organs  are  as  a  rule  well  developed  in  the  Mollusca, 
and  descriptions  of  many  of  them  may  be  more  conveniently 
given  in  connection  with  the  detailed  account  of  the  various 
groups.  The  general  ectoderm  of  the  mantle  and  body-wall 
has  scattered  in  it  numerous  sensory  cells  which  may  become 
specially  aggregated  at  certain  points  to  form  definite  sense- 
organs.  Thus  tentacles  are  frequently  borne  upon  the  head 
which  are  tactile  or  in  some  cases  olfactory  in  nature,  and  at 
the  bases  of  the  gills  special  aggregations  of  sensory  cells  are 


iu 


TYPE  M0LLU8CA. 


283 


125.-  Otocyst    of    Pteio- 
trachea  (after  Ulaus). 


to  be  found  forming  the  osphradia,  also  supposed  to  liave  an 
olfactory  function.  Otocysts  (Fijj;.  125)  are  present  in  nearly 
all  the  groups,  consisting  of  a  vesicle  with  a  membranous  wall, 
the  interior  of  which  is  lined  by  sensory  cells  bearing  bunches 
of  hairs  projecting  into  the  vesicle 
which  contains  one  or  more  calca- 
reous otoliths.  An  auditory  func- 
tion has  usually  been  attributed  to 
these  organs,  but  it  seems  probable 
that,  as  in  the  lower  forms  (see  p. 
82),  they  are  rather  to  be  regarded 
as  organs  of  an  equilibrium -sense, 
and  in  fact  that  they  subserve  such 
a  function  in  part  at  least  has  been 
experimentally  determined  in  the 
Cephalopods. 

Eyes  are  very  frequently  present 
aiid  in  the  Cephalopods  reach  an 
exceedingly  high  development.  They  occur  usually  upon  the 
head,  but  may  also  be  found,  as  in  the  Pelecypoda,  upon  the 
edge  of  the  mantle,  or  even  on  the  dorsal  surface  of  the  body, 
as  in  the  Pulmonate  Onchidivm.  They  vary,  however,  so 
much  in  structure  in  diflerent  groups  that  an  account  of  the 
various  modifications  which  they  present  may  be  postponed. 

Excretory  organs  in  the  form  of  a  pair  of  nephridia  are 
present,  each  nephridium  consisting  (vf  a  tube  which  opens  at 
one  extremity  into  the  mantle-cavity,  while  at  the  other  it 
communicates,  with  the  cavity  of  the  pericardium,  which,  as 
has  been  seen,  is  a  portion  of  the  cislom.  The  relationships 
of  these  structures  are  therefore  the  same  as  those  of  the 
nephridia  of  the  Annelids,  and,  as  in  those  forms,  they  receive 
a  rich  supply  of  blood,  most  of  the  venous  blood  returning  from 
the  tissues  passing  through  their  walls  on  its  way  to  the 
branchiae.  The  reproductive  organs  are  unpaired  in  the 
majority  of  forms  and  in  some  cases  come  into  relation  with 
the  nephridia,  which  seive  as  reproductive  ducts.  More 
usually,  however,  they  open  direct^/  to  the  exterior,  a  con- 
dition which  is  probably  a  secondary  one.  The  majority  of 
the  Mollusca  are   bisexual,  but   hermaphroditism  is  by  no 


284 


IN VERTEBHA  TE  MOlilUlOLOG  Y. 


•i     .', 


f 


yi 


meaus  uucommon,  the  single  reproductive  glaud  produciug 
both  ova  and  spermatozoa  and  being  therefore  au  ovo-testis. 
Accessory  structures  are  frequently  added  to  the  essential 
parts  of  the  rep.oductive  apparatus,  such,  for  instance,  as  al- 
bumiuiparous  glands,  introniittent  organs,  spermatophore- 
sacs,  etc.,  so  that  a  relatively  complicated  arrangement  may 
occur. 

T.  Class  Ahphineuba. 

The  Amphineura  are  Mollusca  in  which  the  primitive  bi- 
lateral symmetry  is  fully  retained  and  which  seem  to  approach 
most  nearly  to  what  may  be  considered  the  primitive  MoUuscan 
condition.  All  the  known  members  of  the  group  are  marine 
in  habitat  and  are  more  or  less  elongated  forms  in  which  the 
elongation  of  the  ventral  surface  or  foot  is  accompanied  by  a 
corresponding  elongation  of  the  visceral  complex,  which  ac- 
cordingly is  not  elevated  at  right  angles  to  the  long  axis  of 
the  foot  to  form  a  visceral  dome.  In  a  general  way,  therefore,  in 
the  form  of  their  body  the  Amphineura  may  be  compared  to  the 
Platyhelmiuths,  especially  to  such  forms,  sometimes  flattened, 
sometimes  more  or  less  cylindrical  and  elongated,  as  are  found 
among  the  Polyclad  Turbellaria.  The  mouth  and  anus  are  situ- 
ated at  the  extremities  of  the  body,  and  to  either  side  of  the  anus 
are  situated  the  one  or  more  pairs  of  plumelike  brauchia)  and  the 
openings  of  the  single  pair  of  uephridia.  The  shell  may  con- 
sist either  of  a  number  of  scattered  calcareous  spicules  im- 
bedded in  or  projecting  from  a  thick  cuticle,  or  else  may  take 
the  form  of  a  number  of  plates  arranged  in  a  longitudinal 
series  upon  the  dorsal  surface  of  the  body,  and  as  a  rule  the 
mantle-fold  is  but  slightly  developed  and  may  be  in  some  forms 
almost  rudimentary.  The  foot,  too,  which  is  so  characteristic 
for  the  Mollusca,  may  in  some  forms  be  practically  un- 
developed, but  in  other  forms  is  a  broad  flat  muscular  surface, 
showing  no  diflferentiatiou  into  special  regions  such  as  are 
found  in  the  higher  Mollusca. 

Little  need  be  said  here  as  to  the  internal  organs  except  to 
emphasize  the  fact  that  both  the  heart  and  the  nephridia  have 
a  perfectly  bilateral  arrangement.  The  nervous  system  is 
characterized  by  the  absence  of  a  deflnito  aggregation  of  the 


TYPE  MOLLUSCA. 


285 


nerve-cells  iuto  coucreto  gangliii ;  they  are  scattered  aloug  the 
lougitudinal  norve-cords,  of  wliich  there  are  two  pairs,  i.e.,  the 
pleuro-visceral  cords,  which  riiu  aloug  the  lateral  portions  of 
the  body,  and  the  pechd  cords,  which  arc  situated  more 
veutrally  and  M'hich,  as  well  as  the  pleuro-visceral,  are  fre- 
quently united  by  cross-couimissuros  which  suggest  an  imper- 
fect metamerism.  In  front  those  cords  unite  together  to  form 
the  circumcesophageal  ring  in  which  the  gangliou-cells  are 
somewhat  more  numerous  tlian  elsewhere,  witht)ut,  however, 
forming  distinct  ganglia.  Henso-organs  are  but  slightly  de- 
veloped throughout  the  group,  wliich  is  divisible  iuto  two  well- 
nuirked  orders. 


Fig.    Vi^.—Nvomcnia  ca- 
riiutta  (Hi'tiT  Nanskn). 

et  —  (ieiiuliiim. 

7«  =  uioutli. 

vg  =  vuntrul  groove. 


1.  Order  Solenogastres. 

The  members  of  this  order  are  for  the  most  part  elongated 
worm-like  animals,  though  some  forms  are  short  (Fig.  12()) 
and  more  nearly  approach  the  typical 
Molluscan  form.  The  exterior  of  the 
body  is  covered  by  an  exceptionally 
thick  cuticle  traversed  by  bands  of 
cells  extending  into  it  from  the  ecto- 
dermal layer  of  the  body  and  termina- 
ting in  cup-shaped  groups  of  cells 
which  secrete  the  calcareous  spicules 
which  are  scattered  through  the  cuti- 
cle {Proneomenia)  or  may  project  upon 

its  surface  {Cha'hxlerma),  and  which  are  the  sole  repre- 
sentatives of  the  shell  of  the  higher  Mollusca.  U])on  the 
ventral  surface  of  the  body  is  a  longitudinal  furrow  (Fig.  12(), 
vg)  at  the  bottom  of  which  lies  tlie  but  slightly  developed 
foot,  represented  by  a  small  ciliated  longitudinal  ridge,  which  in 
Cha'toderma  may  be  quite  undeveloped,  the  furrow  being  in  this 
form  also  barely  indicated  or  entirely  absent.  The  lips  of  the 
furrow  which  enclose  the  foot  probably  represent  the  mautle- 
folds  of  higher  forms,  here  very  much  reduced,  though  more 
extensively  developed  at  the  posterior,  end  of  the  body,  where 
they  project  to  form  a  funnel-like  structure  (Fig.  127)  whose 
cavity — the    cloaca — receives    the   openings  of  the  digestive 


111 


i-'ii 


286 


m VERTEBRATE  MORPHOLOO  7. 


Fig.    137.  —  Diaguam   op   An- 

KANGBMENT     OF     OUGANS     AT 

Hind    End    of     Chmtoderma 
(after  Hubkecht  from  Lankesteb). 

ct  =  cteiiidium. 
n  =  nepliiidium. 
o  =  ovary. 
p  =  pericardium. 
r  =  rectum. 


tract  (r)  and  the  nepbridia  (n)  and  contains  the  branchiae  (ct). 
These  last  are  either  a  single  pair  of  structures  each  consisting 

of  a  central  axis  with  piunately- 
arranged  lateral  appeudages  or  in 
some  cases  are  represented  by 
bunches  of  ciliated  filaments. 

The  ectoderm  rests  upon  a 
layer  of  muscular  tissue  in  which 
both  circular  and  longitudinal 
layers  can  be  distinguished,  and 
numerous  bauds  of  transverse 
fibres,  in  some  cases  arranged  to 
form  septa  placed  at  regular 
intervals,  traverse  the  body- 
cavity.  A  fairly-capacious  peri- 
cardium is  present,  lying  dorsally 
to  the  posterior  portion  of  the  digestive  tract  and  into  its 
upper  portion  the  heart  projects  slightly,  not  being,  however, 
completely  enclosed  by  the  pericardium.  No  auricles  seem 
to  be  developed,  nor  are  any  definite  blood-vessels  present, 
the  circulation  being  throughout  lacunar. 

This  condition  of  the  heart  in  relation  to  the  pericardium  is  interesting 
as  showing  its  original  independence  of  that  portion  of  the  body-cavity. 
Its  enclosure  in  the  pericardium  in  the  higher  Mollusca  is  a  secondary  con- 
dition, the  heart  and  its  cavity  belonging  to  the  schizocoelic  structures 
ratlier  than  to  the  so-called  enterocoelic  pericardium.  This  agrees  perfectly 
with  the  relationships  of  the  blood  vascular  system  of  the  Nemerteans  and 
Annelids.     (See  pp.  165  and  231.) 

The  mouth  is  a  longitudinal  slit  upon  the  ventral  surface 
of  the  body  and  opens  into  a  pharynx  provided  usually  with  a 
radula  and  with  salivary  glands,  though  both  these  structures 
are  absent  in  Neomenia.  The  intestine  pursues  a  straight 
course  towards  the  anal  opening,  being,  however,  in  some 
forms  pouched,  owing  to  its  constriction  at  more  or  less  regu- 
lar and  close  intervals  by  muscular  transverse  septa.  The 
walls  of  the  pouches  thus  formed  are  glandular  and  represent 
the  digestive  gland  of  other  Mollusca,  though  in  Chcetoderma 
there  is  a  simple  outgrowth  of  the  digestive  tract  which  rep- 
resents it  more  perfectly. 


n  I 


TYPE  MOLLUSC  A. 


287 


The  nervous  system  varies  iu  the  details  of  its  arrange- 
meut  iu  the  different  species,  but  is  characterized  iu  general 
by  a  tendency  to  form  ganglia,  although  nerve-cells  are  scat- 
tered along  the  nerve-cords  throughout  their  entire  length. 
In  Proneomenia  there  is  present  a  well-developed  and  closely- 
approximated  pair  of  cerebral  ganglia  from  which  arise  the 
pleuro-visceral  cords  which  extend  backward  along  the  sides 
of  the  body  and  possess  a  number  of  ganglionic  swellings 
near  their  posterior  extremity.  Two  nerve-rings  surround  the 
cesophagus :  (1)  the  cerebro-pedal  connectives,  which  end 
below  in  the  pedal  ganglia,  from  which  two  pedal  cords  extend 
backward  along  the  foot,  in  some  forms  (Dondersia)  connected 
at  regular  intervals  by  transverse  commissures  in  an  almost 
metameric  manner,  ganglionic  enlargements  of  the  cords  being 
developed  in  connection  with  the  commissures ;  and  (2)  the 
cerebro-buccal  connectives,  which  pass  to  two  buccal  ganglia 
lying  below  the  pharynx.  Special  sense-organs  have  not  yet 
been  discovered  in  the  Solenogastres. 

The  nephridia  consist  of  a  pair  of  tubes  which  communi- 
cate internally  with  the  pericardial  cavity  and,  bending  around 


Fig.  128.— Diagrammatic  Longitudinal  Section  of  Chiton  (after  Hallkr). 

e  =  perivisceral  coelom.  n  =  nerve. 

7i  =  heart.  p  =  pericardium. 

m  =  mouth.  ro  =  reproductive  organ. 

1-8  =  sbell-plates. 

the  posterior  part  of  the  digestive  tract,  unite  to  open  into  the 
cloaca  ventral  to  the  anus  by  a  common  orifice.  The  walls  of 
the  tubes  are  glandular  and  probably,  therefore,  excretory  in 
function,  but  the  nephridia  also  serve  as  the  ducts  for  the 
reproductive  elements.  With  the  exception  of  Chcetoderma  the 
Solenogastres  are    hermaphrodite,   the   single   reproductive 


!    I 


11 


288 


INVERTEBRATE  MORPHOLOGY. 


! 


I       1  ::       •> 


I 


gland  produciug  both  ova  and  testes.  This  lie rm aphrodite 
gland  is  a  hollow  sac  divided  into  two  principal  compart- 
ments by  a  longitudinal  partition  and  lies  above  the  digestive 
tract.  It  is  a  hollow  structure  (Fig.  128,  ro\  the  reproductive 
elements  developing  from  the  cells  lining  its  walls  and  pass- 
ing from  its  cavity  into  that  of  the  pericardium  {p),  with 
which  the  reproductive  sacs  communicate.  They  are  in  fact 
simply  prolongations  of  the  pericardial  body-cavity,  and  the 
epithelium  lining  them  is  continuous  with  that  of  the  pericar- 
dium. From  the  pericardial  cavity  the  ova  and  spermatozoa 
pass  to  the  exterior  by  the  nephridia. 

The  Solenogastres  are  especiallj'  interesting  on  account  of  the  many 
structural  peculiarities  of  a  primitive  character  which  they  present  and  in 
consequence  of  which  they  have  been  regarded  as  representatives  of  ances- 
tral Molluscan  forms.  By  others,  however,  this  important  position  is 
denied  them  on  the  ground  that  many  of  their  peculiarities  are  due  to 
degeneration  produced  in  accordance  with  their  life  in  the  mud  at  the  bot- 
tom of  the  ocean.  The  absence  of  a  shell,  the  reduction  of  the  mantle- 
lobes,  foot,  and  radula  may  with  plausibility  be  accounted  for  in  this 
manner,  but  there  are  other  peculiarities  that  are  certainly  primitive 
which  are  not  thus  explicable.  The  relation  of  the  heart  to  the  pericardium 
is  one  of  these,  and  others  are  the  communication  of  the  hermaphrodite 
gland  with  the  pericardium,  and  the  functioning  of  the  nephridia  as  ducts 
for  the  reproductive  organs.  The  Solenogastres  are  unquestionably  primi- 
tive MoUusca;  the  only  question  which  is  yet  to  be  settled  is  to  what  extent, 
if  any,  degeneration  is  responsible  for  their  external  peculiarities,  such  as 
the  absence  of  a  shell,  the  reduction  of  the  mantle-lobes  and  of  the  foot. 
It  must  be  noted  in  this  connection  that  one  form  belonging  to  the  genus 
Dondersia  has  been  described  as  passing  through  in  its  development  a 
stage  in  which  indications  of  a  shell  consisting  of  several  plates  and  simi- 
lar to  that  of  the  Polyplacophora  was  present,  a  condition  which  would 
seem  to  indicate  the  derivation  of  the  members  of  this  group  from  forms 
provided  with  a  distinct  shell. 

2.  Order  Polyplacophora. 

The  Polyplacophora,  like  the  preceding  order,  contains 
only  marine  forms.  For  the  most  part  they  are  somewhat  flat- 
tened animals  with  a  rather  broad  foot  occupying  the  ventral 
surface,  while  from  the  sides  of  the  body  a  slight  fold,  the 
mantle-fold,  projects.  In  one  genus,  CMtonellus,  the  form  of 
the  body  is  more  cylindrical  and  the  foot  is  rather  narrow 


TYPE  M0LLU8CA. 


289 


■  1 


Fig.  129.— ChoBto- 
pleura  apiculata. 


and  situated,  as  in  the  Solenogastres,  at  the  bottom  of  a 
median  ventral  furrow,  the  lips  of  which  correspond  to  the 
more  dorsally  situated  mantle-folds  of  such  forms  as  Chiton^ 
Trachydermon  (Fig.  129),  etc.  In  all  cases,  in  the  groove  be- 
tween the  mantle-folds  and  the  foot  a  number  of  gills,  pinnate 
processes  of  the  body-wall,  are  to  be  found,  in  some  cases 
occurring  at  definite  intervals  along  the  entire  side  of  the 
body,  in  others  {Chitonellus)  limited  to  the  posterior  part  only. 

One  of  the  most  characteristic  features  of  the 
Polyplacophora  is,  however,  the  shell,  which 
consists  of  eight  calcareous  plates  arranged  in 
a  longitudinal  series  along  the  dorsal  surface 
of  the  body  so  that  the  posterior  border  of  one 
overlaps  the  anterior  border  of  the  other. 
The  series  covers  only  the  median  portion  of 
the  surface,  the  more  peripheral  portions  and 
the  outer  surface  of  the  mantle-lobes  possess- 
ing a  large  number  of  scattered  spicules, 
plates  or  granules  imbedded  in  their  wall. 

The  body-wall  has  not  so  definite  an  arrangement  of  the 
muscle-fibres  lying  below  the  ectoderm  as  is  the  case  in  the 
Solenogastres,  but,  on  the  other  hand,  the  body-cavity  is  well 
developed.  Indeed  the  schizocoelic  lacunae  play  a  rather  sub- 
ordinate part  in  the  Chitonidae,  as  the  order  is  sometimes 
termed,  the  enterocoelic  cavity  (Fig.  128)  being  very  large  and 
divisible  into  three  usually  separated  parts  united  by  bauds, 
which  indicate  the  original  continuity.  One  of  the  parts  (c) 
surrounds  the  intestine  and  the  digestive  gland ;  another,  Ij'ing 
rather  towards  the  anterior  end  of  the  dorsal  portion  of  the 
body,  contains  the  reproductive  cells  (ro);  while  the  third  part 
(p),  lying  dorsally  and  posteriorly,  is  the  so  called  pericar- 
dium. 

The  two  auricles  of  the  heart  are  elongated  tubes  which 
open  about  the  middle  of  their  length  into  the  single  ventricle 
and  also  unite  together  posteriori}',  tlie  ventricle,  also  an 
elongated  tube,  again  communicating  with  this  united  portion. 
Anteriorly  the  ventricle  is  continued  into  a  short  aorta  from 
which  the  blood  passes  to  the  lacunar  spaces  of  the  schizocoel. 
Two  vessels  with  distinct  walls  run  longitudinally  in  the  foot, 


■I 

i 


i  e 


\i   V 


290 


INVEHTEBBATE  MORPHOLOGY. 


and  presumably  receive  the  blood  which  they  coutain  more 
or  less  directly  from  the  aorta  and  distribute  it  to  the  lacuuar 
spaces  of  the  foot. 

The  mouth  lies  on  the  ventral  surface,  in  front  of  the 
anterior  end  of  the  foot,  and  leads  into  a  pharynx  provided 
with  a  well-developed  radula  characterized  by  a  somewhat 
complex  arrangement  of  the  teeth.  Into  the  oesophagus 
a  pair  of  glands  opens  in  Chiton  whose  secretion  contains  an 
amylolytic  ferment,  and  in  addition  a  pair  of  small  glands 
open  into  the  mouth-cavity.  The  oesophagus  communicates 
with  a  sac-like  stomach,  into  which  open  the  ducts  of  the 
paired  digestive  gland,  and  the  intestine,  being  considerably 
longer  than  the  body,  is  thrown  into  numerous  coils,  and 
terminates  by  a  short  rectum  which  opens  at  the  posterior 
extremity  of  the  body. 

The  nervous  system  is  characterized  by  the  diffuse 
arrangement  of  the  nerve-cells,  no  well-defined  ganglia  oc- 
curring on  the  principal  nerve-cords.  These  consist  of  a 
strong  circumoesophageal  ring  (Fig.  130),  the  upper  part  of 
which  gives  off  numerous  nerves  and  evidently  corresponds 
to  the  cerebral  ganglia  of  other  Mollusca,  while  the  lower 
part,  corresponding  to  the  pedal  ganglia,  gives  rise  to  two 
nerve-cords  (pc),  the  pedal  nerves,  which  pursue  a  parallel 
course  throughout  the  foot,  giving  off  a  number  of  nerves 
laterally  and  being  connected  by  a  number  of  somewhat  irreg- 
ularly arranged  transverse  commissures,  which  almost  suggest 
a  raetameric  arrangement.  From  the  sides  of  the  circum- 
oesophageal ring  two  other  strong  nerves,  the  pleuro-visceral 
nerve-cords,  arise  and  pass  backwards  along  the  sides  of  the 
body,  uniting  with  each  other  posteriorly  above  the  terminal 
portion  of  the  digestive  tract.  These  cords  (jf>Z),  like  the  cir- 
cumoesophageal ring,  present  no  distinct  ganglionic  enlarge- 
ments, but  contain  the  elements  of  the  pleural,  visceral,  and 
parietal  ganglia,  sending  off  numerous  nerves  to  the  brauchise, 
the  mantle,  and  probably  also  to  the  heart  and  nephridia. 

In  addition  to  these  principal  nerve-cords  others  of  smaller 
size  also  arise  from  the  circumoesophageal  ring.  One  pair  of 
these  pass  to  a  pair  of  ganglia,  the  buccal  ganglia,  lying 
beneath  the  buccal  mass  and  send  nerves  to  the  oesophagus 


:^  _,,*>*«:.= 


TYPE  MOLLUSCA. 


291 


while  another  pair  pass  to  a  pair  of  ganglia  lying  below  the 
radula  and  in  intimate  connection  with  a  peculiar  subradular 
organ,  probably  sensory,  lying  in  this  region. 


Fig.  130.— Diagram  of  Nervous  and  Excretory  Systems  of  Ohiton  siculus 

(cuiubination  of  two  ilgures  by  Haller). 

no  =  nephridial  orifice. 


an  —  anus. 


Br  =  cteuidia. 
go  =  gei)ital  orifice. 
n  —  nephridiuiu. 


oe  =  oesophagus. 

pe  =  pedal  nerve  cord. 

pi  =  pleural  nerve-cord. 


As  regards  sense-organs,  in  addition  to  this  subradular 
organ  whose  function  is  entirely  problematical,  ridges  of 
sensory  epithelium  exist  along  the  sides  of  the  body  in  the 
mantle-cavity.  One  such  ridge  runs  along  the  inner  wall  of 
the  mantle-fold,  while  the  other  is  found  at  tiie  bottom  of 
the  mantle-cavity  passing  over  the  bases  of  the  branchial 
plumes  and  sending  a  short  prolongation  outwards  upon  each 
of  these  structures  and  seeming  thus  to  correspond  with  the 
osphradia  of  other  Mollusca. 


|i 


"*! 


292 


INVERTEBRATE  MORPHOLOGY. 


-I 
[til 


%'■ 


I 


A  much  more  peculiar  series  of  organs,  found,  however,  iu 
their  perfect  form  only  in  certain  species,  is  developed  in 
connection  with  the  shell  of  the  ChitonidsB.  They  consist  of 
club-shaped  structures  contained  in  pores  which  traverse  the 
shell-plates  and  possess  a  definite  arrangement,  being  ar- 
ranged in  groups  of  larger  and  smaller  organs  {megalcesthetes 
and  micrcestheten).  Each  group  is  in  connection  with  a  num- 
ber of  large  glaudlike  cells,  which  terminate  in  the  megal- 
testhete,  covered  externally  by  a  cup-shaped  laj'er  of  chitiu, 
and  from  this  cell-mass  more  or  less  numerous  branches 
arise,  the  micrsesthetes,  which  terminate  in  club-shaped 
swellings  likewise  covered  by  a  chitinous  layer.  Below  the 
group  of  cells  is  in  connection  with  fibrils  which  unite  to 
form  a  nerve  probably  passing  to  the  pleuro-visceral  nerve- 
cords,  and  it  thus  seems  tolerably  certain  that  these  struc- 
tures are  sensory  and  perhaps  tactile  in  function.  In  some 
species  the  megalsesthetes  become  modified  into  eyes  consist- 
ing of  an  external  convex  chitinous  cap,  the  cornea,  below 
which  is  a  lens  and  below  this  a  layer  of  retina-cells  con- 
nected with  nerve-fibrils  and  surrounded  by  a  cup  of  pig- 
ment-cells. No  eyes  other  than  these  occur  in  the  Polypla- 
cophora,  nor  are  tactile  tentacles  or  otocysts,  of  such  fre- 
quent occurrence  in  other  Mollusca,  found. 

The  nephridia  (Fig.  130,  n)  are  paired,  one  lying  on  each 
side  of  the  body  and  consisting  of  a  long  tube  giving  rise  to 
numerous  dendritic  branches.  Posteriorly  the  tube  branches, 
one  of  the  branches  opening  into  the  mantle-cavity  in  its 
posterior  part,  while  the  other  communicates  with  the  peri- 
cardial portion  of  the  enterocoel.  In  function  these  organs 
of  the  ChitonidnB  differs  from  the  corresponding  ones  of  the 
Solenogastres  in  being  excretory  only  and  in  not  serving 
as  ducts  for  the  reproductive  elements.  These  are  developed 
in  a  portion  of  the  enterocoel  which  lies  anteriorly  to  the 
pericardium  and  make  their  way  to  the  mantle-cavity  and  so 
to  the  exterior  by  special  ducts  arising  one  on  each  side  from 
near  the  posterior  part  of  the  reproductive  enterocoel  and 
ending  (go)  on  the  sides  of  the  body  slightly  in  front  of  the 
openings  of  the  nephridia  (no).  The  Polyplacophora  are  with- 
out exception  bisexual. 


TYPE  MOLLUSCA. 


293 


The  structural  peculiaritios  of  the  Polyplacophora  point  strongly  to 
their  primitive  chantcter,  though  in  many  respects  they  are  less  primitive 
than  the  Soleuogustres.  Thus  tiiey  possess  special  reproductive  ducts,  in  all 
probability  a  secondary  acquisition,  and  furthermore  the  reproductive  and 
pericardial  moieties  of  the  enterocojl  no  longer  communicate  freely.  If 
the  Solenogastres  have  been  derived  from  forms  with  Chiton-like  shells 
(see  p.  288),  then  it  must  be  supposed  that  the  two  groups  represent  di- 
verging lines  of  development  from  a  common  ancestor  whose  character- 
istics have  been  partly  retained  in  the  one  group  and  partly  in  the  other. 


iin 


con- 


II.  Class  Gastebopoda. 

The  Gasteropoda  form  a  very  complex  group,  the  various 
members  ditferiug  so  much  iu  the  details  of  their  orgauiza- 
tion  that  it  is  difficult  to  give  a  general  description  which 
will  apply  to  all  the  forms.  Certain  features  may,  how- 
ever, be  considered  typical  of  the  class,  and  these  may  be 
mentioned  here,  reserving  notice  of  the  more  important  varia- 
tions until  the  various  subdivisions  are  being  considered. 

One  of  the  most  characteristic  features  is  the  occurrence  of 
what  may  be  termed  the  "  visceral  hump  "  whose  presence  is 
responsible  for  many  of  the  peculiarities  of  Gasteropod  struc- 
ture. It  consists  of  an  elevation  into  a  dome-like  structure 
of  the  dorsal  region  of  the  body,  the  digestive  tract  and 
gland  being  contained  within  the  elevation.  The  mantle 
arises  as  a  circular  fold  surrounding  the  hump,  but  usually  is 
more  highly  developed,  and  therefore  encloses  a  deeper 
cavity,  upon  the  right  side  or  anterior  surface  of  the  hump, 
and  iu  the  cavity  so  arranged  lie  the  structures  which  usually 
are  associated  with  the  mantle-cavity,  namely,  the  branchise 
and  the  openings  of  the  digestive  tube  and  of  the  nephridia. 
There  is  thus  a  very  decided  asymmetry  in  most  Gasteropods, 
usually  emphasized  by  the  visceral  hump  being  coiled  into  a 
spiral,  a  coiling  which  is  shared  by  the  shell,  usually  present 
and  consisting  of  a  single  tubular  structure  surrounding  the 
visceral  hump,  but  usually  sufficiently  ample  to  permit  of  the 
retraction  within  it  of  the  rest  of  the  body. 

In  a  number  of  forms  the  visceral  hump  may  be  very 
much  reduced,  and  with  this  reduction  there  is  generally  con- 
comitant a  reduction  of  the  shell,  but  such  conditions  are 


'(ri 


I 


294 


INVERTEBRATE  MORPHOLOGY. 


n 


f 


plainly  secondary  inasmuch  as  the  primitive  asymmetry  is 
indicated  in  certain  of  the  organs  in  all  cases.  In  order  to 
understand  the  exact  nature  of  this  asymmetry  it  will  be  nec- 
essary to  consider  what  may  have  been  the  original  form  of 
the  Gasteropoda.  Judging  from  what  is  known  of  the  Amphi- 
ueura,  it  may  be  supposed  that  in  the  i3rimitive  Gasteropod 
(Fig.  131,  A)  the  anus  (a)  was  terminal  and  opened  into  a 
mantle-cavity,  the  mantle  being,  except  posteriorly,  only  a 
slight  fold.  In  this  mantle-cavity  there  was  present  also  a 
single  pair  of  branchial  plumes  (c<),  and  into  it  the  two 
uephridia  opened  {n\  passing  from  the  posteriorly-situated 
pericardium  which  contained  the  heart  provided  with  two 
auricles. 

It  may  be  imagined  now  that  in  such  a  form  the  visceral 
hump  enclosed  by  a  dome-like  shell  became  elevated  to  such 
an  extent  that  it  could  no  longer  be  retained  in  an  erect  posi- 
tion, but  fell  over  to  one  side — it  may  be  supposed  the  left 
side.  The  result  of  this  would  be  an  interference  with  the 
development  of  the  mantle-cavity  towards  the  left  side,  and 
a  prevention  of  the  perfect  growth  of  the  left  branchia  and  of 
the  proper  functioning  of  the  left  nephridium.  There  would 
be  a  tendency  then  for  the  mantle-cavity,  and  with  it  the  anus 
and  indeed  the  entire  posterior  region  of  the  body  with  the 
heart  and  nephridia,  to  be  pushed  over  towards  the  right  side 
(Fig.  131,  B)y  and  Hiis  process  might  in  some  cases  be  con- 
tinued until  the  mantle-cavity  and  the  organs  associated  with 
it  had  been  pushed  round  through  180°  (Fig.  131,  C,  D)  and 
had  come  to  lie  apparently  in  front  of  the  visceral  hump  {D). 
The  anus  in  such  a  case  would  open  into  the  mantle-cavity  in 
the  mid  line,  dorsal  to  the  mouth,  and  what  was  originally  the 
right  branchia  would  lie  upon  the  left  side  of  the  body ;  the 
digestive  tube,  which  may  originally  have  been  practically  a 
straight  tube,  would  now  be  bent  upon  itself,  and  furthermore 
the  original  right  parietal  nerve-ganglion  would  have  passed 
over  to  the  left  side  of  the  body  and  the  original  left  ganglion 
to  the  right  side,  a  crossing  of  the  pleuro-parietal  connectives 
(w)  being  thus  brought  about. 

The   original   pressure   of   the   shell   upon   the  left   half 
of  the  mantle-cavity  would,  however,  as  pointed   out,  have 


TYPE  MOLLUSC  A. 


295 


tended  to  produce  a  retardation  in  the  gtowth  or  even  the 
complete  abortion  of  the  organs  lying  in  that  region.  Accord- 
ingly the  original  left  nephridium  is  in  many  Gasteropoda  com- 
pletely suppressed  as  well  as  the  original  left  branchia,  and 
in  accordance  with  the  disappearance  of  this  latter  structure 


Fig.  131.— Diaoramh  to  iiiLUSTRATE  THE  Rotation  of  the  Mantle-cavity 
AND  ITS  Okgans  IN  A  GasTEKOPOD  (after  flguies  by  BiJTscHLi  and  Lang  from 
KoRscHBLT  aud  Heidbr). 

a  =  anus.  m  =  mouth. 

ao  =  aorta.  n  =  nepliridia]  pore. 

ct  =  cteuidiura.  peg  =  pedal  ganglion. 

eg  =  cerebral  ganglion.  pig  =  pleural  ganglion. 

ve  =  visceral  connective. 

there  is  a  disappearance  also  of  the  left  auricle  of  the  heart 
which  receives  blood  from  it. 

The  visceral  hump  does  not,  however,  retain  its  original 
conical  form,  but,  owing  perhaps  to  unequal  pressure,  grows 
more  rapidly  upon  one  surface,  the  anterior,  aud  so  becc^mes 
coiled  into  a  right-handed  spiral,  the  shell  covering  the 
hump  naturally  assuming  a  similar  form.    In  the  majority  of 


m 


f 


.m 


296 


INVEliTEBRATE  M0UPU0L0G7. 


GaHteropods  cousequeutly  a  shell  coiled  in  a  riglit-lianded 
spiral  occurs,  but  tliiu  rule  lias  uut  a  few  exceptious.  Where 
the  shell  forms  a  left-hauded  spiral  it  is  to  be  explained  by 
supposing  that  in  such  cases  the  visceral  huwp  tended  towards 
the  right  side  of  the  body  rather  than  the  left,  and  this  is 
confirmed  by  the  fact  that  in  most  left-handed  forms  it  is  the 
left  branchia  and  nephridium  that  have  persisted. 

It  must  be  pointed  out,  however,  that  the  extent  to  which 
the  rotation  of  the  mantle-cavity,  the  abortion  of  the  organs 
of  either  the  left  or  right  side  of  the  body,  and  the  crossing 
of  the  pleuro-parietal  nerve-cords  has  been  carried  varies  in 
diiiereut  forms.  In  some  the  rotation  has  been  carried  so  far 
that  the  original  right  branchia,  etc.,  has  passed  the  median 
line  in  front  so  as  to  lie  on  the  left  side  of  the  body,  and  in 
such  cases  the  crossing  of  the  nerve-cords  (chiastoneurism)  is 
completed.  Many  forms,  however,  stop  short  of  this,  and 
numerous  gradations  are  to  be  found.  The  rotation,  however, 
is  present  in  all  forms  to  some  extent  and  forms  a  character- 
istic feature  of  Gasteropod  morphology. 

The  anterior  portion  of  the  body  (Fig.  132)  is  usually  well 
marked  off  by  a  more  or  less  distinct  constriction  or  neck,  and 
consequently  it  is  possible  in  the  Gasteropods  to  speak  of  a 
head  in  contradistinction  to  the  trunk  region  of  the  body  ;  in- 
deed so  prevalent  is  this  character  that  the  term  Cephcdo- 
pJiora  has  been  applied  to  the  group.  Tentacles,  either  one  or 
two  pairs,  are  borne  by  the  head,  and  furthermore  eyes  are 
usually  present  upon  it  either  at  the  bases  of  one  of  the  pairs 
of  tentacles  or  else  borne  at  the  tips  of  these  structures. 

The  foot  is  generally  well  developed  and  usually  has  a  flat 
creeping  sole.  It  undergoes  many  modifications,  however, 
sometimes  becoming  more  keel-like,  or  becoming  differentiated 
into  three  regions  differing  in  form,  the  propodium,  mesopo- 
dium,  and  metapodium,  the  last-named  portion  frequently 
secreting  a  chitinous  plate,  the  operculum  (Fig.  132,  op\  which 
serves  to  close  the  mouth  of  the  shell  when  the  animal  is 
withdrawn  within  it.  In  addition  to  these  portions  an  epipo- 
dium  is  frequently  highly  developed,  consisting  in  its  primi- 
tive form  of  a  fold  arising  from  the  sides  of  the  foot  where  it 
passes  into  the  body- wall.     In  many  cases,  however,  it  loses 


TYPE  M0LLU8CA. 


297 


this  simple  form,  its  margin  becomiug  friuged  or  teutaculate, 
or  else  it  may  be  reduced  to  oue  or  more  separate  lobes  or 
tentacular  processes  ou  either  side  of  the  bod}-.  Opeuiug 
upou  the  surface  of  the  foot  is  frequeutly  to  be  fouud  a  so- 
called  "foot-glaud  "  which  secretes  a  sticky  mucous  fluid  aud 
is  comparable  to  the  byssus-glaud  of  the  I'elecypoda  {q.  v.). 


-si 


Fio.  182. — Buecinum  undatum. 
op  =  operculum.  #t  =  sipbo. 

The  respiratory  organs  (Fig.  133,  d)  consist  in  typical  cases 
of  a  single  pair  of  pinnate  branchial  plumes  lying  in  the 
mantle-cavity,  but,  as  has  already  been  mentioned  in  counec* 
tion  with  the  rotation  of  that  cavity,  oue  of  these  structures  is 
very  frequently  aborted.  Other  changes,  however,  also  occur, 
such,  for  example,  as  the  fusion  of  the  central  axis  of  the 
branchial  plume  throughout  its  entire  length  to  the  inner 
surface  of  the  mantle  {Haliotis),  or  the  disappearance  of  the 
pinnsB  from  one  side  of  the  plume  in  connection  with  such  a 
fusion  {Sycotypus,  Fig.  133).  In  some  forms  accessory  bran- 
cliisB  may  be  produced  as  folds  of  tlie  mantle,  richly  supplied 
with  blood,  and  their  development  may  be  carried  to  such  an 
extent  that  they  may  entirely  supplant  the  branchiae  proper 
{Patella).  From  such  a  condition  as  this  a  passage  is  not  dif- 
ficult to  such  a  condition  as  is  found  in  the  air-breathing  Gas- 
teropods  (Pulmonata)  in  which  the  entire  inner  surface  of  the 
mantle  serves  a  respiratory  purpose,  an  interchange  of  gases 
taking  place  between  the  air  contained  in  the  mantle-cavity 
and  the  blood  which  is  richly  supplied  to  the  mantle. 

The  musculature  of  the  body-wall  does  not  as  a  rule  pre- 
sent the  Annelidan  arrangement  in  layers,  as  in  some  Araplii- 
neura,  but  usually  are  irregularly  arranged  as  dorso-ventral 
and  oblique  bands  traversing  the  scliizocoel.   Special  muscles, 


r 


I 


fl 

ii 


f! 


i  i 


I 


i 


I    '^ 


?98 


INVEHTBDRATE  MORPIIOLOOY. 


however,  are  developed  iu  many  forms,  the  most  important 
being  those  connected  with  the  foot  and  serving  for  locomotion, 
retractor  muscles  in  connection  with  the  head,  proboscis,  and 
tentacles,  and  the  spindle-muscle,  which  has  a  general  vertical 
direction  running  along  the  right  side  of  the  visceral  hump 
from  its  insertion  into  the  shell  to  the  foot  in  whose  wall  its 
fibres  spread  out,  interlacing  as  it  were  with  the  horizontal 
and  transverse  muscles  there  developed  ;  it  serves  to  retract 
the  entire  animal  within  the  shell,  and  its  development  is 
naturally  in  proportion  to  that  of  the  shell,  those  forms  in 
■which  the  shell  is  rudimentary  or  absent  frequently  lacking  it. 

The  euterocoelic  portion  of  the  ccelom  is  much  reduced 
in  comparison  with  what  occurs  in  the  Amphiueura,  being  dis- 
tinctly represented  only  by  a  comparatively  small  pericar- 
dium surrounding  the  heart,  the  auricle  in  some  cases  not 
being  enclosed  by  it.  From  analogy  with  the  Amphineura, 
however,  the  reproductive  organs  must  be  regarded  as  repre- 
senting a  portion  of  the  enteroccel  whose  connection  with  the 
pericardium  has  been  completely  severed.  A  glandular  struc- 
ture, the  pericardial  gland,  is  in  some  Gasteropods  developed 
by  the  folding  of  the  pericardial  walls,  and  has  apparently 
an  excretory  function  acting  as  an  accessory  nephridium  ;  it 
is  not,  however,  as  highly  developed  as  in  some  of  the  other 
Molluscan  groups. 

The  circulatory  organ  possesses  in  some  forms  the  charac- 
teristic Molluscan  structure,  consisting  of  an  unpaired  ven- 
tricle lying  iu  the  pericardium  and  receiving  the  blood  from 
two  lateral  wing-like  auricles.  In  many  cases,  however,  as  al- 
ready pointed  out,  the  asymmetry  produced  by  the  develop- 
ment of  the  visceral  hump  afl'ects  the  heart,  resulting  in  the 
suppression  of  one  of  the  auricles,  that  of  the  left  i^or  right) 
side  (Fig.  133).  In  such  cases  tlie  persisting  auricle  m'ly 
secondarily  assume  a  terminal  position  with  regard  to  the 
ventricle,  and  the  latter,  instead  of  being  continued  into  an  ar- 
tery at  either  extremity,  gives  off  a  single  arter}'  at  the  end  op- 
posite to  that  at  which  the  blood  enters  from  the  auricle,  this 
artery  dividing  into  two  main  trunks  which  distribute  the 
blood  to  the  various  regions  of  the  body.  These  arteries  may 
be  coutiui^ed  as  distinct  tubes  with  definite  walls  for  some 


TYPE  MOLLUSCA. 


299 


onus  lu 


distance  from  the  heart,  but  sooner  or  later  the  blood  passes 
into  the  system  of  lacunar  spaces  constituting  the  schizocd^l, 
whence  it  is  again  returned  to  the  auricle  through  a  series  of 
veins.  The  position  of  the  single  auricle  with  reference  to  the 
body  axis  differs  in  different  orders  of  Gasteropods,  in  accord- 


Fro.  183. — Stuuctuue  of  Sycotypus  canalieulatiis.    The  mantle  is  divided  iu 
the  middle  Hue  aud  turned  aside,  exposing  tlie  mautle-cnvity. 


an  =  anus. 

ct  =  cteniilium. 
dg  =  digestive  glaud. 

i  —  intestine. 

n  =  ueplu'idiiim. 
no  =  nephridial  opening. 

o  =  eye. 
op  —  opeiculiini. 


OS  =  osphradium. 

p  =  perieardial  cavity. 
pe  =  penis. 
pr  =  proboscis. 
ni  =  siplio. 

/  =  tiMitacle. 
te  =  testis. 

V  —  ventricle. 


vd  —  vas  deferens. 
The  arrows  show  tlie  openings  of  nephridium  to  I  lie  mantle-chamber  and  to 

the  pericardium. 

ance  with  the  varying  position  of  the  branchia.  In  those 
forms  in  which  the  branchia  lies  in  front  of  the  heart  the 
auricle  lies  at  the  anterior  end  of  the  ventricle,  while  when 
the  branchia  is  posteriorly  situated  the  auricle  lies  behind 
the  ventricle. 

The  mot^iih  lies  in  all  Gasteropods  at   the  anterior  ex- 
tremity of  the  body,  towards  the  ventral  surface  of  the  head, 


u 


300 


INVERTEBRATE  MORPHOLOGY. 


and  opens  into  a  mouth-cavity  frequently  provided  with  two 
or  more  chitinous  teeth.  The  pharynx  usually  receives  the 
ducts  of  a  pair  of  salivary  glands,  contains  a  well-developed 
radular  organ  in  practically  all  cases,  and  communicates 
posteriorly  with  a  tubular  oesophagus.  In  many  cases  the 
anterior  portion  of  the  digestive  tract  is  capable  of  being 
protruded  as  a  proboscis  (Fig.  133,  pr),  which  lies  when  re- 
tracted within  a  proboscis-sheath,  formed  by  a  circular  infold- 
ing of  the  body-wall  around  the  mouth.  The  intestine  {i)  is 
usually  more  or  less  coiled,  extending  into  the  visceral  hump, 
and  presents  a  stomach-like  enlargement  which  receives 
the  ducts  of  the  digestive  gland  {dg)^  a  structure  usually  well 
developed  and  forming  the  greater  portion  of  the  visceral 
hump.  The  intestine  terminates  in  a  straight  portion,  the 
rectum  (r),  which  passes  forward  to  the  anus  (aw),  which,  as 
has  already  been  indicated,  lies  in  the  mantle-cavity,  slightly 
to  the  right,  but  occasionally  to  the  left,  of  the  middle 
line,  its  position  depending  upon  the  amount  of  rotation 
which  the  mantle-cavity  and  the  associated  organs  have 
undergone.  It  should  be  mentioned  that  in  one  suborder  of 
Gasteropods  the  pericardium  and  ventricle  have  wrapped 
themselves  around  the  rectum  in  euch  a  way  that  the  diges- 
tive tube  seems  to  have  penetrated  the  ventricle,  a  feature 
which  will  later  be  seen  to  be  characteristic  of  one  of  the 
other  groups  of  Mollusca. 

The  nervous  system  has  the  arrangement  which  has  been 
described  as  characteristic  of  the  Mollusca  (Fig.  124),  the 
peculiar  feature  being  the  crossing  of  the  pleuro-parietal  con- 
nectives which  is  found  in  many  forms.  Numerous  modifica- 
tions of  the  typical  condition  are  to  be  found,  consisting 
principally  in  (1)  the  concentration  of  the  ganglia,  more 
especially  the  cerebral,  pleural,  and  pedal,  or  the  pedal, 
pleural,  parietal,  and  visceral  (Fig.  137),  to  form  a  single 
mass ;  (2)  in  the  suppression  in  some  cases  of  one  of  the 
parietal  ganglia ;  and  (3)  in  the  occurrence  of  several  visceral 
ganglia.  In  accordance  with  the  flat  elongated  form  of  the 
foot  in  many  species,  the  nerve-cords  passing  backward  from 
the  pedal  ganglia  may  be  of  considerable  size,  and  further- 
more  may   be    connected    by   regularly-arranged   transverse 


TYPE  M0LLV8CA. 


301 


commissures,  recalling  the  condition  seen  in  the  Chitons,  as 
well  as  the  ladder-like  arrangement  of  the  ventral  nerve-cords 
of  the  Annelida,  though  there  cannot  in  the  Gasteropods  be 
any  question  of  metamerism  in  this  connection. 

Special  sense-organs  are  very  generally  well  developed  in 
the  Gasteropods.     The  tentacles  so  usually  found  upon  the 
head  have  probably  a  tactile  function  as  well  as  the  tentacular 
or  wiuglike   processes  sometimes  found  in  connection  with 
the  anterior  extremity  of   the    foot,  and  the   epipodial   ten- 
tacles which  occur  in  some  forms  {Haliotis).     On  the  ventral 
side  of  the  bases  of  the  epipodial  tentacles  of  some  forms 
special    sensory  thickenings   have   been    found  which   have 
suggested  a  comparison  with  the  sense-organs  of  the  lateral 
line  of  the  Annelida,  a  comparison  which,  however,  at  present 
seems  rather  strained ;  it  seems  probable,  notwithstanding 
their  innervation  from  the  pedal  ganglia,  thac  these  sensory 
patches  are  to  be  placed  in  the  same  category  as  the  osphra- 
dia  and  the  sensory  ridges  of  the  mantle-cavity  of  the  Chi- 
tons.    The  osphradia  (Fig.  133,  os)  in  all  Gasteropods  which 
are  provided  with  branchiaB  are  associated  with  these  organs ; 
and    even    where    one    or    both 
branchisB  have   been   suppressed 
the   osphradia   may  still  persist. 
Eyes  (Fig.  134)  are  very  generally 
present  in  the  Gasteropods,  being 
situated  at  the  base  of   the  ten- 
tacles, or  at  their  summit  in  some 
forms.     They  present  a  very  uni- 
form   structure    throughout    the 
group  and  arise  as  a  depression 
of    the   integument,  the    lips  of 
the    cavity  fusing  and  giving  rise 
to  a  globular  sac  lying  beneath 
the  epidermis,  which  remains  thin  and  transparent,  forming  an 
outer   cornea    {go).     The  cells  of  the  outer  wall  of  the   sac 
likewise  remain  clear,  forming  the  inner  cornea,  while  over 
the   remainder  of   the  wall  of   the   sac  they  are  sensory  in 
function,  pigmented  cells  being  scattered  among  them,  the 
two  together  forming  the  retina  (ret).     The  nerve-flbres  pass- 


PiG.  134. 


-Eye  of  ILiUMis  (after 
Patten). 

CO  =  cornea. 
I  =  lens. 
rt  =  retina. 


&' 


ill 


302 


INVERTEBRATE  MORPHOLOGY. 


^  f  t! 


1  \i 


ing  to  the  eye  from  the  cerebral  ganglia  pass  through  an  optic 
ganglion  lying  beneath  the  optic  sac  and  are  distributed  to 
the  sensory  cells,  and  the  centre  of  the  sac  is  filled  up  by  a 
cuticular  mass  which  serves  as  a  refractive  lens  (Z).  In  some 
forms  {Patdla,  etc.)  the  development  of  the  eye  ceases  while 
it  is  still  in  the  cup  form,  there  being  then  no  formation  of 
corneal  layers  and  no  central  lens,  though  the  retina  is  usu- 
ally covered  by  a  thin  cuticular  layer.  In  some  species  of  a 
peculiar  genus  of  the  air-breathing  Gasteropods,  Onchiduim, 
eyes  are  developed  upon  the  dorsal  surface  of  the  body,  the 
shell  being  lacking  and  the  visceral  hump  undeveloped.  In 
structure  these  eyes  diflter  very  materially  from  those  usually 
occurring  in  the  Gasteropoda  and  will  be  described  later 
(p.  318). 

Otocysts  are  usually  imbedded  in  the  tissues  of  the  foot 
close  to  the  pedal  ganglia,  though  in  all  cases  they  receive 
their  innervation  from  the  cerebral  ganglia ;  they  have  the 
usual  sac-like  form  and  are  lined  with  sensory  hair-bearing 
cells  and  contain  otoliths. 

The  nephridia  of  the  Gasteropods  are  in  nearly  all  cases 
modified  from  the  original  typical  condition  in  accordance 
with  the  asymmetry  of  the  body  (Fig.  133,  re).  In  only  a  few 
forms,  so  far  as  known  {Fissurella,  Patella),  are  two  functional 
nephridia,  opening  on  the  one  hand  into  the  pericardial 
cavity  and  on  the  other  to  the  exterior  through  the  mantle- 
cavity,  perfectly  developed.  In  other  forms,  such  as  Hali- 
otisy  Turbo,  etc.,  both  nephridia  are  present  and  are  struct- 
urally perfect,  though  the  left*  one  has  lost  its  secretory 
function,  but  in  the  majority  of  cases  the  left  (or,  in  forms 
with  a  left-handed  coiling  of  the  visceral  hump,  the  right) 
uephridium  is  completely  aborted. 

The  Gasteropods  are  in  some  cases  bisexual,  in  others 
hermaplirodite.  The  reproductive  sac  (Fig.  133,  t)  is  quite 
unconnected  with  the  pericardial  enteroccel  and  is  an  un- 
paired structure  lying  in  the  visceral  hump.  The  ova  and 
spermatozoa  in  most  cases  reach  the  exterior  by  a  special 


i  ii 


*  The  terms  left  and  right  refer  to  the  position  of  tlie  nepliridia  ns  they  are 
supposed  to  have  been  arranged  in  the  primitive  symmetrical  Gosteropod. 


TYPE  MOLLUSC  A. 


303 


duct  {vd),  having  apparently  no  relation  to  the  nephridia  and 
opening  into  the  mantle-cavity  to  the  right  side  of  the  anus. 
In  the  more  primitive  Gasteropoda,  however,  such  as  Haliotis, 
Fissurdla,  and  Patella,  the  nephridia,  as  in  the  Solenogastres, 
serve  as  reproductive  ducts ;  and  it  has  been  suggested  that 
the  special  reproductive  duct  of  the  remaining  Gasteropods 
may  represent  the  left  uephridiura,  which  is  usually  described 
as  having  disappeared.  The  reproductive  duct,  especially  in 
hermaphrodite  forms,  has  developed  in  connection  with  it 
accessory  glandular  structures  as  well  as  external  copulatory 
organs,  the  whole  reproductive  system  becoming  highly  com- 
plicated. An  account  of  the  more  important  arrangements 
will  be  more  satisfactorily  given  in  connection  with  the 
various  orders. 

1.  Order  Frosobranchia. 

The  Frosobranchia  are,  with  very  few  exceptions,  marine 
Gasteropods,  provided  with  well-developed  shells,  which  are 
usually  spirally  coiled,  the  height  of 
the  spiral  varying,  however,  in  different 
forms.     In  some,  such  as  Patella  and 
Fissurella,    the     shell    has    a    simple 
conical   form,  without  any   indication 
of  a  spiral ;  and  since  these  forms  in 
many  respects  show  primitive  charac- 
ters, it  might  be   supposed  that  this 
type  of  shell  was  also  primitive.    These 
very  forms,  however,   show   also   that 
asymmetry  of  parts,  which  is  character- Pig.  135.— Shells  of  Puoso- 
istic   for   the   Gasteropods,  and  which    buanch      Gasteuopods. 
accompanies  the  rotation  of  the  mantle- 
cavity,  and  furthermore,  in    Fhsiirella 
at   least,    a    distinct   indication    of    a 
spiral  coiling,  is  present  in  the  shells  of 

young  animals.  It  seems  more  probable,  accordingly,  that 
these  conical  shells  are  to  be  regarded  as  secondary  modifi- 
cations of  an  originally  spirally-coiled  shell. 

The  mantle-cavity  is  situated  in  front  of  the  well-devel- 
oped visceral  hump,  and  is  usually  somewhat  capacious,  com- 


A,  Acmcea  teatudinnlia 
(after  QottLn)  ;  B,  Haliotis 
(after  Lkunis);  C,  Tun'itella 
(after  Leunis). 


304 


INVERTEBRATE  MORPHOLOGY. 


W 


H 


M    '- 


muuicating  with  the  exterior  freely.  In  some  forms  the 
mautle  is  slit  from  its  margin  upwards  and  backwards,  a 
corresponding  slit  occurring  in  the  shell  {Emarginvla).  In 
Haliotis  and  Pleurotomaria  the  slit  in  the  shell  becomes 
closed  at  regular  intervals,  producing  a  row  of  round  perfora- 
tions, beneath  which  lies  the  mantle-slit,  and  through  Avhich 
water  iinds  a  ready  exit  from  the  mantle-cavity,  and,  in  /Ys- 
surelJa,  in  which  at  an  early  stage  the  margin  of  the  shell 
possesses  a  slit,  by  the  subsequent  growth  and  obliteration  of 
the  spiral  coiling  the  slit  becomes  converted  into  an  aperture 
which  lies  almost  at  the  :..pex  of  the  conical  shell  and  leads 
into  the  mantle-cavity,  functioning  as  a  means  of  exit  of  the 
water  and  excrementa  from  that  cavity.  In  the  greater  num- 
ber of  forms,  however,  such  slits  or  apertures  do  not  exist ; 
but  one  finds  frequently  the  margin  of  the  mantle  produced 
at  one  point  on  the  left  side  into  a  projecting  narrow  lobe 
whose  edges  may  be  brought  into  opposition,  thus  producing 
a  tube  or  siphon  through  which  water  may  pass  into  the 
mantle-cavity.  Where  this  siphon  is  well  develuj^ed  a  dis- 
tinct notch  is  found  in  the  margin  of  the  shell,  through  which 
it  may  bft  protruded,  or  else  the  lips  of  the  notch  are  pro- 
longed so  as  to  form  a  grooved  process,  the  siphonal  canal,  in 
which  the  siphon  lies,  being  by  these  arrangements  able  to 
function  even  when  the  mouth  of  the  shell  is  closed  by  the 
operculum.  In  many  forms  the  mantle-folds  are  sufficiently 
large  to  allow  of  their  being  reflected  over  the  outer  surface 
of  the  shell  when  the  body  is  fully  protruded. 

The  foot  is  as  a  rule  adapted  for  creeping,  but  in  many 
cases  is  differentiated  into  pro-,  meso-  and  metapodium,  the 
last  usually  bearing  a  chitinous  or  more  or  less  calcified 
operculum.  In  certain  forms  belonging  to  a  group  of  pelagic 
forms,  however,  which  were  formerly  associated  together  as  a 
distinct  order,  the  Heteropoda  (Fig.  138),  the  pro-  and  meso- 
podium  are  modified  into  a  keel-like  structure  and  bear  a 
peculiar  sucker.  The  epipodium  is  frequently  developed  in 
the  Prosobranchia,  especially  in  the  more  primitive  species — 
most  frequently,  however,  being  reduced  to  tentacle-  or  lobe- 
like processes  arising  from  the  sides  of  the  foot. 

Ill  the  majority  (Fig.  1 33)  of  forms  there  is  but  a  single 


,v«3i»>s*i*"*-' 


TYPE  MOLLUaCA. 


305 


lace 


IS  a 

iSO- 

a 
iu 

I — 

rle 


brancbia  wbicli  lies  in  front  of  tbe  beart,  wbence  tbe  name  of 
tbe  order,  but  in  a  few  genera  tbe  original  left  gill  also  per- 
sists. In  many  forms  a  gland  is  developed  in  tbe  floor  of  tbe 
mantle-cavity  close  to  tbe  rectum — bence  called  tbe  adrectal 
gland — wbicb  iu  some  forms,  e.g.  Murex  secretes  a  purple 
pigment.  Tbe  rotation  of  tbe  mantle-cavity  and  tbe  associated 
organs  bas  called  fortb  a  crossing  of  tbe  pleuro-parietal  nerve- 
cords,  a  feature  wbicb  is  lacking  in  tbe  otber  orders  and 
tberefore  forms  a  cbaracteristic  of  tbe  Prosobraucbs. 

In  all  but  a  few  cases  tbe  members  of  tlie  order  are  bisex- 
ual, tbe  unpaired  reproductive  gland  lying  in  tbe  visceral 
bump.  Tbe  oviduct  bas  iu  connection  witb  it  one  or  more 
receptacula  seminis  and  dilates  into  a  glandular  uterus  in 
wbicb  tbe  eggs  are  supplied  witb  tbe  albumen  in  wbicb  tbey 
are  usually  imbedded  and  also  surrounded  by  a  sbell.  In 
tbe  males,  except  in  tbe  more  primitive  forms,  tbere  is  present 
a  well-developed  introraittent  organ  or  penis  (Fig.  133,  j9e), 
situated  upon  tbe  rigbt  side  of  tbe  bead  or  neck  and  tbere- 
fore removed  at  some  distance  from  tbe  opening  of  tbe  vas 
deferens  into  tbe  mantle-cavity.  A  groove  or  tube  extends, 
bowever,  from  tbe  reproductive  orifice  to  tbe  grooved  or  tubu- 
lar penis,  and  along  tbis  groove  or  tube,  by  tbe  ciliary  action 
of  tbe  cells  lining  it,  tbe  seminal  fluid  is  carried. 

1.  Suborder  Diotocardia. 

Tbis  suborder  includes  tbe  more  primitive  Prosobrancbs, 
in  wbicb,  altbougb  a  considerable  rotation  bas  occurred,  yet 
nevertbeless  tbe  abortion  of  tbe  organs  of  tbe  original  left 
side  of  tbe  body  bas  not  been  carried  very  far.  Tims,  except 
in  Patella  and  some  allied  forms,  tbere  are  two  auricles  to  the 
beart,  altbougb  iu  Turbo,  TrocJms,  Neritina,  and  allied  genera 
tbat  of  tbe  rigbt  side  (i.e.,  tbe  original  left  one)  does  not  com- 
municate witb  tbe  ventricle.  Attention  may  again  be  called 
to  tbe  fact  tbat  in  tbose  forms  wbicb  possess  two  functional 
auricles  tbe  ventricle  and  pericardium  bave  wrapped  tbem- 
selves  round  tbe  rectum  wbicli  seems  to  perforate  tbe  ventri- 
cle. Sucb  forms  as  Haliotis,  Fissurelht,  and  Phurofomaria  pos- 
sess two  brancbiaB,  but  in  tbe  majority  of  tbe  members  of  tbe 


■ '  Li 


it: 


1* 

i 

'1 

t^ 

1 

\ 

m 

Y 

■i 

306 


IN  VEHTEBHA TE  MORPUOLOG  Y. 


n 


group  only  one  is  present,  while  in  Patella  both  have  disap- 
peared, their  place  having  been  taken  by  respiratory  folds 
of  the  mantle.     Both  kidneys  are  invariably  present. 

The  primitive  character  of  the  suborder  is  further  shown 
in  the  absence  of  certain  structures  found  in  more  specialized 
forms.  Thus  the  foot  is  flat  and  undifferentiated  into  pro-, 
meso-,  and  metapodiura  ;  the  anterior  part  of  the  digestive  tract 
is  not  evertible  as  a  proboscis ;  there  is  no  siphonal  prolonga- 
tion of  the  mantle,  and  no  notch  or  siphonal  groove  ou  the 
margin  of  the  shell ;  and  there  is  no  penis.  On  the  other  hand 
the  epipodium  is  usually  well  developed,  as  are  also  the  pedal 
nerve-cords,  which  are  connected  by  numerous  cross-commis- 
sures. 

A  further  distinguishing  feature  of  the  suborder  is  the 
arrangement  of  the  teeth  of  the  radula.  Each  transverse  row 
of  teeth  presents  an  indefinite  number  of  marginal  teeth,  usu- 
ally a  single  lateral,  a  single  median,  and  a  varying  number 
of  admedian  teeth,  an  arrangement  known  as  rhipidoglossate. 
Thus  in  Haliotis  the  arrangement  is  indicated  by  the  formula 


Fig.  136.— Dentition  op  T'roc/ms  (after  Lankester). 

X,  1,  5, 1,  5,  1,  a? ;  in  Fissurella  by  x,  1,  4, 1,  4,1,  x;  and  in  Tro- 
clms  (Fig.  136)  and  Turbo  by  x,  0,  5,  1,  5,  0.  x,  the  single  lateral 
tooth  being  absent  in  these  forms.  In  Patella,  however,  an- 
other arrangement  is  found  characterized  by  the  occurrence 
of  only  a  small  number  of  marginal  teeth  and  by  the  absence 
of  the  median,  the  formula  being  3,  1,  2,  0,  2,  1,  3 ;  this  ar- 
rangement is  termed  docoglossate. 


2.   Suborder  MonotocarcUa. 

In  this  suborder  the  effect  of  the  pressure  of  the  visceral 
hump  on  the  organs  of  the  left  side  of  the  mantle-cavity  is  more 
pronounced  than  in  the  Diotocardia.     The  heart  possesses  a 


TYPE  MOLLUSC  A. 


307 


single  auricle  only,  except  in  Cypr(va,  where  the  rudiment  of  a 
second  occurs,  and  throughout  the  group  but  a  single  nephrid- 
ium  is  present.  There  is  never  more  than  a  single  gill,  which 
is  usually  more  or  less  united  to  the  mantle-wall  and  bears 
lateral  branches  only  upon  one  side. 

The  foot  is  in  some  cases  flat  and  broad,  as  in  the  Dioto- 
cardia,  and  in  such  cases  may  possess  the  parallel  pedal  nerve- 
cords  with  transverse  commissures  as  'n  Cyprcea  and  I\di(din((, 
but  usually  it  becomes  more  or  less  differentiated,  a  propodi- 
um  being  in  many  cases  well  defined  {iStrGmhus,  Natica),  while 
a  chitinous  or  calcareous  operculum  is  usually  carried  by  the 
metapodium,  and  the  pedal  nerve- cords  are  very  much  re- 
duced or  wanting,  the  pedal  ganglia  being  on  the  other  hand 
more  highly  developed  than  in  the  Diotocardia.  The  epipo- 
dium  is  usually  entirely  wanting,  and  when  present  is  but 
slightly  developed,  reaching  its  fullest  development  as  a  con- 
tinuous fold  upon  the  sides  of  the  foot  only  in  lanthina.  In 
Pidtidina  it  is  represented  by  two  anteriorly-situated  tentacle- 
like lobes,  and  in  Calyptrcea  by  a  semicircular  fold  on  each 
side  of  the  neck  region. 

The  Mouotocardia  are  further  distinguished  by  the  fre- 
quent occurrence  of  a  well-developed  siphon  and  a  more  or 
less  developed  siphon  groove  at  the  margin  of  the  shell,  and 
furthermore  a  well-developed  penis  is  usually  present. 

The  anterior  portion  of  the  digestive  tract  is  in  many  forms 
capable  of  being  protruded  as  a  proboscis.  The  arrangement 
of  the  teeth  of  the  radula  varies  considerably  in  diflerent 
forms,  but  the  rhipidoglossate  arrangement  is  not  represented. 
In  one  group,  including  the  genera  Cyprcea,  Natica  (Fig.  137, 
A)j  Littorina  (the  periwinkles),  Calyptrcea,  Strombus',  etc.,  the 
tffiuioglossate  arrangement  is  found,  represented  by  the  for- 
mula 2  or  3, 1, 1, 1,  2  or  3,  the  admedian  teeth,  however,  being 
very  similar  to  the  lateral.  In  other  cases  but  a  single  median 
tooth  or  the  median  with  a  single  admedian  on  each  side  is 
found,  as  in  Fusm,  Buccinum  (the  whelks),  Nassa  (Fig.  137,  B), 
3Iurex,  Purpura,  Oliva,  Manjinella,  etc.,  forming  the  raclii- 
glossate  arrangement  represented  by  the  formulas  — ,  1,  — ,  or 
1, 1, 1.  In  Terd)ra,  Conus,  Pleurotoma  (Fig.  137,  C),  and  allied 
genera  the  median  tooth  is  absent,  and  the  single  admedian 


il 


I 


308 


IN  VEIiTEBHA TE  MORPHOLOO  T. 


tooth  on  either  side  peculiarly  long,  forming  the  toxiglossate 
arrangement  with  a  formula  1,  0,  1 ;  and  finally  certain  forms, 
such  as  lanthina,  Scaluria  (Fig.  137,  iV),  /Solarium,  etc.,  have  a 
ptenoglossate  arrangement  in  which  the  median  is  wanting 
but  in  which  there  are  a  large  number  of  admediaus,  x,  0,  x. 

The  suborder  is  relatively  very  rich  in  species,  and  conse- 
quently considerable  variety  of  form  is  found.  The  majority 
are  marine,  but  a  few  are  fresh-water  or  even  terrestrial  in 
habitat.  In  these  latter  adaptations  to  their  mode  of  life  are 
found  in  modifications  of  the  respiratory  processes.    In  Am- 


FiQ.  137.— id,  Dentition  of  Natiea  ;  B,  of  Naaaa ;  C,  of  Pleurotoma  ;  D, 

OF  Scalaria  (from  Bronn). 

pullaria  the  single  branchia  persists,  but  in  addition  a  com- 
paratively capacious  "  lung-cavity  "  is  formed  by  a  fold  of  the 
mantle,  its  walls  being  richly  supplied  with  blood-vessels  and 
its  cavity  being  in  communication  with  the  exterior,  so  that 
air  can  be  taken  into  and  expelled  from  it.  The  species  of 
this  genus  live  partly  in  fresh  water  and  partly  are  terrestrial, 
but  in  other  forms,  such  as  Cyclostoma,  which  are  purely  ter- 
restrial, the  branchia  has  entirely  aborted,  respiration  being 
aerial  and  performed  by  the  highly  vascular  wall  of  the 
mantle-cavity. 

The  majority  of  the  marine  Monotocardia  have  a  creep- 
ing habit,  but  a  number  are  pelagic  and  form  a  group  pre- 
senting many  adaptive  peculiarities  which  obtained  for  it  the 
dignity  of  an  order  in  older  classifications.  The  members  of 
this  group,  Heteropoda,  are  more  or  less  transparent  animals, 
some  of  which,  with  this  exception,  present  few  diflferences 


TYPE  MOLLUSCA. 


309 


from  the  other  Mouotocardia,  while  others  are  extensively 
modified.  The  geuus  Atalnnta  possesses  a  large  transparent 
shell  within  which  the  animal  can  be  completely  retracted. 
The  foot  is  no  longer  adapted  for  creeping,  but  is  differenti- 
ated into  a  laterally  flattened  keel-like  pro-  and  mesopodium 
which  bears  a  sucker  on  its  posterior  surface,  and  a  metapo- 
dium  provided  with  an  operculum.     In  Carinaria  (Fig.  138) 


Fig.  138. — STnucTUUE  of  Carinaria  mediterranea  {a.tiw  O'tiks). 


ao  =  aorta. 

b  =  buccal  mass. 
eg  =  cerebral  ganglion. 
ct  =  ctenidium. 

7i  =  heart. 

i  —  intestine. 

I  =  liver. 
mp  =  mesopodium. 


0  =  eye. 

p  =  penis. 
peg  =  pedal  ganglion. 

a  =  salivary  gland. 
8U  =  sucker. 

ie  =  testis. 
vd  =  vas  deferens. 
vg  =  visceral  ganglion. 


vs  =  seminal  vesicle. 

the  visceral  hump  is  reduced  to  a  comparatively  small  mass 
upon  the  dorsal  surface  of  the  elongated  body  and  is  enclosed 
in  a  transparent  shell  shaped  like  a  liberty-cap.  The  pro-  and 
mesopodium  have  the  form  of  a  plate  hanging  down  from 
about  the  middle  of  the  under-surface  of  the  body,  and  the 
metapodium  is  directed  backwards,  forming  in  reality  the 
posterior  portion  of  the  body.  The  same  relationships  of  the 
foot  are  found  in  Pterotrachea,  which  presents  the  extreme  of 
modification  found  in  this  group  ;  in  this  form  the  visceral 
hump  is  still  more  reduced  than  in  Carinaria,  forming  only  a 
small  oval  mass  imbedded  in  the  dorsal  surface  of  the  body 
and  being  destitute  of  any  shell.    Considering  these  two  forms, 


310 


INVERTEBRATE  MORPIIOLOOT. 


Pterotrachea  and  Carinaria,  by  themselveH,  the  formatiou  of 
a  separate  order  for  their  reception  would  perhaps  be  justi- 
fiable, but  Atalanta  shows  their  close  relationships  with  the 
Prosobrauchia  aud  indicates  their  true  position  as  Monoto- 
cardia. 


\t  : 


2.  Order  Opisthobranohia. 

The  Opisthobranchs  are  exclusively  marine  forms,  present- 
ing numerous  modifications  of  shape  and  structure,  but  all 
agreeing  in  certain  important  particulars.  The  rotation  of 
the  mantle-cavity  has  not  proceeded  quite  so  far  as  in  the 
Prosobranchs,  the  cavity  and  its  organs  lyiug  upon  the  right 
side  of  the  body,  but  at  the  same  time  the  abortion  of  the 
organs  of  the  primitively  left  side  of  the  body  has  occurred. 
Thus  in  those  forms  which  possess  respirator}-  organs  homol- 
ogous with  the  branchitfi  of  the  Prosobranchs,  but  one  (that 
of  the  right  side)  is  present,  aud  with  this  character  is  associ- 
ated the  occurrence  in  the  heart  of  but  a  single  auricle, 
which  lies  behind  the  ventricle.  Only  one  nephridium  occurs, 
and  a  distinction  from  the  Monotocardiate  Prosobranchs  is 
found  in  the  fact  that  the  branchia  when  present  lies  as  in- 
dicated by  the  position  of  the  auricle,  behind  the  heart — the 
name  bestowed  upon  the  order  being  suggested  by  this  pecul- 
iarity. 

A  more  important  distinguishing  character  perhaps  is, 
however,  to  be  found  in  the  arrangement  of  the  nerve-cords. 
The  rotation  of  the  mantle-cavity  and  its  associated  parts  has 
not  been  carried  to  such  an  extent  as  to  produce  a  crossing  of 
the  pleuro-visceral  connectives,  wl>ioli  run  more  or  less  par- 
allel with  one  another  and  present  vvi.at  is  termed  an  ortho- 
neurous  arrangement  in  contradii-'ti.iction  to  the  chiastoneu- 
rism  of  the  Prosobranchs.  In  addition  to  this  character  a 
tendency  towards  an  aggregation  of  the  various  ganglia  to 
a  complex  mass  lying  behind  the  pharynx  may  also  be  con- 
sidered a  characteristic  of  the  Opisthobranchs.  One  or  both 
parietal  ganglia  may  disappear,  and  in  some  cases  where 
there  is  a  marked  concentration  of  the  ganglia  the  visceral 
ganglion  may  also  be  unrepresented,  though  usually  from  one 


TYPE  MOLLUSC  A. 


311 


to  three  such  ^auglia  may  be  clistiu^uishecl.  In  th'^  6gure  of 
the  uervous  system  of  Fiona  (Fij^.  139)  the  couceutratiou  of 
the  guuglia  is  well  marked,  but  a 
decided  asymmetry  is  made  evi- 
dent iu  some  forms  bv  the  exist- 
euce  of  a  siugle  parietal  gaugliou 
aud  of  three  visceral  gauglia.  In 
Fiona,  however,  the  gauglioiiic 
couceutratiou  has  beeu  carried 
still  farther,  aud  at  the  same  time 
by  the  suppression  of  the  parietal  j,^^  139.-NEi1vc.rK  Svrtkm  <,f 
ganglion  as  a  distinct  mass  of  Fiona  atlantica  mh^v  mnoH  trom 
cells  an  apparent  symmetry  has  oeoenbaur). 
resulted  "^  ~  cercbro-plcuro-visceml   guii- 

With   regard   to   many    other   ^  ^  {,„ecal  ganglion, 
features   of    their    anatomy   con-  /)  =  gastro-cesophageal  ganglion, 
siderable    variations    are    to    be    e  =  pedal  commissure, 
found.      Thus    in    some  forms   a  «*=  visceral  commissure, 
well-developed   spirally-coiled  visceral   hump   is   developed, 
while  in  others  it  loses  its  spiral  arrangement,  and  in  others 
again  is  elongated  in  the  direction  of  the  foot  and  can  hardly 
be  said  to  exist.     So,  too,  with  the  occurrence  of  the  shell, 
mantle,  and  branchise ;  all  are  well  developed  in  some  forms, 
but  entirely  absent  in  others.    These  peculiarities  will  be  more 
conveniently  referred  to  in  connection  with  the  various  groups, 
aud  it  is  only  necessary  here  to  refer  to  another  feature  in 
addition  to  those  already  given,  which  is  common  to  all  the 
members  of  the  order — i.e.,  the  hermaphroditic  character  of 
the  reproductive  gland. 

This  forms  part  of  the  visceral  mass  and  is  usually  com- 
posed of  numerous  lobes,  these  again  being  divided  into 
secondary  lobes  or  acini,  the  lining  epithelium  of  Avhicli 
give  rise  to  both  ova  and  spermatozoa.  In  some  forms, 
such  as  Bulla  and  Aplysia,  both  elements  are  formed  iu  ail 
the  acini ;  but  iu  others,  such  as  Doris,  Janus,  Pteropoda,  etc., 
the  epithelium  of  the  terminal  acini  gives  rise  to  ova  only  ;  the 
epithelium  of  the  lobes,  i.e.,  the  central  portious  of  the  gland, 
producing  spermatozoa.  Whether  or  not,  however,  there  be 
such  a  separation  of  the  epithelium  into  male  and  female 


312 


IN\  miTEBRA TE  MORPUOLOG Y. 


5 


areas,  the  reproductive  elemeuts  make  their  way  into  a  com- 
mou  hermaphnHlUeiiuet,  wliich  presents  variations  of  structure 
in  different  forms  and  receives  the  secretion  oi  certain  acces- 
sory ghinds.  In  its  simplest  form,  as  seen  for  instance  in 
Aphjmiy  the  duct  runs  forward,  pursuing  a  somewhat  tortuous 
course  .lud  becomes  surrounded  by  an  albumhupnrom  gland, 
from  which  it  receives  a  viscid  secretion,  within  which  the 
ova  become  imbedded  just  in  front  of  the  point  where  the 
ghmd  opens  into  the  duct.  The  hitter  has  attached  to  it  a 
pouch-like  structure,  the  cesicula  seniimiUs,  and  is  continued 
on  as  a  somewhat  wider  tube  to  open  to  the  exterior  at  the 
genital  pore  situated  on  the  right  side  of  the  body,  shortly 
before  reaching  the  pore,  however,  receiving  a  duct  from  a 
globular  sac,  the  spermatheca.  From  the  anterior  edge  of  the 
pore  a  groove,  the  seminal  groove,  extends  along  the  right 
side  of  the  body  to  the  neck  region,  where  it  eiid^  in  a  mus- 
cular evertible  penis,  situated  near  the  anterior  right  tentacle. 
It  seems  probable  that  the  spermatozoa  mature  before  the 
ova,  and  passing  to  the  vesicula  are  stored  up  there.  During 
copulation  the  seminal  fluid  is  transferred  through  the  penis 
to  the  spermatheca  of  another  individual  (perhaps  the  trans- 
ference is  a  mutual  interchange),  and  when  later  the  ova  pass 
along  the  duct  they  are  impregnated  by  the  spermatozoa 
so  stored  away,  a  cross-fertilization  being  thus  brought 
about. 

This  arrangement  of  the  reproductive  duct  is  found  in  the 
more  primitive  Opisthobranchs,  i.e.,  in  those  in  which  the 
mantle-lobe  still  persists,  and  in  the  group  Pteropoda\  in 
the  more  highly-modified  forms,  such  as  Doris,  JEolis,  etc., 
and,  among  the  more  simple  forms,  in  Pleurohrancluva  the 
hermaphrodite  duct  divides  into  an  oviduct  and  a  vaa  deferens. 
The  former  after  receiving  the  spermathecal  duct  opens  into 
a  genital  atrium,  with  which  communicate  also  the  albuminipa- 
rous  gland  and  a  imhimentul  gland,  which  manufactures  the 
outer  shell-like  investment  of  the  ova.  The  vas  defei'eua,  after 
a  more  or  less  tortuous  course,  enters  the  muscular  saclike 
penis-sheath  which  communicates  with  the  genital  atrium;  the 
enlarged  termination  of  the  vas,  the  penis,  being  thus  capable 
of  eversion  through  the  pore  by  which  the  atrium  communicates 


TYPE  MOLLUSCA. 


313 


with  the  extorior.  This  eoiulitiou  sooms  to  be  a  socomlai y 
inoditicjitiou  of  one  iu  which  the  oviduct  iiiul  vas  tleforeiis 
open  inilepenihnitly  at  widely  se})arateil  ])oints — a  couditiou 
which  is  represented  by  a  few  Opisthobranchs. 


Suborder    Ted ibranch  ia. 


The  Tectibrauchiates  are  those  Opisthobranchs  which 
present  the  smallest  amount  of  '.uoditication  from  what  has 
been  considered  the  typical  Gasteropod  structure.  A  more 
or  less  developed  mautle-ft)ld  is  usually  present,  sometimes 
sufHciently  voluminous  to  cover  in  the  single  branchia  which 
persists  {Bulla),  but  frequently  represented  only  by  a  slij^ht 
fold,  which  leaves  the  branchia  exposed  {Aplysia,  Gafiterop- 
tt't'on).  A  shell  is  very  generally  present,  sometimes  well  de- 
veloped {Bulla),  but  iu  other  cases  reduced  to  a  plate-like 
structure  enclosed  within  the  mantle  which  has  been  reflected 
over  it  and  the  lips  of  the  reflected  portion  meeting  and  fusing 
{Aplysia,  /Veurohranchua).  The  visceral  hump,  however,  is  as 
a  rule  low  and  elongated  in  the  direction  of  the  long  axis  of 
the  body  instead  of  at  right  angles  to  it,  as  in  the  majority  of 
Prosobrauchs.  In  many  members  of  the  group  the  foot  pos- 
sesses a  broad  creeping  surface,  but  its  margins  are  prolonged 
into  broad  thin  wings,  the  parapodia,  which  may  be  bent  up- 
Avaids,  as  in  Aplysia,  so  as  almost  to  enclose  the  body. 

The  Tectibrauchiates  are  divisible  into  two  groups  accord- 
ing to  their  habits,  in  accordance  with  which  the  form  of  the 
foot  and  especijilly  of  the  parnjxxlia  is  moditi(Hl.  Those  forms 
which  possess  n  broad  Hat  sole  to  the  ft)ot  have  a  croe[)ing 
habit ;  but  iljere  are  many  fm-ms  which  are  pelagic  in  habit 
and  present  many  modifications  of  structure  in  adaptation  to 
this  mode  of  li^o,  and  were  c(Misoi[uently  classitied  at  one  time 
as  a  distinct  order,  the  PrKnoroDA,  and  consei^uentiy  call  for 
special  mention.  One  of  the  most  charat'jristic  features  of 
this  group  is  the  foot,  which  is  limited  to  the  anterior  })ortiou 
of  the  body  and  consists  of  a  small  nieuiau  portion  and  two 
lateral  wing-like  parapoilia  arising  fnni  the  sides  of  the 
median   portion,   and    by  the    rapid   iiapping   of   which    the 


314 


INVERTEBHATE  MORPHOLOGY. 


\ 


)■!  S  J 


I 


animals  are  propelled  through  the  water.  In  their  general 
form  much  diversity  is  observable.  In  accordance  with  their 
pelagic  habits  the  majority  are  more  or  less  transparent ;  and 
some,  the  Gymnosomafa,  e.g.  Pneumodermay  Clione,  etc.,  are 
entirely  destitute  of  a  shell,  mantle,  and,  except  in  Pneumo- 
derma  and  its  allies,  of  a  branchia.  Others,  the  Thecosomata, 
possess  these  structures,  however — tL  j  shell  in  Limacina  being 
spirally  coiled,  the  mantle-cavity  situated    in  front  of  the 


■  ^ 


Pig.  140. — Hyalea  complanaia  (after  6e(;enbaur,  from  Hertwio). 

a  =  anus.  m  =  mantle. 

br  =  braucbiae.  oe  =  a?sophagus. 

c  =  heart.  re  =  nephridia. 
O  =  reproductive  organs.  »  =  stomach. 

h  =  digestive  gland.  //  =  pedal  ganglion. 

visceral  hump  being  without  a  branchia ;  in  Styliola  the  shell 
is  not  coiled,  but  is  cone-shaped  and  bilaterally  symmetrical, 
the  mantle-cavity  containing  a  gill ;  while  in  Cymhidiopsis  the 
original  shell  is  replaced  by  a  cartilaginous  case  formed  by 
the  subepidermal  tissues  of  the  mantle,  and  the  voluminous 
mantle-cavity  contains  no  gill.  The  head  of  the  Gymnoso- 
mata  carries  a  non-retractile  proboscis,  at  the  extremity 
of  which  is  situated  the  mouth,  and  it  may  furthermore  bear 
in  addition  to  the  tentacles  usually  present  peculiar  tentacle- 
like processes,  sometimes  provided  with  suckers  and  perhaps 


Wl^^SSSm 


TYPE  M0LLU8CA. 


315 


general 
th  their 
ut;  and 
)tc.,  are 
^neumo- 
isomata, 
la  beiug 
of  the 


3  shell 
itrical, 
s?,§  the 
ed  by 
linous 
iiioso- 
eniity 
bear 
itacle- 
rhaps 


\i 


modifications  of  portions  of  the  foot,  being  innervated  from 
the  pedal  ganglia.  In  these  forms  also  fringed  or  simple  pro- 
cesses of  the  posterior  portion  of  the  body  occur  which  serve 
as  respiratory  organs,  though  they  are  not  homologous  with 
the  true  branchia  which  in  Pneumoderma  coexist  with  them. 

Suborder  Nudihraiichia. 

In  the  Nudibranchs   the   visceral   hump   has   undergone 
elongation  parallel  with  the  long  axis  of  the  foot,  from  which  it 
is  not  distinctly  marked  off,  and  an  apparent  bilateral  sym- 
metry is  manifested  by  the  body.     This  condition,  however, 
is  evidently  entirely  secondary,  as  is  shown  by  the  structure 
of  the  heart  and  nephridium,  in  which  the  usual  asymmetry'  is 
well  marked.    There  is  no  shell,  mantle,  or  cteuidia.    Adaptive 
branchiae  are,  however,  frequently  developed,  as  in  Pleurophyl- 
Udia,  where  they  form   a   series  of  folds 
which  lie  in  a  groove  at  the  side  of  the 
body  and  recall  somewhat  the  arrange- 
ment in  the  Chitonidse,  or  in  Boris,  where 
they   surround   the   anus,   which   has   a 
dorsal  position,  and  form  a  circle  of  pin- 
nate processes.     In  the  pelagic  Phyllirhoe 
and  in  the  creeping  Limapontia,  however, 
there  is  no  trace  of  respiratory  organs. 
Many  forms   (Fig.    141),  such  as  ^oJis, 
FacelUna,  and  their  allies,  bear  upon  the 
<lorsal  surface  of  the   body  numbers  of 
fluger-like  processes  usually  arranged  in 
bunches,  and  frequently  brightly  colored. 
These  cerata  frequently  enclose  branches 
ii'om  the  intestine  which  correspond  to 
the  digestive  gland  of  other  forms,  and  p^^    ^^^ 
bear  at  their  extremities  a  sac  in  which 
are  developed  nematocysts.    These  organs 
are  usually  richly  provided  with  blood- 
vessels,  and   are    probably   respiratory  in    function,  though 
the  presence   of  nematocysts  renders   it  probable  that  they 
are    also    protective — an  idea  which    is   confirmed  by   their 


-  NuDinnAN- 

CniATE        Ol'ISTHO- 
BRANCH  i^olid). 


IM 


316 


INVERTEBRATE  MORPHOLOGY. 


i 


I  m'' 


\\ 


'ti 


m 


ill!  11 
if  u 


usually  brilliant  coloration.  The  foot  in  the  pelagic  Phyl- 
lirhoe  has  entirely  disappeared,  but  is  usually  elongated  and 
provided  with  a  broad  flat  surface,  in  accordance  with  the 
creeping  habits  of  the  Nudibranchs.  Parapodial  folds,  such 
as  occur  in  the  Tectibranchs,  are  never  developed. 

Order  Fulmonata. 

The  Pulmonates  diflfer  from  all  the  other  groups  of  Gas- 
^  teropods  in  that  they  are,  with  the  exception  of  a  single  genus, 
Onchidium,  either  terrestrial  or  aquatic ;  and  in  adaptation  to 
this  assumed  habit  certain  well-defined  changes  have  occurred. 
In  some  genera,  more  especially  the  aquatic  forms,  such  as 
Zimncea,  Physa,  and  Planorhis,  the  visceral  hump  has  its  typi- 
cal Gasteropod  development,  and  is  spirally  coiled ;  but  in 
many  terrestrial    j         such  as  Limax  (Fig.  142,  .4),  ArioUy 

and  Vuginida,  it  is  low  and  elon- 
gated parallel  to  the  long  axis 
of   the   foot  with   which   it  is 
fusei.     The   mantle   is  in   all 
forms  well  developed,  but  pre- 
sents the  peculiarity  that  it  is 
fused  by  its  edges  to  the  body- 
wall  except  at  one  point  upon 
the  right  side,  where  an  open- 
ing is  left  by  which  the  other- 
wise completely-closed  mantle- 
cavity  communicates  with  the 
exterior  and  through  which  air 
may  be  taken  into  the  cavity.    The  position  of  the  mantle- 
cavity,  when  not  interfered    with  by   secondary  changes,  is 
upon  the  right  side  of  the  body  and  somewhat  in  front  of 
the  visceral   hump  when  this  is  present.     A   spirally-coiled 
shell   is   present   in  all  forms  in  which  the   visceral   hump 
is  well  developed,  as  in  Limmva,  Physa,  Helix  (Fig.  142,  P), 
and   Planorhis,   but    in    the    elongated    terrestrial    forms   a 
rudimentation  of  the  shell   accompanies   the   diminution  of 
the   visceral   hump.     Thus   in    Dmidehardia,   in  which   only 
a  slight  trace  of  the  hump  persists,  the  shell  has   become 


Fig 


142.—^,  Limax  maximus;  B, 
Helix  (after  Howes). 


TYPE  M0LLU8CA. 


317 


quite  small,  though  still  showing  plainly  a  spiral  form  ;  but  in 
Umax  it  is  represented  only  by  a  partially  calcified  plate,  im- 
bedded in  the  roof  of  the  mantle-cavity  by  the  closure  over 
it  of  a  fold  of  the  mantle.  In  Arion  only  a  few  isolated  parti- 
cles of  carbonate  of  lime  persist,  while  in  Vaginula  and  Onchi- 
dium  all  trace  of  it  has  disappeared. 

A  marked  characteristic  of  the  Pulmonata  is  found  in  the 
character  of  their  respiratory  organ.  A  cteuidium  is  entirely 
wanting,  the  only  trace  of  its  existence  being  the  occurrence 
in  some  of  the  aquatic  forms  {Limncea,  Physa,  etc.),  of  an  os- 
phradium  near  the  mantle-pore.  Its  place  is  taken  by  the 
roof  of  the  mantle-cavity,  which  receives  a  rich  vascular  net- 
work and  functions  as  a  lung,  the  mantle-cavity  containing 
air  which  can  be  renewed  through  the  mantle-pore.  The 
heart  is  situated  far  back  in  the  mantle-cavity,  its  auricle 
lying  in  front  of  the  ventricle  and  receiving  the  blood  from 
the  more  anteriorly-situated  lung,  so  that  the  relation  of  the 
respiratory  organ  to  the  heart  is  the  same  as  obtains  in  the 
Opisthobranchs.  In  the  immediate  neighborhood  of  the 
heart  lies  the  single  nephridium,  opening  into  the  mantle- 
cavity  or  else  into  the  terminal  portion  of  the  rectum  {Helix), 
this  structure  opening  on  the  right  side  of  the  body  in  close 
proximity  to  the  mantle-pore. 

As  in  the  Opisthobranchs,  the  rotation  of  the  mantle-cavity 
and  it«  organs,  as  indicated  by  its  position  on  the  side  of  the 
body,  has  not  extended  as  far  as  in  the  Prosobranchs,  and 
consequently  there  is  no  crossing  of  the  pleuro-visceral  con- 
nectives. The  Pulmouates  are  orthoueurous.  The  ganglia 
are  present  in  typical  number,  and  are  massed  together,  as  in 
some  Opisthobranchs  and  Prosobranchs,  behind  the  buccal 
mass. 

Special  visual  organs  are  invariably  present  with  the  struct- 
ure which  has  already  been  described.  In  some  forms  they 
arf,  situated,  as  in  the  Prosobranchs,  at  the  bases  of  the  ten- 
tacles ;  while  in  others  they  are  found  at  the  tips  of  these 
structures — the  Pulmonates  being  divisible,  according  to  the 
situation  of  the  eyes,  into  the  Bmommatophora,  including  such 
forms  as  lAmnwa,  Phyaa,  IHanorbis,  and  in  general  the  aqua- 


"1  I 


J   i 


if 


1  h 


III  ill 


li 


'1. 

if.  ■*  *■ 


bin 
I 


:   '  ft. 


w 

m 


318 


INVERTEBRA  TE  MORPIIOLOO  T. 


tic  forms,  and  the  Stylommatophora,  which  includes  the  ter- 
restrial  forms,  and  Onchidium, 

This  last  genus  in  addition  to  the  usual  eyes  borne  upon  the  tentacles 
is  in  some  species  further  provided  with  a  number  of  eyes  situated  upon 
the  back  and  differing  from  the  typical  eye  in  the  arrangement  of  the  retinal 
cells.  As  has  been  seen,  the  optic  nerve  in  typical  eyes  on  entering  the  eye 
spreads  out  in  a  layer  to  form  the  retina,  the  terminal  optic  cells  being 
situated  on  that  surface  of  the  retina  which  is  turned  towards  the  light. 
The  dorsal  eyes  of  Onchidium,  however,  present  a  somewhat  different 
arrangement,  the  cells  in  which  the  nerve-fibres  teiminate  having  their 
distal  ends  turned  away  from  the  light,  which  to  affect  them  must  pass 
through  the  layer  of  nerve-fibres  formed  by  the  spreading  out  of  the  optic 
nerve.  Compared  with  the  retinae  of  typical  eyes,  those  of  the  dorsal  eyes 
of  Onchidium  are  inverted  and  have  assumed  an  arrangement  exceedingly 
rare  in  Invertebrates,  but  typical  for  the  lateral  eyes  of  the  Vertebrata. 

Otocysts  are  always  present,  and  the  tentacles  borne  by  the 
head  are  probably  tactile  in  function.  In  the  Stylommatoph- 
orous  Pulmonates  there  are  in  some  cases  {Helix)  two  pairs 
of  such  tentacles,  the  eyes  being  situated  upon  the  posterior 
pair,  both  pairs  furthermore  being  capable  of  being  invagi- 
nated  for  protection  into  the  body-cavity,  a  peculiarity  not 
presented  by  the  tentacles  of  the  Basommatophora.  As 
stated  above,  the  osphradiura  is  represented  in  certain  aquatic 
forms,  but  in  the  Stylommatophora  it  has  disappeared  with 
the  suppression  of  the  cteuidium. 

The  Pulmouata  are  hermaphrodite,  the  epithelium  of  the 
reproductive  gland  (lig.  143,  kg)  differentiating  into  both 
spermatozoa  and  ova,  there  being  no  localization  of  the  for- 
mation of  either  one  or  the  other  in  a  special  portion  of  the 
gland,  as  happens  in  some  Opisthobrauclis.  In  the  Basom- 
matophora and  certain  terrestrial  Pulmonates,  such  as  Vagi- 
nnla  and  Qnchidium,  the  common  duct  {hd)  for  the  spermatozoa 
and  ova  divides  and  passes  to  the  exterior  by  two  distinct 
and  separate  apertures.  Thus  in  LimiKea  tlie  hermaphro- 
dite duct  shortly  after  leaving  the  gland  divides,  and  into  one 
of  the  branches  immediately  after  the  division  there  opens  a 
well-developed  albuminiparous  gland  (al),  and  it  then  becomes 
somewliat  folded,  forming  what  is  termed  the  uterus  (ut). 
Beyond  this  structure  the  duct,  now  known  as  the  oviduct  (od), 
receives  the  duct  of  a  nidamental  gland  and  dilates  into  a 


.■tois>*ilw**^ 


TYPE  MOLLUSC  A. 


319 


'  the 

both 

e  for- 

f  the 

h,som- 

Vagi- 

ozoa 

;inct 

hro- 

one 

ms  a 

)mes 

(ut). 

{od\ 

to  a 


large  pyriform  structure,  which  tapers  somewhat  to  form  a 
vagina  opening  to  the  exterior  and  receives  a  duct  from  the 
receptaculum  semiuis.     The  vas  deferens  (fd)  shortly  after 
its  separation  from  the   hermaphrodite  duct  dilates  into  a 
glandular  structure,  the  prostate  gland,  from  which  the  nar- 
row duct  passes  onward  to  terminate  in  an  enlarged  penis- 
sheath  {pe)  which  contains  the 
muscular    protrusible    penis 
and    opens    to    the    exterior 
quite     independent     of     the 
opening  of  the  vagina. 

In  the  majority  of  the 
Stylommatophora  (Fig.  143), 
however,  the  two  ducts  open 
into  a  common  atrium  so  that 
only  one  genital  orifice  occurs, 
as  in  some  of  the  Opistho- 
branchs  (see  p.  312).  Other- 
wise the  arrangement  is  simi- 
lar to  what  has  been  de- 
scribed for  the  Basomniato- 
phora,  except  that  in  some 
forms,  as  HeliXy  one  or  two 
additional  accessory  struc- 
tures are  added.  Thus  the 
atrium  has  communicating 
with  it  a  sac  which  contains 
a  sharp  calcareous  rod,  the 
"dart,"  which  serves  as  a 
stimulus  during  copulation, 
being  plunged  into  the  body 
of  the  other  party  to  the  act ; 
and  again  just  at  the  point  where  the  vas  deferens  opens 
into  the  penis  it  has  communicating  with  it  an  elongated 
tubular  structure,  the  "  flagellum,"  which  perhaps  furnishes 
the  material  of  which  the  capsule  of  the  spermatophoies  is 
composed. 

Development  and  Affinities  of  the  Gasteropods.—Uhe  devel- 
opment of  the  Gasteropods  is  made  interesting  on  account  of 


Fig.  143.— Repkoddctive  Organs  of 

Liniax  maximus  (aftei-  Simroth). 
al  =  tilbuminiparousglaud. 
hd  =  lieimapbrodite  duct. 
hg  =  hermiiphrodite  gland. 
li  =  ligament. 
od  =  oviduct. 
pe  =  penis-sbeatb. 
rs  =  receptaculum  seminis. 
ut  =  uterus. 
vd  =  vas  deferens. 
V8  =  vesicula  seminalis. 


320 


INVERTEBRATE  MORPHOLOGY. 


{ 


the  occurrence  in  the  majority  of  forms  of  a  larva  known  as 
the  Veliger  (Fig.  144)  which  presents  many  interesting  affin- 
ities to  the  Annelid  Trochophore.  In  the  early  stages  of  de- 
velopment the  embryo  is  strictly 
bilateral,  with  the  mouth  and 
anus  at  the  extremities  of  the 
longitudinal  axis.  Upon  the  dor- 
sal surface  posteriorly  is  a  de- 
pression lined  with  columnar  cells 
which  secrete  the  larval  shell  [Sh), 
and  in  front  of  this  is  an  area 
enclosed  by  two  rows  of  cells 
bearing  stout  cilia  and  forming 
the  velum  (F).  This  band  of 
cilia  is  prsBoral  (Pro)  in  position, 
and  in  addition  to  it  a  second 
band  of  smaller  cilia  is  to  be 
found  which  passes  ventrally  to 
the  mouth  and  constitutes  a  post- 
oral  band  (Poo),  the  groove  be- 
tween it  and  the  prseoral  baud 
being  occupied  by  the  adoral  cilia.  On  the  ventral  surface 
is  found  a  prominence  which  represents  the  foot. 

In  later  stages  the  lateral  edges  of  the  velum  are  drawn 
out  so  as  to  form  a  broad  lobe,  sometimes  divided  into  two 
arms,  projecting  on  each  side  of  the  head ;  the  prseoral  and 
postoral  bands  of  cilia  extending  round  the  margin  of  the 
fold,  not,  however,  completely  enclosing  the  velar  area,  but  re- 
maining open  on  the  dorsal  surface.  The  shell  area  increases 
markedly  in  size,  the  shell  becoming  spirally  coiled,  the  vis- 
ceral hump  which  develops  in  the  shell  area  likewise  assum- 
ing the  coiled  form.  At  the  margins  of  the  shell  area  a  fold 
appears,  the  rudiment  of  the  mantle,  which  gradually  increases 
in  size  as  the  shell  area  extends,  and  at  the  same  time  the 
anus  becomes  rotated  forwards  from  its  original  terminal  posi- 
tion along  the  right  side  of  the  body  to  a  greater  or  less  ex- 
tent. As  these  changes  progress,  the  embryo  gradually  ap- 
proaches more  and  more  to  the  adult  form,  diflfering  from  it 
mainly  in  the  existence  of  the  velum,  by  means  of  which  it 


Fig.  144.— Veliqkb  Lakva. 
F=  foot. 
M  =  mouth. 
Oc  =  eye. 
Poo  =  postoral  bund  of  cilia. 
Pro  =  praforal  band  of  cilia. 
8h  =  shell. 
'T  =  tentacle. 
V  =  velum. 


TYPE  MOLLUSCA. 


321 


leads  a  free-swimming  pelagic  existence,  assuming  the  adult 
habit  only  after  a  further  growth  which  is  accompanied  by  a 
reduction  of  the  velum. 

Such  a  Vt'liger  larva  occurs  in  the  life-history  of  the  majority  of  the 
Gasteropoda,  though,  as  might  be  expected,  it  undergoes  certain  modifica- 
tions more  especially  in  terrestrial  forms,  though  even  in  these  there  are 
ample  indications  of  its  existence.  Indeed  the  Veliger  is  so  frequent  in  its 
occurrence  that  the  conclusion  is  almost  unavoidable  that  it  has  an  ances- 
tral significance  and  represents  in  a  more  or  less  modified  condition  a 
primitive  form  from  which  the  MoUusca  have  descended.  A  comparison  of 
the  Veliger  with  the  Annelid  Trochopliore  brings  out,  as  already  men- 
tioned, numerous  similarities.  These  are  especially  noticeable  in  the  ar- 
rangement of  the  ciliary  bands,  which  res'-Tible  those  of  the  Trochopliore 
part  for  part,  even  to  the  dorsal  bre".k  in  their  continuity.  It  is  difficult 
to  believe  that  such  marked  similarities  should  have  been  acquired  inde- 
pendently in  the  larvae  of  two  different  groups  of  animals  and  have  become 
so  characteristic,  a  difficulty  rendered  all  the  greater  by  the  occurrence  of 
other  points  of  similarity,  such  as  the  development  of  the  mesoderm,  in 
some  forms  at  least,  from  a  pair  of  mesoblasts  situated  at  the  posterior 
extremity  of  the  blastoc«l ;  the  existence  of  a  thickening  of  the  ectoderm 
in  the  centre  of  the  velar  area  in  some  forms,  corresponding  to  the  apical 
plate  of  the  Trocophore  ;  and  the  occurrence  of  a  larval  excretory  organ  or 
nephridium  in  some  Veligers  which  may  be  compared  to  the  larval  ne- 
phridium  or  head-kidney  of  the  Trochophore.  The  probable  significance  of 
this  larval  form  will  be  more  suitably  discussed  at  the  conclusion  of  this 
chapter  ;  it  re'nains  to  be  said  here  regarding  it  that  the  occurrence  among 
the  Pteropods  of  larvae  with  several  bands  of  cilia  surrounding  the  visceral 
hump  is  pi'obably  to  be  explained  as  a  secondary  adaptation,  just  .as  the 
mesotrochal  Annelid  larvae  are  probably  secondary  modifications  of  a  Tro- 
chophore. 

As  regards  the  relationships  of  the  various  groups  of  Gasteropods 
among  themselves,  there  is  little  doubt  but  that  the  Diotocardiate  Proso- 
branchs  are,  on  the  whole,  the  most  primitive  of  all  the  groups  and  stand 
nearest  to  the  Amphineura,  and  from  them  the  Monotocardia  have  devel- 
oped. The  Opisthobranchs  and  Pulmonates  are  apparently  closely  re- 
lated, the  latter  group  having  been  derived  from  Tectibranchiate  ancestors 
somewhat  more  generalized  probably  than  any  Opisthobranch  now  living. 
The  orthoneurous  character  of  the  nervous  system  and  the  structure  of  the 
reproductive  system  in  the  two  groups  indicates  their  affinity,  and  it  seems 
probable  that  the  Pulmonates  are  to  be  regarded  as  Opisthobranchs  which 
have  accommodated  themsel  ves  at  first  to  an  amphibious  life,  somewhat 
similar  to  that  now  led  by  Onchidium,  and  later  to  one  purely  terrestrial, 
at  the  same  time  differentiatinj»  an  organ  for  aerial  respiration.  Such  an 
origin  would  imply  that  the  aquatic  species  have  secondarily  taken  to  fresh 
water  as  a  habitat,  having  originally  been  terrestrial,  an  idea  which  on 


11    !: 
,1:  h:, 


§ 


322 


INVERTEBRATE  MORPUOLOOY. 


a  priori  grounds  seems  improbable  ;  but  there  seems  to  be  no  good  reason, 
if  the  aquatic  forms  are  derived  directly  from  marine  ancestors,  why  their 
ctenidia  should  have  become  replaced  by  a  lung,  since  in  the  aquatic  Pro- 
sobranch  Paludina  tlie  ctenidium  is  still  retained.  On  the  other  hand,  it 
may  be  again  mentioned  that  the  terrestrial  Prosobranchs  such  as  Cycle- 
stoma,  Acivula,  etc.,  have  lost  their  ctenidium  and  resemble  a  Pulmonato 
in  their  mode  of  respiration. 


III.  Class  Scaphopoda. 

The  class  Scaphopoda  coutains  a  small  number  of  closely- 
related  genera  of  marine  Mollusca,  Dentalium,  Siphonodenta- 
lium,  Cudulus,  etc.,  living  imbedded  in  the  sand  in  depths  of 
from  10  to  100  fathoms  and  possessing  but  slight  powers  of 
locomotion.  They  resemble  the  Gasteropoda  in  possessing  a 
visceral  hump  which  is  relatively  enormously  elongated  but 
does  not  undergo  a  spiral  twisting,  nor  has  it  fallen  over  to 
the  right  or  left  side  of  the  body.  Consequently  the  Scaph- 
opods  are  bilaterally  symmetrical  and  stand  in  marked  con- 
trast in  this  respect  to  the  Gasteropods. 

The  mantle-folds  are  two  in  number,  arising  from  the 
anterior  surface  of  the  visceral  hump  and  extending  around 
the  body  so  as  to  completely  enclose  it,  meeting  posteriorly 
and  fusing  together,  except  for  .  short  extent,  dorsally  and 
veutrally,  and  forming  thus  a  tube  to  the  anterior  wall  of 
which  the  body  is  as  it  were  attached.  This  tube  is  open  at 
either  end,  the  ventral  opening  being  somewhat  larger  than 
the  dorsal  one,  and  the  whole  is  enclosed  within  a  tubular 
shell  (Fig.  145,  sli)  whose  shape  corresponds  essentially  to  that 
of  the  mantle.  From  the  ventral  opening  the  foot  (/)  pro- 
jects to  a  greater  or  less  extent,  being  in  Dentalium  a  cylin- 
drical structure,  terminating  in  a  conical  process  provided 
with  two  lateral  lobes. 

The  mouth  (m)  is  situated  at  the  extremity  of  a  cylindrical 
proboscis  (not  to  be  confounded  with  the  protrusible  proboscis 
of  a  Gasteropod)  and  is  surrounded  by  a  number  of  leaflike 
tentacles,  while  at  the  base  of  the  snout  there  is  upon  each 
sidrt  a  bunch  of  long  filamentous  tentacles  {t)  capable  of  being 
protruded  from  the  mouth  of  the  shell  and  of  being  withdrawn 
within  it.     Each  tentacle  terminates  in  a  spoon-shaped  struct- 


TYPE  MOLLUSC  A. 


323 


81) 


mc> 


ure  whose  concave  surface  is  furuislied  with  ciliated  cells  and 
also  towards  the  margin  with  unicellular  glands.  These 
structures  have  been  supposed  to 
rei^resent  the  ctenidia  of  the  other 
MoUusca,  but  this  view  cannot,  in  the 
present  condition  of  our  information 
concerning  their  structure  and  devel- 
opment, be  accepted  without  reser- 
vati<jn.  The  mouth  opens  into  a 
short  oesophagus  provided  with  a 
single  chitinous  jaw-tooth  apparently 
formed  by  a  fusion  of  two  chitinous 
masses,  and  behind  this  there  is  a  m. 
pharynx  provided  with  a  radula  and 
opening  posteriorly  into  the  some- 
what U-shaped  more  or  less  convolu- 
ted intestine  {i)  which  terminates  in 
the  anus  (a)  lying  in  the  mid-ventral 
line  behind  the  foot.  Into  the  intes- 
tine at  the  turn  of  the  U  there  open 
the  ducts  of  the  digestive  gland  (?), 
and  into  the  posterior  portion  of  the  Fig.  145.  —  Structuhe  of 
intestine,  the  rectum,  there  open  in  Dentalium  (after  leuckart). 
Dentalium  several  ducts  from  a  rectal 
gland  which  surrounds  this  portion 
of  the  digestive  tract  and  whose  sig- 
nificance is  quite  obscure. 

The  nervous  system  presents  the 
majority  of  the  ganglia  characteristic 
of  the  Gasteropoda,  and  the  pleuro- 
visceral  connectives  do  not  cross  one 
another.  The  cerebral  ganglia  (ce) 
lie  at  the  base  of  the  proboscis  an- 
terior to  the  aisophagus  and  have 
closely  associated  with  them  the  pleural  ganglia,  the 
cerebro-pedal  and  pleuro-pedal  connectives  fusing  with  one 
another  to  pass  downwards  and  forwards  to  the  pedal  ganglion 
(pe)  situated  in  the  foot.  Posteriorly  in  the  vicinity  of  the 
rectum   lie   the   two   visceral   ganglia  (vi)  from  which   long 


a  =  anus. 

ce  =  cerebral  ganglion. 
/  =  foot. 
i  =  intestine. 
I  —  liver. 
m  =  mouth. 
mc  =  mantle-cavity. 
pe  =  pedal  ganglion. 
r  =  reproductive  organ. 
rn  =  right  nephridium. 
(i7i  =  shell. 

<   -  tentaclfc. 
■vi  =  visceral  j? .  igliou. 


824 


IN VEKTEDRA  TE  MOliPUOLOO  T. 


nerves  pass  dorsally,  but  no  special  parietal  ganglia  occur. 
Two  pairs  of  buccal  ganglia  are  also  present.  Otocysts  are 
present  imbedded  in  the  foot  in  the  neighborhood  of  the 
pedal  ganglia,  but  no  other  special  organs  of  sense,  unless 
the  bunches  of  tentacles  be  considered  such,  occur. 

No  special  respiratory  organs  are  developed,  the  mantle 
probably  subserving  the  respiratory  function.  The  heart,  a 
simple  invagination  of  the  wall  of  the  pericardial  cavity,  lies 
in  the  posterior  region  of  the  body,  on  the  dorsal  surfac  * 
the  intestine.  It  possesses  no  auricle,  but  receives  the  bi 
through  small  slits  in  its  walls.  There  are  no  special  blood- 
vessels, but  the  blood  circulates  through  a  series  of  sinuses 
traversing  the  body  in  various  directions. 

A  pair  of  nephridia  occurs  in  the  posterior  region  of  the 
body,  opening  to  the  exterior  by  a  pore  on  either  side  of  the 
anus,  but  a  communication  with  the  pericardial  cavity  is  said 
to  be  wanting.  However  this  may  be,  the  right  uephridium 
{rn)  serves  for  the  exit  of  the  reproductive  elements,  though 
the  exact  method  by  which  these  latter  make  their  way  into 
the  duct  is  unknown.  Between  each  nephridial  pore  and  the 
anus  there  is  a  pore  which  seems  to  be  the  opening  of  a  she 
tube  which  communicates  directly  with  the  schizoccelic  si 
surrounding  the  terminal  portion  of  the  intestine  and  places 
it  in  communication  with  the  surrounding  water,  a  peculiar 
arrangement  which  recalls  the  dorsal  pores  of  the  oligo- 
chsetous  Annelids.  The  Scaphopods  are  bisexual,  and  the 
reproductive  organs,  ovaries  or  testes,  are  single,  consisting  of 
long  completely  closed  sacs  with  lateral  diverticula,  lying 
along  the  posterior  wall  of  the  visceral  hump.  As  already 
stated,  the  reproductive  elements  after  the  rupture  of  the  wall 
of  the  reproductive  gland  make  their  way  to  the  exterior 
through  the  right  nephridium. 

Development  and  Affinities  of  the  Seaphopoda. — The  larva  of  Denta- 
lium,  though  presenting  considerable  resemblance  to  the  Trochophore, 
differs  from  it  nevertheless  in  several  points  of  detail.  It  possesses  a  dis- 
tinct apical  tuft  of  cilia  and  the  prototroch  is  present,  though  represented 
by  three  or  more  circles  of  cilia-bearing  cells.  The  mantle-folds  develop  at 
a  relatively  early  stage  as  two  lateral  folds,  quite  separate  along  the  ventral 
line,  the  fusion  characteristic  of  the  adult  only  appearing  later.  It  is  this 
early  development  of  the  mantle-lobes  and  the  multiplication  of  the  proto- 


teAia><*»*i»>' •"'*"■ 


TYPE  M0LLU8CA. 


325 


troch  bauds  which  obscure  the  Trochophore  characters,  a  still  earlier  larva 
presenting  greater  similarities  to  the  annelid  larva. 

By  the  earlier  writers  the  Scaphophods  were  considered  more  closely 
related  to  the  Pelecypoda  than  to  the  other  Molluscan  ^'roups,  this  relation- 
ship being  indicated  more  especially  by  the  symmetrical  form,  the  appiiiorit 
lateral  arrangement  of  the  mantle-folds  and  the  absence  of  eyes.  On  the 
other  baud,  there  are  a  large  number  of  differences  between  the  members 
of  the  two  groups,  as  for  instance  the  univalve  character  of  the  shell,  and 
especially  the  occurrence  of  a  radula  and  jaw.    This  latter  feature  suggest  .1 


FiC.  146.— DiAOBAMS  TO  SHOW  THE  OriOIN  OF  THE  SCAFHOPODS  FROM  A  ^- 

sureUar^\ie  Ancestor  (after  Plate). 
et  =  ctenidium.  go  =  reproductive  organ. 

/  =  foot.  m  =  mouth. 

$h  =  shell. 

the  Gasteropods,  and  it  seems  most  probable  that  it  is  to  this  group  that 
tlie  Scaphopods  should  be  considered  as  related.  They  must,  however,  be 
referred  to  the  more  primitive  Gasteropods,  those  in  which  the  rotation  of 
the  mantle-chamber  had  not  occurred.  An  elongation  of  the  dorsal  hump 
of  a  Fissiirella-Vike  ancestor  unaccompanied  by  a  twisting  to  one  side,  as 
represented  in  Fig.  146,  would  bring  about  a  condition  from  which  -fc  does 
not  seem  a  great  step  to  reach  the  Scaphopods. 

Among  recent  Gasteropods  it  is  with  the  Diotocardiates  that  the 
Scaphopods  seem  to  be  most  affiliated,  and,  as  we  shall  later  see,  it  is  from 
the  primitive  members  of  this  order  that  the  Pelecypods  have  probably 
been  derived,  and  thus  any  similarities  which  may  exist  between  the 
Scaphopods  and  Pelecypods  is  readily  explicable  on  the  basis  of  a  similar 
ancestry,  both  groups  being  derived  from  Prosobranch-like  forms.  The 
absence  of  a  larva  corresponding  closely  to  the  Gasterojiod  Veliger  would 
seem  to  oppose  such  a  view,  but  it  must  be  remembered  that  the  Veliger  is 
characteristic  only  of  the  more  highly-differentiated  Prosobranchs— such 


i  \ 


i  1 


326 


INVERTEBRATE  MORPUOLOGT. 


forms  as  Patella,  for  instance,  having  a  larva  destitute  of  some  of  the 
more  cliaracteristic  Veliger  features  and  more  closely  resembling  the 
Annelid  Trochophore  and  the  Scaphopod  larva. 


IV.  Class  Felectfoda. 

The  class  Pelecj'poda,  also  kuowu  as  the  Lamdlihrancliiay 
coutaius  a  uumber  of  fresh-water  genera,  though  the  majority 
are  marine,  aud  all  its  members  retain  the  primitive  bilateral 
symmetry  of  form,  no  visceral  hump  being  developed.  The 
body  is  more  or  less  laterall}'  compressed  and  two  large 
mautle-folds  (Fig.  149,  m)  are  developed,  arising  one  on  each 
side  a  short  distance  veutrad  of  the  dorsal  mid-line  and  extend- 
ing downward  so  as  to  meet  below.  They  thus  enclose  a  wide 
space,  the  mantle-cavit}-,  between  their  inner  surfaces  and  the 
body-wall,  within  which  lie  the  ctenidia  (Fig.  148,  ct)  and  the 
foot  {p).  Upon  the  mantle-edge  in  many  forms  tentacles, 
papillae,  glands,  and  eyes  are  developed,  and  in  many  cases 
the  edges  of  the  two  lobes  may  fuse  more  or  less  completely, 
openings  being,  however,  left  for  the  entrance  and  exit  of 
water  into  the  mantle-cavity,  and  also  for  the  protrusion  of 
the  foot.  All  gradations  of  fusion  are  represented :  thus  in 
JVucida,  Ostrea,  etc.,  there  is  no  fusion  whatever ;  in  Unio 
(Fig.  149)  and  other  forms  the  posterior  edges  of  the  mautle- 
folds  are  modified,  so  that  while  the  edges  of  the  folds  are  in 
contact  throughout  the  greater  portion  of  their  extent  two 
openings  are  left,  through  the  uppermost  of  which,  the  exha- 
lent  opening  (eo),  water  carr3ing  with  it  the  excreta  and  the 
reproductive  elements  finds  an  exit,  while  through  the  lower 
one,  the  inhalent  opening  (/a),  fresh  water  passes  in ;  in  the 
next  gradation  the  point  of  separation  between  these  two 
openings,  which  in  C^nio  was  simply  formed  by  the  contact  of 
the  mantle-edges,  becomes  permanent  by  the  fusion  of  these 
latter  parts,  and  a  further  stage,  seen  in  Venus  for  example, 
is  formed  by  the  fusion  of  the  mantle-edges  ventral  to  the 
branchial  opening,  a  fusion  which  may  extend  forward  a  con- 
siderable distance.  In  this  last  condition  there  are  three 
openings  which  place  the  mantle-cavity  in  communication 
with  the  exterior,  one  anterior,  through  which  the  foot  is  pro- 


..,*^>i,..,*«i»is*^'' 


TYPE  MOLLUaCA. 


327 


truded,  and  two  posterior,  the  braucliial  aud  aual  openings. 
The  mantle  around  these  latter  frequently  becomes  prolonged 
so  that  two  tubes,  or  siphons  as  they  are  termed,  are  formed, 
sometimes  in  contact  with  one  another  {Phohvi),  sometimes 
quite  sepairate  ( Venus)^  sometimes  capable  of  n^tractiou  witliiu 
the  shell,  sometimes  so  large  as  to  be  incapable  of  retraction 
{Myd). 


Fig.  147.— .4,  Mya  arenarin  with  the  siphons  slightly  expanded;  li,  iiintT  sur- 
face of  tlie  right  valve  of  the  shell  of  Mf/a. 

aa  =  impression  of  anterior  ad-  jm  =  impression  of  posterior  ad- 
ductor muscle.  ductor  muscle. 

I  =  ligament.  pi  =  pallial  line. 

m  =  mantle  edge.  a  =  siphon. 

p  =  foot.  si  =  siphonal  impression. 

In  conformity  with  the  form  of  the  mantle-lobes  the  shell 
consists  of  two  similar  portions  or  valves,  lying  on  the  sides 
of  the  body  and  united  along  the  dorsal  mid-line  by  a  hinge. 
The  hinge  is  formed  by  a  ligament,  as  it  is  termed,  which  is 
really  a  portion  of  the  shell  substance,  and  consists  of  an 
external  portion  continuous  with  the  epidermis  of  the  shell 
and  an  iuterntil  elastic  portion,  frequently  calcified  to  a  cer- 
tain extent,  and  continuous  with  the  middle  layer  (prismatic 
layer)  of  the  shell.  When  at  rest  the  two  valves  of  the  shell 
are  kept  apart  along  the  ventral  line  by  the  elasticity  of  the 
hinge-ligament,  and  it  is  only  by  the  application  of  force  that 


328 


IN  VERTEBRA  TB  MORPHOLOG  T. 


the  two  valves  can  be  brought  togetlier,  the  ligament  being 
then  compressed.  The  hinge  is  fiequeutlj  complicated  by 
the  development  of  tooth-like  processes  and  corresponding 
sockets  so  that  the  two  valves  may  be  firmly  locked  together. 
Upon  the  inner  surface  of  the  valves  are  certain  impressions 
produced  by  the  softer  parts  and  of  considerable  value  in 
systematic  conchology.  A  short  distance  from  the  margin  of 
each  valve  and  parallel  to  it  is  a  distinct  line,  the  pallial 
impression  (Fig.  147,  'pl\  produced  by  the  attachment  of  the 
muscle-fibres  which  bind  the  mantle-lobes  to  the  shell.  In 
some  forms,  such  as  Anodon,  this  pallial  line  follows  the  shell 
margin  throughout  its  entire  course,  but  in  those  genera 
which  possess  well-developed  and  retractile  siphons  it  is 
deeply  incurved  in  the  posterior  portion  of  its  .rse.  Other 
markings  of  the  shells  are  produced  by  the  insertion  into 
them  of  a  number  of  muscles.  The  largest  and  most  import- 
ant of  these  are  the  adductor  muscles  of  the  shell  (aa),  large 
muscles  passing  from  one  valve  to  the  other,  by  their  contrac- 
tion overcoming  the  elasticity  of  the  hinge-ligament  and  clos- 
ing the  shell.  In  the  majority  of  forms  there  are  two  such 
muscles,  situated  towards  the  anterior  and  posterior  portion 
of  the  body,  but  not  unfrequently,  as  in  Ostrea  and  Pecten, 
but  one,  corresponding  to  the  posterior  adductor  of  other 
forms,  is  present.  In  the  immediate  vicinitj'  of  the  adductor- 
impressions  other  Smaller  muscle-impressions  are  usually 
observable,  produced  by  the  protractor  and  retractor  muscles 
of  the  foot  and  siphons. 


Although  in  the  Pelecypod  shell  the  two  valves  are  typically  similar 
and  symmetrical,  yet  in  a  number  of  cases  a  marked  dissimilarity  is  found 
in  tlieir  shape.  Thus  in  Ontrea  the  valve  upon  which  the  animal  rests, 
usually  the  left  valve,  is  large  and  concave,  while  the  other  is  smaller 
and  flattened,  and  a  similar  relation  is  found  in  other  forms  which  be- 
come temporarily  fastened  to  rocks,  etc.  Occasionally  additional  cal- 
careous plates  are  added  to  the  usual  shell,  as  in  the  boring  moll usk  P/to/as, 
in  which  three  accessory  calcareous  plates  are  developed  on  the  dorsal  sur- 
face of  the  body.  In  the  Ship-worm,  or  Teredo,  which  bores  extensively 
into  timber  and  is  in  some  cases  exceedingly  destructive,  the  true  shell- 
valves  are  very  small  and  situated  at  the  anterior  end  of  the  body,  and  the 
mantle  projects  backwards  far  beyond  them  and  secretes  a  thin  calcareous 
tube  which  lines  the  interior  of  the  passages  excavated  by  the  animal.    A 


.i^a*-**"*^ 


TYPE  MOLLUSCA. 


329 


r- 

y 
1- 

10 
IS 


similar  peculiarity  is  found  in  the  Aspergilliim.  Here,  too,  the  true  shell- 
valves  are  exceedingly  small  and  are  united  together  by  and  imbedded  in 
a  calcareous  tube  secreted  by  the  mantle,  which  projects  far  beyond  the 
shell  proper  and  is  fused  throughout  the  greater  portion  of  its  extent. 
The  calcareous  tube  is  open  behind  for  tlie  passage  <jf  the  two  siphons, 
but  anteriorly  is  closed  by  a  perforated  plate,  the  margins  of  the  perfora- 
tions being  sometimes  prolonged  into  tubes  which  may  branch  dichoto- 
mously.  The  animal  lives  imbedded  in  the  sand,  the  posterior  ex- 
tremity of  the  shell  being  directed  upwards,  and  seems  to  have  been 
derived  from  forms  originally  possessing  a  boring  habit,  such  as  is  seen  in 
Teredo. 

Tlie  foot  of  the  Pelecypoda  is  as  a  rule  very  simple.  In 
the  most  primitive  members  of  the  group,  such  as  Nuciila 
(Fig.  151),  it  is  a  flat  disk-like  structure,  recalling  somewhat 
the  foot  of  the  Gasteropoda,  but  more  usually  it  is  a  keel- 
shaped  structure  (Fig.  149,  p).  The  modifications  in  shape 
which  it  undergoes  are,  however,  numerous  and  it  may  even 
in  some  cases  be  almost  absent,  as  in  the  Oyster  (Ostrea),  but 
special  developments,  such  as  epipodia,  are  never  found  in 
connection  with  it.  A  "  byssus-gland "  is  a  characteristic 
development  of  the  Pelecypod  foot,  consisting  of  a  cavity 
with  usuallj'^  greatly  folded  walls  lying  in  the  tissues  of  the 
foot  and  connected  with  the  exterior  by  a  canal  opening  on 
the  sole  of  the  foot.  By  the  cells  lining  the  cavity  threads  of 
a  horny  consistency  are  secreted  by  means  of  which  the 
animal  is  enabled  to  fasten  itself  to  stones,  etc.,  or  even  in 
some  cases,  as  Mytilus^  to  move  about  in  the  absence  of  a 
well-developed  foot,  throwing  out  byssus  filaments,  attaching 
them,  and  then  drawing  itself  forward  towards  them. 

The  respiratory  organs  (Fig.  149,  hr)  of  the  Pelecypoda 
consist  of  a  pair  of  platelike  structures  situated  on  each  side  of 
the  body,  and  being  attached  along  their  dorsal  margins  hang 
down  between  the  mantle  and  the  body-wall.  Notwithstand- 
ing their  platelike  form  they  are  modifications  of  the  plumose 
ctenidium  of  the  Gasteropods.  If  the  typical  bii)iunate 
cteuidium  be  imagined  to  be  directed  parallel  to  the  long 
axis  of  the  body  and  the  median  axis  to  have  fused  with  the 
body-wall,  so  that  the  two  rows  of  pium©  are  bent  down  so 
as  to  lie  parallel  to  one  another,  the  simplest  form  of  the 
Pelecypod  cteuidium,  such  as  occurs  in  Nucida  (Fig.  151),  will 


330 


INVERTEBRATE  MORPHOLOGY. 


be  obtained.  In  the  majority  of  forms,  however,  the  arrange- 
ment is  much  more  complicated  than  this.  Thus  in  Mytilus 
it  will  be  found  that  the  various  pinnae  composing  each 
plate  are  held  together  by  a  series  of  patches  of  strong  cilia 


Fio.  148. — A,  diagrammatic  section  tlirough  Pecten,  and  B,  tbrough  Anodon; 

C,  section  tlirougli  gill-lamella  of  Pecten,  and  D,  of  Anodon. 
au  =  auricle.  ol  =  outer  lamella  of  outer  gill. 

/  =  foot.  pc  =  pericardial  cavity. 

gf  =  gill-fllament.  p  =  pore. 

ill  =  inner  lamella  of  inner  gill.  «  =  blood-sinus. 

il  =  interlamellar  junction.  sbi'  =  suprabrauchial  cbaniber. 

ne  =  nepbridium.  ah  =  sbell. 

which  interlock  forming  the  "  ciliated  junctions,"  and  further- 
more the  pinnae  are  at  their  free  ends  bent  abruptly  upon 
themselves,  those  of  the  outer  row  outwards  and  those  of  the 
inner  row  inwards,  so  that  each  gill-plate  is  composed  of  two 
lamellee  (Fig.  148,  ^l).  This  condition  may  be  regarded  as  the 
next  step  in  the  modification,  which  is  continued  even 
further  by  the  permanent  union  of  the  outer  and  inner  limbs 


^,,atii*i««**''^ 


TYPE  MOLLUSCA. 


331 


s 


of  the  piuuee,  or  gill-filaments  as  they  may  be  called,  by  hollow 
processes,  the  "  interlamellar  juuctions "  (Fig.  148,  C,  il). 
A  still  greater  departure  from  the  primitive  condition 
is  found,  however,  in  the  greater  numbyr  of  existing  Pelecy- 
pods,  consisting  of  a  fusion  of  all  the  filaments  of  each  lamella 
into  a  plate  (Fig.  148,  Z>),  small  openings  {p)  only  being  left  here 
and  there  between  adjacent  filaments ;  furthermore  the  inter- 
lamellar junctions  become  very  well  developed,  so  that  the 
two  lamellsB  of  each  gill  become  firmly  united  together  to  form 
a  plate,  containing  in  the  interior  a  cavity,  the  interlamellar 
space. 

In  addition  to  these  various  modifications  which  lead  to 
the  formation  of  a  true  lamellate  gill,  the  edge  of  the  external 
lamella  of  the  outer  plate  fuses  with  the  inner  surface  of  the 
mantle,  and  the  internal  lamella  of  the  inner  plate  fuses  sim- 
ilarly with  the  side  of  the  foot  (Fig.  148,  B),  and  the  mantle- 
cavity  thus  becomes  divided  into  two  chambers.    Into  the  ven- 
tral chamber  the  iuhaleut  siphon  opens,  and  the  water  which 
enters  by  it  passes  through  the  openings  left  between   the 
filaments  and  so  reaches  the  interlamellar  spaces  which  com- 
municate above  with  the  dorsal  or  suprabranchial  chamber 
(«6r),  whence  it  passes  to  the  exterior  through  the  exhalent 
siphon.     In  the  region  of  the  foot  the  suprabranchial  cham- 
ber is  of  course  divided  into  two  portions,  one  of  which  lies 
on  each  side  of  the  base  of  the  foot,  and  each  of  these  is  again 
divided  longitudinally  into  an  inner  and  an  outer  portion  by 
the  line  attachment  of  the  gills  to  what  may  be  considered 
the  roof  of  the  mantle-cavity.  Behind  the  foot  the  inner  cavities 
of  the  two  sides  unite  and  in  some  forms  open  ventrally  into 
the  mantle-cavity  proper  ;  in  others,  however,  the  inner  lamellsB 
of  the  inner  gill-plates  fuse  with  one  another  along  the  middle 
line  so  that  a  distinct  partition,  formed  by  the  gills,  sepa- 
rates the  suprabranchial  chamber  from  the  ventral  mantle- 
chamber  throughout  its  entire  length.     In  a  few  forms,  such 
as  Cmptdaria,  the  gills  become  reduced  to  simple  muscular 
partitions  perforated  by  pores  and  separating  the  two  cham- 
bers,  practically  all   indication    of    the   original    ctenidium 
characters  having  disappeared. 

The  muscular  system  of  the  Pelecypoda  reaches  a  some- 


332 


IN  VERTEBRA  TE  MORPHOLOO  T. 


what  extensive  development  in  connection  with  the  presence 
of  the  bivalved  shell.  The  mantle-folds  are  as  a  rule  some- 
what richly  provided  with  muscle-fibres  especially  near  the 
margin;  and  where  siphons  are  developed  some  of  the  fibres 
are  specialized  into  retractors  for  these  organs.  For  the  closure 
of  the  shell-valves,  hoAvever,  more  extensive  muscular  bands  are 
present  which  seem,  like  the  siphonal  retractors,  to  be  special- 
ized portions  of  the  mantle  musculature.  Of  these  shell- 
adductors  there  may  be  one,  as  mOstrea  and  Peden,  or  two,  as  in 
Anodon  (Fig.  149,  aa  and  pa),  which  pass  transversely  across 
the  body  from  one  shell-valve  to  the  other,  in  the  form  of 
stout  compact  muscular  bands.  In  connection  with  the  foot 
special  bands  are  also  developed  which  function  as  protrac- 
tors {pp\  retractors  (rp),  and  elevators  arranged  in  pairs  and 
extending  from  the  inner  surfaces  of  the  shell-valves  to  spread 
out  below  in  the  foot.  These  various  bundles  seem  to  cor- 
respond to  the  spindle-muscle  of  the  Gasteropods. 

The  ccelom  presents  an  arrangement  similar  to  that  of 
other  Mollusca,  both  schizocoelic  and  enterocrelic  portions 
being  distinguishable.  To  the  former  portion  belong  the 
numerous  lacunar  spaces  which  traverse  the  body  and  mantle- 
folds,  and  to  the  latter  the  pericardial  cavity  (Fig.  149,^)  and 
the  cavity  of  the  reproductive  glands.  The  blood-vascular 
system  consists  of  a  heart  provided  with  two  lateral  auricles 
and  lying  in  the  pericardium.  In  the  majoritj'  of  forms  the  ven- 
tricle (<')  seems  to  be  traversed  by  the  terminal  portion  of  the 
digestive  tract,  a  condition  produced  by  its  having  folded  itself 
longitudinally  around  the  rectum,  and  which  recalls  what 
occurs  in  certain  Diotocardiate  Gasteropods  (see  p.  305).  Tliis 
arrangement  does  not,  however,  obtain  in  all  forms,  some  of 
the  more  primitive  (Niicida,  Area)  having  the  ventricle 
entirely  dorsal  to  the  intestine,  as  it  is  in  the  Amphineura,  for 
example,  while  in  a  few  others  {Ostrea)  it  has  assumed  a 
secondary  position  ventral  to  the  intestine.  From  both  the 
anterior  {ao)  and  posterior  extremities  of  the  ventricle  arteries 
arise  which,  after  branching  a  number  of  times,  pour  the  blood 
into  the  schizocoelic  lacunar  system.  Traversing  this  the 
venous  blood  is  returned  to  a  longitudinal  sinus  lying  in  the 
middle  line   of  the   body  just  below  the   pericardium  (Fig. 


jij^^gjiiauiis.*-'  / 


TYPE  MOLLUSCA. 


333 


148,  B,  «),  whence  the  j^reater  portion  passes  into  the  compli- 
cated network  of  the  uephriclia  and  thence  to  a  bh>od-vessel, 
the  branchial  artery,  runninj^  along  the  base  of  the  gill  of  each 
side.  After  traversing  the  gill-tilameuts  it  becomes  arterial 
and  is  returned  to  the  branchial  veins  which  run  parallel  to 
the  branchial  arteries  and  thence  is  returned  to  the  auricles 
of  the  heart. 

The  digestive  tract  has  a  much  simpler  structure  than  in 
the  majority  of  the  Mollusca,  lacking  all  trace  of  a  radula 
and  muscular  pharynx.  On  each  side  of  the  mouth  are  two 
usually  triangular  plates,  the  so-called  labial  palps,  the  ui)})er- 
most  of  which  meet  above  the  mouth  forming  a  sort  of  upper 
lip,  while  the  lower  ones  similarly  form  a  lower  lip.  At  the 
bottom  of  the  space  separating  the  two  palps  of  each  side  is 
a  groove  which,  starting  at  the  sides  of  the  mouth,  runs  back- 
wards along  the  sides  of  the  body  to  the  gills.  This  groove 
serves  for  the  conduction  to  the  mouth  of  the  particles  of  food 
brought  into  the  mantle-cavity  by  the  action  of  the  cilia  of 
the  gills,  the  food  of  the  Pelecypods  consisting  of  diatoms 
and  other  minute  organisms  capable  of  being  captured  in  this 
manner.  The  oesophagus  opens  into  a  stomach  (Fig.  149,  *•) 
which  receives  by  numerous  openings  the  secretion  of  the 
usually  voluminous  digestive  gland  (?),  the  so-called  liver,  and 
passes  posteriorly  into  the  intestine  (i),  which,  usually  in  sev- 
eral convolutions,  lies  imbedded  in  the  tissues  of  the  base  of 
the  foot.  In  the  wall  of  the  anterior  portion  of  the  intestine 
is  a  groove,  frequently  converted  into  a  canal,  which  may 
open  into  the  stomach  by  an  independent  opening ;  the  epi- 
thelium of  this  groove  or  canal  secretes  a  substance  which 
forms  a  transparent  glass-like  rod  lying  in  the  canal  and  pro- 
jecting  into  the  lumen  of  the  intestine.  The  function  of  this 
crystalline  style,  as  it  is  termed,  has  been  the  subject  of  much 
speculation,  the  most  plausible  theory  being  tluit  the  secre- 
tion serves  to  surround  sharp-edged  particles  of  sand  or  simi- 
lar substances,  taken  into  the  intestine  with  food,  with  a  jelly- 
like coating  which  will  prevent  them  from  injuring  the  delicate 
walls  of  the  intestine.  Towards  its  posterior  end  the  intes- 
tine bends  upwards,  i.e.  dorsally,  to  a  point  in  front  of  the  heart 
and  then  passes  directly  backwards  to  terminate  in  the  anus 


334 


INVERTEBRATE  MORPHOLOGY. 


(a)  which  opens  into  the  suprabranchial  chamber  (shr)  iu  the 
viciuity  of  the  exhalent  siphon.  The  relations  of  this  rectum 
to  the  lieart  have  already  been  noted  (p.  33^), 

The  nervous  system  of  the  Pelecypoda  differs  somewhat 
apparently  from  that  of  the  Gasteropods,  a  smaller  number 


-Structdue 


a  = 

aa  = 
ao  = 
In'  = 
eg  = 

eo  = 

/  = 

go  = 

i  = 
to  = 

I  = 
m  = 
ne  = 


anus. 

anterior  adductor. 

aorta. 

gill. 

cerebral  gauijlion. 
exhaleut  uritice  of  siphon, 
foot. 

genital  orifice, 
intestine. 

inlialent  orifice  of  siphon, 
liver, 
mantle, 
nephridium. 

uephridial  opening  into  pericar- 
dial cavity. 


np^  = 

P  = 
pa  = 

Pff  = 
PP  = 

r  = 
rp  = 

«  = 

sbr  = 

sh  = 

V  = 
vi  — 


OF  Anodon. 
nephridiul  opening  into  supra- 
branchial  cLumbcr. 
pericardial  cavity, 
posterior  adductor, 
pedal  ganglion, 
protractor  pedis, 
reproductive  organ, 
retractor  pedis, 
stomach. 

.suprabranchial  chamber, 
shell, 
ventricle, 
visceral  ganglion. 


of  ganglia  being  discernible.  Above  the  oesophagus  a  short 
distance  behind  the  mouth  is  on  either  side  a  well-marked 
ganglion  (Fig.  149,  eg)  connected  with  its  fellow  of  the  oppo- 
site side  by  a  transverse  commissure.  In  the  more  primitive 
forms  {Nucda)  two  ganglia  are  found  on  either  side,  of  which 
one  evidently  corresponds  to  the  cerebral  and  the  other  to  the 
pleural  ganglion  of  the  Gasteropods.  Where,  therefore,  as  in 
the  majority  of  the  Pelecypods,  but  a  single  ganglion  occurs  on 


t> 


^,^«U-J«Uit«tS' 


le 
m 

3r 


TYPE  MOLLUSCA, 


335 


each  side,  it  is  to  be  regarded  as  a  cerebro-pleural  ganglion. 
From  each  of  these  a  pedal  connective  passes  downwards  into 
the  foot  to  terminate  in  a  paired  pedal  ganglion  (pg),  and  a 
second  strong  connective  passes  backwards  on  each  side  of 
the  base  of  the  foot  to  terminate  in  a  large  ganglion  {vi),  sit- 
uated below  the  rectal  portion  of  the  intestine  and  frequently 
in  close  proximity  to  the  posterior  adductor  muscle,  and  which 
from  its  relations  is  evidently  to  be  regarded  as  representing 
both  the  parietal  and  the  visceral  ganglia  of  the  Gasteropods 
and  hence  may  be  termed  the  viscero-parietal  ganglion. 

The  sense-organs  are  of  essentially  the  same  nature  as  in 
the  Gasteropods.  Tactile  cells  exist  scattered  over  the  sur- 
face of  the  body,  and  are  especially'  numerous  in  certain  lo- 
calities, as  upon  the  siphons  when  these  are  present.  A  pair 
of  osphradia  are  also  present  situated  above  the  viscero-parie- 
tal ganglion  close  to  the  insertion  of  the  bases  of  the  gill- 
plates  into  the  side  of  the  body ;  and  imbedded  in  the  tissues 
of  the  foot,  usually  in  close  proximity  to  the  pedal  ganglia, 
though  innervated  by  the  cerebro-pleural,  are  a  pair  of  oto- 
cysts  having  the  usual  structure  (see  p.  283).  In  a  number  of 
forms  paired  elevations,  evidently  of  a  sensory  nature,  have 
been  found  in  the  neighborhood  of  the  inner  ends  of  the 
siphons,  or  on  the  sides  of  the  body  a  little  in  front  of  the 
anus ;  the  function  of  these  is  doubtful,  though  it  has  been 
suggested  that  they  are  olfactory. 

Eyes  are  present  in  a  number  of  forms  and  present  various 
degrees  of  complexity.  In  some  cases  a  perception  of  sudden 
variations  in  the  intensity  of  light  is  present,  as  in  the  siplious 
of  some  forms,  without  any  distinct  optic  sense-organs  being 
developed.  Sensory  and  pigment  cells  are  present,  however, 
and  may  be  regarded  as  forming  a  diffuse  optic  organ.  No 
eyes  occur  upon  the  head,  nor  are  tentacles  developed  in  any 
of  the  Pelecypods,  but  large  numbers  of  eyes  are  developed 
upon  the  edge  of  the  mantle  of  many  forms,  such  as  Pecten 
and  Spondylus.  These  eyes  may  be  simple  depressions  of  the 
mantle-margin,  the  bottom  of  the  depression  being  lined  with 
pigmented  and  sensory  cells,  a  cuticle  of  varying  thickness  cov- 
ering this  retinal  surface.  Another  form  of  eye  (Fig.  150)  also 
occurs  upon  tentacular  processes  Avliich  presents  an  arrange- 


336 


INVERTEBRATE  MORPHOLOGY. 


ment  unusual  for  Invertebrates.  The  extremity  of  the  pro- 
cess is  occupied  by  a  uumber  of  clear  transparent  cells  which 
serve  as  a  cornea  {co)  and  which  are  continuous  with  a  zone  of 
pigmented  cells  (/)</)  analogous  to  an  iris,  and  which  pass  grad- 
ually over  into  ordinary  ectodermal  cells.  Upon  the  inner 
surface  of  the  cornea  is  a  mass  of  transparent  cells  constitut- 
ing a  lens  (^,  and  below  this  lies  the  sensory  portion  of  the 
eye.     The  optic  nerve  as  it  comes  towards  the  eye  branches ; 

1 


■''■S;'M* 


Pig.  150. — Eye  of  R?<^^«n  (modified  sliKhtly  from  Patten). 
CO  =  cornea.  op,  op'  =  optic  nerve. 

I  =  lens.  pg  =  pigment-cells. 

la  =  blood-lacuna.  rt  =  retina. 

tl  =  tapetum  lucidtiin. 

one  branch  {op'),  passing  to  one  side  of  the  eye,  bends  inwards 
towards  the  axis  of  the  eye  between  the  retina-cells  {rt)  and 
the  lens.  The  sensory  portion  of  the  eye  consequently  is  in- 
verted, the  retina-cells  being  turned  away  from  the  light  which 
must  pass  through  the  fibres  of  the  optic  nerve  to  reach  them. 
Below  the  retina  and  separated  from  it  by  a  space  is  a  layer 
of  tissue,  the  tapetum  Ivcidum  {tl),  which  serves  as  a  reflector 
and  gives  the  metallic  lustre  which  is  characteristic  of  the 


^ssarssss**" 


nJIdirtMilihlMlMfN--'" 


<.rtaMl-JW»' 


TYPE  MOLLUSCA. 


337 


f^ 


eye  of  Peden,  and  below  this  again  comes  a  pigment-layer 

iPO)' 

In  a  small  number  of  forms,  e.g.  Area,  peculiar  compound 

eyes  are  also  found  on  the  edge  of  the  mantle.  They  form 
slight  rounded  elevations  and  consist  of  a  number  of  conical 
retinal  cells,  each  surrounded  by  a  sheath  of  six  cylindrical 
pigment-cells.  Each  of  these  groups  of  retinal  and  pigment 
cells  is  known  as  an  ommatidium  and  is  separated  from  the 
adjoining  ones  by  slender  intermediate  cells,  so  that  on  sur- 
face view  the  composite  character  of  the  eye  is  very  distinct. 

The  uephridia  (Fig.  149,  ne)  of  the  Pelecypoda  are  always 
paired,  and  each  consists  of  a  tube  bent  upon  itself  lying  im- 
mediately beneath  the  pericardial  cavity  into  which  one  of 
the  limbs  opens  (w/>'),  while  the  other  communicates  with  the 
suprabranchial  chamber  {np'),  and  so  with  the  exterior.  In 
the  simplest  forms  the  entire  extent  of  both  limbs  is  glandu- 
lar, but  in  the  majority'  the  limb  which  opens  to  the  exterior 
loses  its  glandular  character  and  surrounds  to  a  certain  ex- 
tent the  glandular  or  proximal  limb.  In  addition  to  these 
nephridia,  frequently  known  as  the  organs  of  Bojauus,  peri- 
cardial glands  are  of  common  occurrence  in  all  but  the 
simplest  Pelecypods,  and  apparently  assist  the  nejihridia  in 
their  excretory  function.  They  are  known  also  as  Keber's 
organs  and  consist  either  of  outpouchiugs  of  the  anterior 
portion  of  the  pericardial  wall  into  the  space  between  the 
two  walls  of  the  mantle  {Unto,  Venus)  or  of  similar  evagina- 
tions  of  the  walls  of  the  auricles  into  the  pericardial  cavity 
{Mytilus),  both  methods  of  formation  usually  being  associated. 

The  reproductive  organs  (Fig.  149,  r)  are  i)aired,  lying  usu- 
ally in  the  tissue  forming  the  base  of  the  foot,  though  extend- 
ing in  some  cases  into  the  lacunar  spaces  between  the  walls 
of  the  mantle  (Mytiltis).  They  are  very  richly  branched  and 
usually  contain  in  any  one  individual  only  ova  or  sperma- 
tozoa, as  the  case  may  be,  though  a  number  of  forms  are 
hermaphrodite — such,  for  example,  as  the  members  of  the 
genus  Cf/clas  and  some  species  of  the  genera  Ostrea  and 
Pecten.  The  ducts  which  convey  the  reproductive  elements 
to  the  exterior  open  into  the  nephridia  near  their  proximal 
ends  in  Nucvla  and  a  few  other  primitive  genera,  but  more 


338 


INVERTEBRATE  MORPHOLOOT. 


usually  opeu  directly  into  the  Huprabraucliial  chamber  quite 
ueur  the  opeuiiigs  of  the  uephridiu  (Fig.  149,  go) ;  couditious 
cuuuectiug  these  two  extremes  are  to  be  fouud,  us  iu  Pecten^ 
where  the  reproductive  ducts  commuuicate  with  the  uephridia 
uear  their  distal  euds,  uud  iu  Cyclas  aud  OstreUj  where  both 
uephridial  aud  reproductive  openiugs  are  coutained  iu  a 
conimou  groove.  No  complex  accessory  structures  are  de- 
veloped iu  couuectiou  with  the  reproductive  orgaus,  as  iu 
some  of  the  Gasteropods,  uor  is  there  an  iutromitteut  organ 
iu  the  male,  the  ova  aud  spermatozoa  being  usually  extruded 
to  the  exterior,  where  fertilization  takes  place,  or  else  the  ova 
pass  from  the  suprabranchial  chamber  into  the  interlamellar 
spaces  of  the  gill-plates  aud  are  fertilized  there. 

The  structure  of  the  gills  forms  a  suitable  character  for  a 
classification  of  the  Pelecypoda. 

1.  Order  Frotobranchia. 

The  gill  is  a  true  ctenidium  attached  by  its  axis  to  the 
roof  of  the  mantle-cavity  in  its  posterior  part.     In  addition  to 

P 


Fig.  151.— Nucula  nucleus  for  the  Left  Side  after  the  Remov/t.  op 
THE  Left  Shell  and  Left  M antle- lobe  (after  Pklsknkeb). 
aa  =  auteiior  adductor.  /  =  foot. 

ar  =  anterior  retractor  pedis.  g  =  reproductive  orir'"- 

c  =  cteuidium.  p  =  labial  palp. 

ep  7=  levator  pedis.  pa  =  posterior  adductc. 

pr  =  posterior  retractor. 

this  primitive  feature  the  foot  has  a  creeping  surface,  the 
pleural  gauglia  are  not  completely  uuited  with  the  cerebral, 


^ 


TYPE  MOLLUSC  A. 


339 


and  the  reproductive  duct8  cominuuicate  with  tlie  proxiituil 
portion  of  the  nephridiu.  To  this  order,  which  represents  the 
most  primitive  Pelecypods,  belong  the  genera  Nucula  (Fig. 
151),  Yoldia,  and  others. 

2.  Order  Filibranohia. 

In  this  group  the  gill-filaments  have  elongated  consider- 
ably and  commenced  to  bend  upwards  at  their  ends  to  form 
the  outer  and  inner  lamellsB  {Anomia;  Mytilus,  Modiolaria,  the 
mussels ;  Area). 

3.  Order  Fsendolamellibranohia. 

In  this  the  gill-filaments  show  a  tendency  to  become 
united  together  and  the  inner  and  outer  lamellee  are  united 
{Pecteriy  the  Scallop ;  Oatrea,  the  Oyster). 

4.  Order  Eulamellibranchia. 

In  which  the  gill-filaments  are  united  to  form  continuous 
lamelliB.  To  this  order  belong  the  majority  of  forms,  such 
as  the  fresh-water  mussels  Unio  and  Anodon,  the  small  fresh- 
water CydaSj  the  hard-shell  clam  or  Quahog  Vemis,  the  soft- 
shell  clam  Jfi/ttf  the  razor-shell  Ensatella,  the  boring-shell 
J*holas,  the  ship-worm  Teredo,  and  a  very  large  number  of 
other  genera. 

5.  Order  Septibranchia. 

A  smr-,11  group  in  which  ^he  gills  are  reduced  to  a  muscu- 
lar perf(  rated  septum  dividing  the  suprabrauchial  chamber 
from  the  more  ventral  mantle-chamber  (Ompidaria). 

Development  and  Affinities  of  the  Pelecypoda. — The  larva  wliich  is 
characteristic  of  the  Pelecypods  resembles  a  lr<.?'iophoro  very  closely  in- 
deed and  may  be  described  as  a  Trochophore  proviUeu  ,v'th  a  bivalved 
shell.  In  certain  forms  the  characteristic  ciliary  bands  riay,  however, 
be  very  much  reduced,  and  in  the  fresh-water  mussels  {Unio,  Anodon) 
a  remarkable  secondary  larva  known  as  the  Qlochidimn  is  developed. 
The  ova  undergo  their  development  in  the  interlamellar  spaces  of  the  gill 
plates,  and  the  shell-valves  assume  a  somewhat  triangular  shape,  the  apex 
usually  constituting  a  somewhat  cijrved  tooth,  while  smaller  teeth  may 
also  be  present  on  the  edges.  Hach  mantle-lol)e  is  provided  with  four 
tactile  papillae  on  each  side;  the  sjlightest  stimulation  of  which  causes  the 


840 


IN  VEHTEDHA  TE  MOliPUOLOO  Y. 


slioll-valvcs  to  close  with  considernblo  force  and  tlio  teeth  to  adhere  to  any 
soft  object  they  coiue  in  contact  witli.  By  those  arrangements  the  CJlo- 
chidia  are  able  to  fasten  thein.sclves  to  tlie  skin  of  tiie  tins  or  to  the  gills  of 
tishes,  wliere,  setting  up  un  intl.-innnation,  they  become  enclosed  in  u  cyst 
within  which  tiie  organs  assuiii«>  tlic  adult  form,  the  embryo  iussuming  a 
truly  parasitic  habit  and  drawing  nourishment  from  the  tissues  of  its  host. 
When  sufliciently  developed  tiie  young  mussel  makes  its  way  out  of  the 
cyst  and  jissumes  a  free  mcxle  of  life. 

As  regards  the  affinities  of  the  relecy|)oda  there  can  be  no  doubt  that 
between  the  Proiobranchiate  forms  and  the  Diotocardiate  (lasteropods 
there  are  not  a  few  re^^emblances.  They  possess  a  creeping  foot,  true 
ctenidia  function  as  gi'is,  the  heart  is  traversed  by  the  rectum,  the  pleural 
and  cerebral  ganglia  are  distinct,  and  the  nephridia  serve  to  transmit  the 
reproductive  elements  to  the  exterior,  all  of  which  are  also  to  be  fouml  in 
certain  of  the  Diotocardiates,  such  as  Ilaliotis  and  Fisunrttla.  It  may  be 
snp|)osed  that  the  Pelecypods  arose  from  the  Gasteropod  stem  before  the 
asymmetry  became  develoiwd,  and  suLsecjuently,  by  assuming  a  fixed  or 
limicolous  mode  of  life,  a  certain  amount  of  degeneration,  such  as  the  loss 
of  the  radula  and  of  tentacles  and  cephalic  eyes,  supervened.  The  plough- 
share-shaped foot  is  undoubtedly  an  adaptation  to  the  limicolous  habit, 
and  the  great  development  of  the  gills  stands  in  relation  to  their  mo<le  of 
obtaining  food,  the  cilia  of  the  gills  being  responsible  for  the  production  of 
currents  sufficiently  strong  to  carry  with  them  diatoms  and  other  small 
organisms  upon  which  the  Pelecypods  feed. 


V.  Class  Cephalopoda. 

The  Cephalopoda  are  iu  some  respects  the  most  specialized 
of  all  the  Mollusca,  but  nevertheless  })reseut  the  primitive 
bilateral  sAinmetry  and  arrauf^ement  of  the  structures  associ- 
ated with  the  mantle-cavity.  The  visceral  hump  is  enor- 
mously developed  (Fij^.  lo'i),  so  that  the  true  anterior  and 
posterior  surfaces  of  the  body  are  very  lon<;,  whereas  the 
ventral  surface  is  comi)aratively  short,  the  j^eneral  form  of 
the  body  beiuj;  not  unlike  that  oi  the  Scaphopoda.  Unlike 
the  members  of  this  latter  class  the  Oephalopods  lead,  how- 
ever, a  somewhat  active  existence,  some,  such  as  the  Squids, 
swimmiu^^  actively  about  in  the  sea  to  which  they  are  exclu- 
sively confined,  while  «)thers,  such  as  the  Cuttlefishes,  have  a 
more  creeping;  habit,  thouj^h  capable  of  swimming  freely. 
While  swimming  the  animals  assume  a  position  iu  which  their 
longest  axis,  i.e.  the  dorso-ventral  axis,  is  more  or  less  hori- 
zontal, the  true  morphological  anterior  surface  thus  becoming 


.Vi'.A-jkwis*^'*?**'' 


TYrfa  MOLLUSCA. 


341 


physic )lo{»icjilly  tlie  dorsal,  aud  the  posterior  the  ventral  sur- 
face. In  the  followiuj^  description  the  surfaces  will  be  cou- 
sidered  in  their  morphological  relations. 

The  head  is  usually  somewhat  distinctly  marked  off  from 
the  body  proper  by  a  neck  constriction  ami  bears  two  usually 
remarkably-developed  and  highly-specialized  eyes.  A  j>ecul- 
iar  feature  of  the  Cephalopods,  and  the  one  which  has  sug- 


rr-O 


Fl(».  ISa,— DiAOUAM  TO  SHOW  OkkKIIAI,  Pl.AN  «>F  STUUCTtIRE   OKA   CkIMIA- 

I.OI'UU  (sliKlilly  iiitiilKU-il  froiii  Lamd. 


an  =  iiixia. 

ft  =  bticcal  mnas. 
CiV  =  cjucuin  of  stomach. 
ct  =  clenidluin. 

e  =  eye. 
(]o  =  reproductive  organ. 

i  =  iiik-lmg. 
m  =  iimntlc. 


me  =  inautlo-cuvily. 
iw  =  uephridiiim. 

a  =■  uvsopliHgiis. 

•  =  stoiiiuch. 
ah  =  slu'Il. 

Hi  =    fuilIH'l. 

t  =  It-niacle. 

e  =  valve  of  funnel. 


gested  the  name  applied  to  the  class,  is  the  fusion  with  the 
head  of  a  portion  of  the  foot.  The  mouth  thus  becomes  situ- 
ated at  the  bottom  of  a  funnel-like  depression,  formed  by  the 
foot,  whose  margins  are  drawn  out  into  a  number  of  tentacle- 
bearing  lobes  or  into  eight  or  ten  long  arndike  i)rocesst'8 
(Fig.  152,  t)  provided  with  suckerH,  uud  serving  us  powerful 


342 


INVERTEBRA TE  MORPHOLOG  Y. 


orgaus  of  prehension.  A  second  portion  of  the  foot  lies  in 
the  neck  region  on  the  ventral  surface  and  has  the  form  of 
two  folds  (si),  whose  edges  may  be  approximated  or  even  fused 
to  form  a  tube,  through  which  the  water  contained  in  the 
mantle-cavity  may  be  violently  expelled,  the  animal  being 
thereby  propelled  through  the  water  in  a  direction  of  their 
long  dorso-ventral  axis.  This  portion  of  the  foot  is  termed 
the  funnel  and  is  perhaps  equivalent  to  the  epipodium  of  the 
Gasteropods.  In  the  majority  of  forms  there  projects  into 
the  lumen  of  the  funnel  a  fold  {v)  arising  from  the  body-wall 
and  termed  the  valve  of  the  funnel.  It  is  probably  homolo- 
gous with  the  posterior  portion  of  the  foot,  the  metapodium, 
of  the  Gasteropoda,  so  that  all  portions  of  the  Gasteropod 
foot  are  represented  in  the  Cephalopods,  the  propodium  and 
mesopodium  by  the  arm  bearing  portion,  the  metapodium  by 
the  valve  just  mentioned,  and  the  epipodium  by  the  funnel. 
In  many  forms  two  depressions  are  to  be  found  on  the  outer 
surface  of  the  funnel,  which  receive  two  corresponding  eleva- 
tions on  the  inner  surface  of  the  mantle,  which  thus  becomes 
locked  as  it  were  to  the  funnel  during  the  expulsion  of  water 
from  the  mantle-cavity. 

The  mantle  (w)  forms  a  circular  fold  surrounding  the  vis- 
ceral hump,  but  upon  the  anterior  sui'face  it  has  usually  only 
a  very  slight  development,  while  posteriorly  there  is  a  wide 
space,  the  mantle-cavity  (»tc),  between  it  and  the  body-wall. 
Within  this  space  lie  the  ctenidia  {ci),  and  into  it  the  uephri- 
dia  {no  and  the  digestive  tract  open,  the  excreta  being  ex- 
pelled from  it  during  the  expulsion  of  water  from  the  funnel. 
The  mantle-fold  is  rather  thick  as  a  rule,  owing  to  the  pres- 
ence in  it  of  abundant  muscle-fibres,  by  the  contraction  of 
which  the  mantle-cavity  may  be  considerably  reduced  in  size, 
and  frequently  there  is  a  special  muscular  thickening  around 
the  edge  of  the  mantle  whereby  the  mouth  of  the  cavity, 
widely  open  during  the  intaking  of  water,  may  be  firml}'  ap- 
pressed  upon  the  funnel  tluriiig  the  expulsive  act.  In  the 
majority  of  Cephalopods  the  integument  covering  the  outer 
surface  of  the  mantle  and  of  the  visceral  hump  is  provided 
with  abundant  pigment-cells  or  chromatophores  each  of 
which  is  provided  with  a  muscular  arrangement  by  which  its 


.-^^i.:wi£«S^ 


sa^ytu^m iiritMif* 


yrP^  MOLLUSCA. 


343 


size  may  be  rapidly  dimiuished,  remarkable  flushes  of  color 
passing  over  the  surface  of  the  liviug  animal. 

In  the  Nautilm  (Fig.  160)  a  chambered  calcareous  shell  is 
present  having  a  rather  complicated  structure  which  will  be 
described  later,  and  in  one  or  two  other  living  forms,  such 
as  A  rgonmda  and  Spirula,  an  external  shell  also  exists,  but 
in  the  majority  of  forms  the  edges  of  the  mantle  close  over 
the  shell,  which  thus  becomes  internal  and  takes  the  form  of  a 
plate  lying  along  the  anterior  surface  of  the  body,  being  some- 
times calcareous  as  in  the  common  Cuttlefish  bone  of  com- 
merce obtained  from  the  /Sepia  (Fig.  l''>2,  sh),  or  else  chitiuous 
as  iu  the  common  Squid,  Loligo.  In  connection  with  the 
mantle  there  are  also  frequently  developed  fiulike  expansions 
with  a  cartilaginous  support  and  provided  with  muscles, 
sometimes  running  along  the  sides  of  the  visceral  hump  or 
in  other  cases  situated  near  its  dorsal  extremity. 

The  respiratory  organs  or  cteuidia  (Fig.  154,  ct)  are  present 
as  either  one  or  two  {Nautilus)  pairs  of  pinnate  structures 
lying  in  the  mantle-cavity.  Each  consists  of  a  central 
axis  attached  throut^hout  its  entire  length  to  the  body-wall, 
forming  a  rather  high  ridge  upon  it  and  containing  near  its 
outer  edge  two  blood-vessels  running  throughout  its  entire 
length.  The  vessel  nearer  the  summit  of  the  ridge  is  the 
branchial  vein  carrying  the  aerated  blood  back  to  the  body, 
and  between  it  and  the  branchial  artery  is  a  cavity  or  canal 
which  communicates  with  the  mantle-cavity  between  each 
pair  of  branchial  pinnir.  These  structures  arise  from  near 
the  free  edge  of  the  axial  ridge,  but  each  is  bound  to  the  ridge 
by  a  thin  triangular  membrane  so  that  they  possess  the  form 
of  lamellu'  rather  than  of  [)inu)C.  Near  the  line  of  attachment 
of  the  axial  ridge  to  the  body-wall  is  a  cord  of  cellular  tissue 
richly  supplied  with  blood  coming  from  the  branchial  artery, 
forming  what  is  termed  a  blood-gland,  from  which  the  blood 
is  collected  into  two  longitudinal  canals  which  conduct  it  back 
to  the  heart. 

The  Cd'loni  of  the  Ce[)halo|»ods  is  characterized  by  the 
great  developn»ent  of  the  pericardial  cavity  (Fig.  153,  pc), 
which  recalls  the  couiiitiou  found  in  the  Amphineura,  and 
may  perhaps  be  better  termed  the  viscero-pericardial  cavity. 


344 


IN  VERTRBBA  TE  MORPIIOLOQ  T. 


In  tbe  majority  of  forms  it  is  a  large  sac  occupying  a  con- 
siderable portion  of  the  apex  uf  tbe  visceral  bump  and  ex- 


Pig.  158.— Diagram  op  Body  cavity  of  /Ss/n'a  (after  Grobbe»). 
bh  =  branchial  heart.  od  =  oviduct. 

/  =  funnel.  Ov  —  ovary. 

ffo  =  reproductive  opening  into  coelom.  p  =  pancreas. 
//  =  heart.  pa  =  partition  partially  dividing  coe- 

1  =  intestine.  iom. 

76  =  ink-bag.  pc  =  coelom. 

I  =  liver.  «  =  stomach. 

Id  =  liver-duct.  8h  =  sbtll. 

mc  =  mantle  cavity.  u  =  external  opening  of  nephridium. 

N=  nephridium.  D^  =  opening  of  nephridium  into  coe- 

lom. 

tending  ventrally  a  considerable  distance,  tbe  more  ventral 
portion  being  incompletely  separated  from  tbe  more  capacious 


rMXMMIMRMI^ 


TYPE  MOLLUSC  A. 


346 


an- 

3X- 


dorsal  portion  by  a  transverse  fold  or  partial  partition  {jki). 
In  yautilus  it  is  placed  in  direct  communication  with  the  man- 
tle-cavity by  two  minute  pores,  but  in  other  forms  such  direct 
communications  do  not  occur.  With  a  ventral  prolongation 
of  the  ventral  cavity  the  uephridia  {N)  communicate,  and  the 
walls  of  the  cavity  fold  themselves  around  the  heart  (//)  in 
the  usual  manner,  and  in  addition  also  enclose  the  branchial 
hearts  {bh)y  becoming  thickened  and  considerably'  folded  in 
this  region  so  as  to  form  the  so-called  appendages  of  the 
branchial  hearts,  which  are  homologous  with  the  pericardial 
glands  of  the  Lamellibranchs.  The  wall  of  the  dorsal  cavity 
is  in  a  similar  manner  folded  over  the  viscera  present  in  that 
region,  and  more  or  less  completely  encloses  the  reproductive 
organs  (ov)  so  as  to  form  around  them  a  capsule,  sometimes 
Avith  muscular  walls,  into  the  cavity  of  which  the  reproduc- 
tive elements  are  shed  when  mature.  In  one  group  of  Ce- 
phalopods,  however,  the  Octopoda,  the  arrangement  departs 
slightly  from  this  owing  to  the  reduction  of  the  viscero-peri- 
cardial  cavity  to  a  number  of  comparatively  small  canals 
which  constitute  the  so-called  water  vascular  canals  of  the 
older  authors.  Three  of  these  canals  are  found  on  either  side 
of  the  body,  meeting  together  in  a  common  centre,  the  ue- 
phridia communicating  with  one,  another  passing  to  the 
branchial  heart  of  its  side  to  form  the  pericardial  gland,  while 
the  third  extends  dorsally  to  dilate  with  its  fellow  of  the 
opposite  side  into  the  capsule  surrounding  the  reproductive 
organs  (Fig.  158,  wc).  The  general  relationships  of  these 
canals  are  evidently  comjjarablo  with  those  of  the  viscero- 
pericardial  cavity  of  the  majority  of  the  Ce})hulop()ds,  but 
they  dift'er  in  one  very  marked  peculiarity,  i.e.,  the  heart  is 
not  enclosed  within  their  lumen.  The  tubelike  condition  of 
the  cavity  is  evidently  a  secondary  condition,  and  the  exclu- 
sion of  the  heart  can  be  understood  as  a  result  of  the  diminu- 
tion of  the  extent  of  the  cavity,  when  the  manner  in  which  it 
is  enclosed,  as  exemplitied  by  the  Solenogastr  s,  is  considered. 
The  schizocculic  })ortion  of  the  coelom  takes  the  form  partly 
of  lacunar  spaces,  but  partly  of  blood-vessels  with  definite 
walls.  To  a  certain  extent  the  blood  system  is  completely 
closed,  an  unusual  condition  among  Mollusca  ;  well-defined 


■i 


346 


JNVBRTEBBATB  MOBPUOLOOT. 


veius  return  the  blood  carried  by  the  arteries  to  various  por- 
tions of  the  body,  deiiuite  capillaries  couuectiug  the  two  sets 
of  vessels.  A  lacunar  system  also  exists,  however,  so  that, 
while  showing  a  much  greater  differentiation  than  the  other 
MoUusca,  the  Cephalopods  yet  retain  indications  of  the  more 
primitive  arrangement. 

The  heart  consists  of  a  tubular  ventricle  (Fig.  154,  'o), 
usually  arranged  with  its  long  axis  directed  dorso-ventrally^ 


Pig.  154.— CmcuLATouY  Apparatus  of  (Se;)m  (after  Hunter  from  Owen). 

ao  =  (Ulterior  iiorta.  Iv  =  latenil  vein. 

ao'  =  abdoniiiml  aorta.  ne  =  excretory  appeudage. 

an  =  auricle.  pg  =  pericardial  glaud. 

bfi  =  braiicliial  hetirt.  v  =  ventricle. 

et  =  ctenidium.  va  —  abdominal  vein. 

vc  =  cephalic  vein. 

though  iu  the  Octopoda  it  is  transverse,  and  has  opening  into 
it  at  each  side  one  or  two  (Ndiihlm)  auricles  {an)  which  re- 
ceive the  blood  from  the  branchiae  {cf).  Two  principal  artti- 
ries  arise  from  the  ventricle,  a  larger  one  ruuuihjj;  ventrally 
(ao),  and  a  smaller  one  which  runs  towards  the  ti[)  of  the 
visceral  hump  and  supplies  the  viscera  of  tliat  region  {ao'). 
As  already  stated,  these  arteries  pass  into  a  fine  capillary  uet- 
Avork  from  which  the  veius  arise,  sinuses,  however,  interven- 
ing iu  some  cases  in  the  course  of  the  latter,  and  possibly 
some  arterial  branches  may  terminate  iu  such  sinuses.     The 


kt-^iCSBSiSStf"^**^ 


TYPE  MOLLUSC  A. 


347 


•r- 
ts 

it, 
3r 
i-e 


principal  venous  trunk  is  the  cephalic  vein  (w),  which  lies  on 
the  posterior  side  of  the  oesophagus,  and  passing  dorsally 
divides  into  two  branches,  the  venie  cavie,  with  each  of  which 
an  abdominal  vein  (t'a)  unites,  the  conjoined  trunk  on  each 
side  passing  into  a  contractile  dilatation,  the  branchial  heart 
(/>/<),  at  the  base  of  the  ctenidium  of  that  side.  The  veuie  cava) 
and  the  abdominal  veins  are  covered  by  a  much-folded  mass 
of  tissue,  the  venous  appendages  (hp),  which  are  portions  of 
the  nephridia  and  will  be  considered  in  the  description  of 
those  organs.  Mention  may  also  be  made  here  of  the  peri- 
cardial glands  (/)^)  attached  to  the  branchial  hearts,  which 
have  already  been  described  in  connection  with  the  viscero- 
pericardial  cavity. 

Slight  variations  from  the  arrangement  here  described 
may  be  found  in  various  forms,  of  which  the  most  important 
is  that  found  in  N^autiJtis,  in  which,  in  accordance  with  the 
presence  of  two  pairs  of  ctenidia,  ejicli  vena  cava  divides  into 
two  branches,  one  passing  to  each  ctenidium.  No  branchial 
hearts  occur,  and,  as  lias  been  already  mentioned,  the  ventri- 
cle has  opening  into  it  two  pairs  of  auricles  instead  of  the 
single  pair  usually  present. 

In  the  mesodermal  tissue  of  the  Oephalopods  in  various 
portions  of  the  bod}-  there  are  developed  plates  and  nodules 
of  a  consistency  resembling  cartilage  and  like  it  consisting  of 
a  hyaline  t)r  ])artl3-  fibrous  matrix  through  which  numerous 
cells  with  branching  processes  are  scattered.  These  cartilagi- 
nous structures  resemble  the  tissue  which  is  developed  in  the 
pharynx  of  the  Gasteropods  below  the  radula,  but  reach  a 
much  more  extensive  development  in  the  Cephalopods,  serving 
as  a  protection  for  some  oi  the  more  important  organs,  and 
also  as  a  point  tVuppui  for  the  various  muscles,  and  therefore 
constituting  a  true  endoskeleton.  In  the  Nimtiluti  there  is  but 
a  single  cartilage  which  lies  on  the  posterior  surface  of  the 
cesophagus,  being  deeply  grooved  for  the  reception  of  the 
brain  and  optic  ganglia.  In  other  forms,  however,  the  carti- 
lages are  more  numerous.  There  is  a  well-developed  cephalic 
cartilage  forming  a  deeply-concave  disk  perforated  by  the 
<BSophagU8,  and  partially  enclosing  the  brain,  being  also  ex- 
panded at  the  sides  and  hollowed  out  so  as  to  form  a  support 


348 


INVRRTEBltATE  MOliPllOLOG  Y. 


for  the  eyes,  which  are  further  covered  by  a  pair  of  plates 
which  project  anteriorly  and  laterally  from  the  auterior  margin 
of  the  disk.  At  the  base  of  the  arms  a  brachial  cartilage, 
sometimes  united  with  the  cephalic  mass,  is  found  which 
serves  for  the  origin  of  the  brachial  musculature,  and  further- 
more a  nuchal  plate  is  present  lying  below  the  anterior  sur- 
face of  the  body  just  behind  the  head.  In  connection  with 
the  infundibulum  plates  and  nodules  are  developed,  the  most 
important  of  wluch  is  the  infundibular  cartilage  on  the  pos- 
terior (strictly  speaking  the  ventral)  surface  of  the  body  in 
the  floor  of  infundibulum,  nodules  being  found  below  the  de- 
pressions on  the  side  of  the  infundibulum  and  the  corre- 
sponding elevations  of  the  mantle  which  have  already  been 
described  as  interlocking  during  the  expulsion  of  water 
through  the  funnel.  Finally,  it  may  be  mentioned  that  the 
centre  of  each  tin  is  occupied  by  a  cartilaginous  plate  which 
serves  for  the  origin  of  the  muscles  which  move  the  fin. 

In  harmony  with  the  peculiar  modification  of  the  foot 
there  is  a  considerable  amount  of  differentiation  of  special 
muscles  in  the  Cephalopods,  which  pass  from  cartilage  to 
cartilage  or  from  the  shell  to  the  various  cartilages.  Leaving 
aside  the  general  musculature  of  the  mantle  and  of  the  arms, 
mention  may  be  made  of  the  three  or  four  strong  retractor 
muscles,  which  pass  from  the  shell  to  the  cephalic  cartilage 
and  are  sometimes  fused  together  to  form  a  single  strong 
muscle  which  serves  to  retract  the  head ;  the  collaris,  which 
runs  on  either  side  of  the  neck  from  the  infundibular  cartilage 
to  be  inserted  into  the  sides  of  the  nuchal  cartilage  ;  and 
finally  the  adductors  and  depressor  of  the  funnel,  which  pass 
respectively  from  the  cephalic  cartilage  and  the  shell  to  be 
inserted  into  the  infundibular  cartilage.  Considerable  varia- 
tion is  to  be  found  in  the  arrangement  of  muscles  in  various 
forms,  but  the  typical  arrangement  may  be  regarded  as  being 
somewhat  as  described. 

Like  the  other  organs  the  digestive  system  presents  a  con- 
siderable amount  of  differentiation.  The  mouth  opens  in  the 
centre  of  the  disk  which  bears  the  arms  or  tentaculiferous 
lobes,  and  is  guarded  by  two  strong  chitinous  or  partly  calca- 
reous {Nautilvs)  jaws  resembling  in  form  the  beak  of  a  parrot. 


TYPE  MOLLUSCA. 


349 


It  leads  into  a  muscular  pliaryux  (Fig.  152,  ft),  upou  the  floor 
of  which  lies  the  characteristic  molluscau  radula,  while  iuto 
its  cavity  the  ducts  of  oue  or  two  pairs  of  salivary  j^laiids 
opeu.  Succeediuj^  the  pharyux  is  a  tubular  uesophagus  (a^) 
which  in  some  forms  is  provided  with  a  lateral  diverticulum, 
the  crop,  aud  which  termiuates  below  iu  the  larj^e  pyriform 
stomach  (s).  The  iutestiue  leaves  the  stomach  close  to  the 
entrauce  of  the  oesophagus,  aud  a  pouchlike  structure,  iu  some 
forms  prolonged  iuto  a  spiral  ciccum  {civ),  is  to  be  found 
either  communicatiug  with  the  stomach  close  to  this  i)oiut  or 
else  openinc;  into  the  proximal  portion  of  the  intestine  {Nauti- 
lus). Into  this  ctBcum  the  two  ducts  from  the  large  digestive 
glands,  or  so-called  liver,  open,  their  walls  beiug  in  the  ma- 
jority of  cases  provided  with  sacculations  arrauged  in  bunches 
and  constituting  the  pancreas,  a  structure  which  in  Loligo 
(Fig.  153,^)  is  imbedded  in  the  thickened  walls  of  the  ducts  or 
else,  as  in  Odopits,  attached  to  the  digestive  gland  iu  the  region 
where  its  ducts  arise.  From  its  origin  in  the  stomach  the 
intestine  passes  ventrally,  the  entire  tract  having  thus  a 
V-shaped  arrangement,  and  opens  iuto  the  nmntle-cavity  on 
the  summit  of  a  papilla  situated  a  short  distance  from  the 
dorsal  end  of  the  infundibulum.  From  each  side  of  the  anal 
papilla  a  fleshy  appendage  arises,  the  anal  valve,  which  in 
some  forms  may  be  drawn  down  so  as  to  completely  close  the 
jinal  opening. 

In  connection  with  the  posterior  portion  of  the  digestive 
tract  there  is  found  in  all  Cephalopods  except  Nautilus  a  sac- 
like gland  (Fig.  152,  i)  which  secretes  a  dark  pigment  and  is 
known  as  the  ink-bag,  the  animal  discharging  the  ink  into 
the  surrounding  water  to  conceal  its  retreat  when  alarmed. 
It  arises  as  a  saclike  diverticulum  of  the  rectum  close  t  its 
termination  and,  elongating,  becomes  differentiated  it  '  duct 
of  considerable  length  opening  into  the  terminal  j*  rtion  of 
the  rectum  and  closed  by  a  circular  band  of  muscle-fibres 
which  surround  it  near  its  opening.  The  more  or  less  globu- 
lar extremity  of  the  diverticulum  becomes  differentiated  into 
(1)  a  cavity  which  serves  as  a  reservoir  for  the  inky  secretion 
manufactured  in  (2)  a  special  glandular  region,  traversed  by  a 
series  of  trabecuhe  lined  by  the  secreting  cells. 


860 


INVEHTKBltA TE  MOIIPHOLOG  Y. 


The  uervous  system  of  the  Cephalopods  shows  a  high 
dep;ree  of  couceutrutiou,  the  various  gauglia  being  more  or  less 
fused  with  one  auother  to  form  a  mass  surrouuiliug  the  (eso- 
phagus just  behiud  the  pharyngeal  mass.  In  Xautilus  this 
mass  takes  the  form  of  two  rings  surrounding  the  fesophagus, 
united  in  front  but  widely  divergent  behind,  and  in  which  the 
various  ganglia  are  but  indistinctly  indicated,  the  condition 
which  occurs  in  (Jhiion  in  this  res})ect  being  recalled.  That 
portion  of  the  ring  which  lies  in  front  of  the  oesophagus  rep- 

,  resents   the    cerebral   ganglia ;    the 

lateral  portions  of  the  more  ventral 
of  the  two  rings  found  on  the  pos- 
terior surface  are  the  pedal  ganglia, 
giving  rise  to  the  nerves  to  the  pedal 
lobes  and  the  infundibulum  ;  while 
the  more  dorsal  posterior  ring  rep- 
resents the  combined  visceral,  pa- 
rietal, and  pleural  ganglia.  In  other 
forms  the  ganglia  become  more 
perfectly  marked  off  and  at  the  same 
time  more  concentrated.  A  cerebral 
ganglion  (Fig.  155,  c)  is  always  dis- 
tinguishable, and  with  it  are  con- 
nected pleuro-parieto-visceral  {pi 
and  v)  and  pedal  (p  and  p')  ganglia ; 
the  latter,  however,  are  usually  divi- 
ded into  two  portions — a  more  ven- 
tral mass  {p')  which  sends  branches 
to  the  armlike  prolongations  of  the  pro-  and  mesopodium  and 
which  is  hence  termed  the  brachial  ganglion,  and  a  more  dorsal 
one  (/))  which  sup])lies  the  infundibulum  and  is  known  as  the 
pedal  ganglion  proper.  A  study  of  a  number  of  different 
forms  shows  clearly  that  the  brachial  ganglion  is  merely  a 
separated  portion  of  the  })edal,  and  that  the  arms  are  to  be  con- 
sidered portions  of  the  foot  and  are  not  cephalic  appendages. 
At  the  sides  of  the  cerebral  ganglia  there  are  to  be  found  a 
pair  of  large  ganglia  (op)  which  stand  in  relation  to  the  eye  and 
are  termed  the  optic  ganglia;  they  are  undoubtedly  spe- 
cializations of  the  cerebral  ganglia,  owing  their  separate  exist- 


PP'      B 
J^o.  155. — Nkkv«h'8  Ganglia 
OF  (A)  Ijoligo  AND  (Z?)  Octopus 
(after  Prlhrnrkr). 

b  =  buccal  ganglion. 

c  =  cerebral  ganglion. 

p  =  pedal  ganglion. 
p'  =  brachial  ganglion. 
op  =  optic  ganglion. 

V  =  plt'uro  -  parieto  -  visceral 
ganglion. 


TYPE  MOLLUSC  A. 


351 


ence  to  the  remarkable  tlevelopmeut  ami  iliflfereutiatlou  of 
the  eye  which  is  found  in  the  niujority  of  the  Cei)haloj)otls. 

A  sympathetic  system  of  nerves  is  well  tlevuh)j)e(l  and  con- 
sists of  one  or  two  j)airs  of  buccal  ganglia  (b)  innervating  the 
large  pharyngeal  mass  and  united  to  the  cerebral  ganglia  by 
connectives  and  giving  rise  to  a  strong  nerve  which  runs 
dorsally  along  the  o'soj»hagus  to  end  iu  a  largo  gastric 
ganglion  from  which  nerves  pass  to  the  viscera.  Mention 
should  also  be  made  of  two  other  ganglia,  the  ganglia  stellata, 
which  belong  to  the  central  system  and  are  situated  in  the 
lateral  portions  of  the  mantle,  being  united  with  the  phiuro- 
■visceral  ganglia  by  stnnig  nerves;  they  correspond  probably 
with  the  parietal  ganglia  of  the  Gasteropods,  sending  branches 
to  the  tissues  of  the  mantle. 

The  special  sense-organs  are  exceedingly  well  developed, 
and  especially  is  this  the  case  Avith  the  eyes.  In  N(udUn.% 
however,  the  eye  (Fig.  156,  A)  stands  ou  a  much  lower  grade 


Fia.  156. — A,  Eye  of  Nuntilus  (modified  from  Henbrn);  and  B,  of  Loligo. 
e  —  cartilage.  I  =  lens. 

CO  =  cornea.  n  =  nerve-layer. 

g  =  layer  of  gauglion-cells.  op  =  optic  nerve  and  retinal  ganglion. 

ii-  =  iris.  pff  =  i)ignu'ul-layer. 

r  =  layer  of  rods. 

of  organization  than  that  of  the  other  Cephulopods,  con- 
.sisting  of  a  cup  lined  by  a  retina  composed  of  several 
layers  and  richly  sui)i)lied  with  nerves.  The  outermost 
layer  consists  of  rodlike  bodies  (r)  below  which  is  a 
layer  of  pigment  (pg),  below  which  again  lies  a  layer  of 
ganglion-cells  (g).  No  refractive  structures  are,  however, 
present,  the  cavity  of  the  cup  communicating  freely  with  the 


352 


IN VKHTEBHA  TK  MOliPUOLOG  Y. 


external  water  tliroiigli  a  small  circular  opening  in  the  front 
flattened  wall  of  the  cup.  The  eye  is  a  camera  constructed  on 
the  "  pin-hole  "  type,  the  image  being  detiued  by  the  exclusion 
of  all  the  more  divergent  rays  of  light  which  pass  in  from  the 
object  towards  the  eye. 

In  the  remaining  forms  the  eyes  (Fig.  15G,  Ti)  are  large 
globes  imbedded  in  an  orbit  formed  by  the  lateral  p(jrtions 
of  the  cephalic  cartilage  and  its  })rocesses.  The  retinal  ])()r 
tion  of  the  eye  ch)s«'ly  resembles  tiiat  of  A'antilns,  consisting 
of  an  external  layer  of  rods  (/•)  bounded  beneath  by  a  pigment- 
layer  (pg)  beneath  which  is  a  nerve-layer  (n)  enclosed  within 
a  c(mnective  tissue-sheath  in  which  cartilage  (c)  is  developed. 
The  optic  nerve  dilates  into  a  retinal  ganglion  before  being 
distributed  to  the  retina,  the  rods  of  which,  it  will  be  noted, 
are  turned  towards  the  source  of  light.  The  e^'e-cup  differs, 
however,  from  that  of  Nitutlhis  in  being  completely  closed, 
and  the  cells  which  form  the  outer  and  inner  layers  of  the 
outer  wall  of  the  cup  secrete  chitiuous  material  which  acta 
as  a  lens  (?),  forming  a  powerful  biconvex  cimdenser.  In  ad- 
dition  to  this  the  eye  is  further  complicated  by  the  develop- 
ment of  a  series  of  folds  from  the  skin  in  its  neighborhood. 
One  such  fold  is  developed  from  the  front  wall  of  the  optic 
sac,  surrounding  the  region  occupied  by  the  lens  and  fori.i- 
ing  an  iris  (ir),  the  circular  opening  in  its  centre  correspond- 
ing to  the  pupil  of  the  Vertebrate  eye.  A  second  likewise 
forms  nearer  the  base  of  the  optic  sac  and,  growing  forward, 
may  enclose  a  space  bounded  behind  by  the  iris  and  lens, 
resembling  the  anterior  chamber  of  the  \ertebrate  eye,  the 
portion  of  the  fold  immediately  in  front  of  the  lens  becoming 
transparent  and  forming  a  cornea  (co).  The  anterior  chamber 
is  not,  however,  closed  in  all  forms,  but  remains  in  communi- 
cation with  the  exterior  by  an  aperture  produced  by  a  failure 
of  the  edges  of  the  fold  to  unite  completely.  Finally,  in  some 
forms  other  folds,  which  from  anah)gy  have  been  termed  eye- 
lids, develop. 

The  resemblance  of  such  an  eye  to  that  found  in  the  Vertebrates  is  ex- 
ceedingly striking,  but  a  detailed  study  of  the  structure  and  mode  of  origin 
of  the  various  parts  deraonstrat.es  conclusively  that  the  similarities  are  ana- 
logical only  and  not  homological.    One  of  the  most  important  of  the  diflFer- 


[ 


TYPE  MOLLUSCA. 


353 


ences  is  found  in  thoftrrangenjont  of  tlio  layers  of  the  retina,  tlio  rods  l)einj^ 
turned  towards  the  lij,'ht  as  is  usual  in  Invertebrate  eyes,  while  in  the  Verte- 
brates they  are  reversed,  the  nerve-tibre  layer  lying  above  them,  the  light 
of  neeessity  penetrating  it  Ijeforu  reaching  the  rods.  The  stnicture  of  the 
lens  is  again  very  ditlerent,  being  cellular  and  formed  as  an  invagination  of 
the  ectoderm  in  the  Vertebrates,  while  in  the  Cephalopods  it  is  a  (luticular 
structure.  These  are  fundamental  ditVerences  and  may  sutlice  to  .show 
what  is  meant,  but  many  other  dissimilarities  may  readily  Ix;  found. 

Otocysts  also  occur  imbedded  iu  a  ca])Hiilo  forniiiij^  part 
of  tlie  cephalic  cartilage.  They  have  the  clianicteriHtic  Mol- 
luscaii  form  aud  receive  a  large  nerve  arising  from  the  cere- 
bral gaugliou.  Osphradia  occur  oidy  in  Nuntilm,  \vher»»  they 
form  a  ])air  of  sensory  ])apilhu  one  of  which  lies  ut  the  base 
of  each  of  the  more  ventral  ctenidia.  Other  Ce}»lialoi)ods, 
though  lacking  these  structures,  are  yet  provide«l  with  special 
olfactory  organs  iu  the  form  of  a  pair  of  fossn'  or  grooves 
lined  by  ciliated  aud  sensory  cells  and  situated  above  the  eye 
iu  the  position  occupied  by  the  eye-tentacles  of  Noiitilns 
(seep.  368),  from  which  they  may  possibly  have  been  d(U-ived. 

The  excretory  organs  consist  of  two  comj)arativi!ly  large 
sac-like  nephridia  except  in  Nontilu.s,  iu  which,  in  harmony 
with  the  number  of  ctenidia  aud  auricles,  there  are  four.  In 
Octopus  and  the  other  members  of  the  group  Octopoda  the 
two  nephridia  are  quite  separate  from  one  another,  but  in  the 
group  Decapoda,  to  which  LoUgo  aud  Sepia  belong,  the}'  are 
placed  in  communication  with  one  another  by  transverse 
canals  one  of  which  may  be  produced  dorsally  into  a  large 
sac  occupying  a  great  portion  of  the  anterior  region  of  the 
body.  The  vena?  cavte  aud  branchial  veins  lie  between  the 
walls  of  this  anterior  sac  aud  the  paired  i)osterior  nephridia, 
aud  along  the  course  of  the  veins  the  walls  of  the  excretory 
sacs  are  richly  folded  (Fig.  IS'l,  ne),  constituting  the  venous 
appendages,  for  a  loug  time  considered  to  be  the  excretory 
organs  in  their  entirety.  The  posterior  paired  ne])hridia 
present  the  same  relations  to  the  exterior  and  to  the  entero- 
ccel  which  exist  in  other  Mollusca,  o])ening  by  two  distinct 
apertures  into  the  mantle-cavity  on  the  one  hand,  aud  on  the 
other  communicating  with  the  large  enterocitd  which  has 
been  shown  to  be  the  equivalent  of  the  pericardial  cavity  of 
the  Gasteropods  and  Feiecypods. 


354 


IN  VEHTEBliA  TE  MOlirilOLOG  Y. 


The  reproductive  orguiis  are  Hitiuited  near  the  dorsal  ex- 
tremity of  the  visceral  hump.  The  sexes  are  always  sepa- 
rated iu  difl'ereut  individuals,  there  beiug  occasionally  well- 
marked  difl'erences  between  the  two  sexes  of  the  same  species, 
as  in  ArgomiutK,  the  feuiale  of  which  possesses  a  well-devel- 
oped shell  which  the  male  lacks.  The  ovary  (Fig.  157,  ov)  is 
single  and  is  enclosed  in  a  capsule  (c)  formed  by  the  walls  of 
the  euterocd'I  or  viscero-pericardial  cavity,  into  which  the 
organ  seems  to  project,  though  morphologically  it  is  entirely 
outside  it. 

The  germ-producing  region  is  nearly  always  the  anterior 
surface  of  the  organ,  the  stalked  ova  surrounded  by  their 
follicle-cells  projecting  forward  into  the  capsule,  into  the 
cavity  of  which,  i.e.  into  the  viscoro-poricardial  cavity,  tliey 
burst  when  mature.     In  some  forms  the  germ-producing  sur- 


ov 


Fig.  ir)7.— Fkmalk  Kki'koductivk  Oikians  of  Treiiwctopiis  violnceus 

(after  Hrixk). 
c  --  i-apsuk'.  ov  =  ovary. 

oil  =  oviduct.  r«  =  seminal  receplacle. 

og  =  ovidiical  gland.         wc  =  (■(rloiiiic  canal. 

face  becomes  more  hij'hiv  folded  and  more  or  less  dendritic 
iii  form,  the  area  over  which  the  ova  are  formed  brconiiiig 
thus  much  greater.  The  ov.i  reach  the  exterior  after  tliey 
have  passed  into  the  cavity  of  the  cai)sule  by  means  of  one 
or  two  ci>mplicated  ducts  {(mI)  ojtening  into  the  mantle-cavity. 
In  N<iHt{liis  two  ducts  are  ])resent,  that  of  the  Ic^ft  side,  howev«'r, 
being  non-functional,  and  in  the  Octopoda  and  some  Decapods, 
such  as  UmmdHtrvphi-H,  both  ducts  are  present.  In  other  forms 
but  a  singh'   duct  p»!rsists,  which,  contrary  to  what  occurs  iu 


^ifrfiiUta*!-"*^- 


TYPE  MOLLUSC  A. 


355 


Nautihia^  is  that  of  the  left  side.  The  oviduct  opeiiH  into  thn 
iiiaiitle-eavity  nt  the  extremity  of  a  well-murked  papilla,  its 
tei'iuiual  portion  beiu^  richly  sup})liinl  with  glands,  and  in 
addition  in  some  forms  two  small  puar-sliapcd  glands  iirv, 
attached  to  it  in  this  region.  In  connection  with  tlio  fcmalo 
ducts  there  should  be  mentioned  a  })air  of  glands  which  take 
})art  in  the  formation  of  the  investments  of  the  ova,  but  which 
open  (piite  separate  from  the  oviduct  into  the  mantle-cavity. 
These  are  the  uidamental  glands  which  are  present  in  the 
majority  of  forms,  e.\cluding  the  Octopoda,  and  consist  of  a 
pair  of  large  pyriform  structures  lying  on  the  posterior  sur- 
face of  the  visceral  mass  ;  in  connection  with  tlu'm  in  some 
forms  are  developed  accoK-tory  uidamental  glamls  consisting 
of  a  central  and  two  latev;;-  portions  whoso  ducts  open  into 
the  mantle-cavity  in  close  proximity  to  those  of  the  uidamen- 
tal glands  })roper.  An  stated,  the  gelatinous  mass  within 
which  the  ova  are  imbedded  is  probably  manufactured  by 
these  glands. 

The  testis  in  its  gtiueral  ridations  rost>nd)les  the  ovary, 
being  single  and  enclostul  in  a  capsule  which  is  a  portion  of  the 
viscero-pcricardial  cavity.  The  organ  is  attache<l  to  the  wall 
of  th«^  capsule  by  a  thin  band  of  tissue  and  is  in  most  cases 
almost  completely  surroundeil  by  the  capsule,  into  the  cavity 
of  which  the  spermatozoa  are  sIumI  when  mature.  From  the 
wail  of  the  capsule  th«5  vas  defenujs  arises  and  is  usually  a 
single  tube  op«Miing  upon  the  left  side  of  the  boily  inlt)  the 
mantle-cavity.  In  A^fiiitihts  there  are,  as  in  tlui  ft^nale,  two 
ducts,  th*'  right,  however,  being  I'linctionless,  but  in  «)tlu'r 
forms  -A  paired  arrangement  is  vt^ry  rare.  Tie-  pi')ximal  por- 
tion of  the  duet  is  a  coiled  vas  deferens,  which  opens  into  a 
thick-walled  glandular  seminal  v«'sicle  which  on  its  part  by 
nus'ins  of  a  iianow  duct  passes  into  ii  saclike  structuie  known 
as  Needham's  pouch  which  linally  pass».'S  into  the  muscular 
penis.  In  most  roiins  the  dnct  connecting  tin*  seminal  vesicle 
with  Netulhain's  pouch  rtureives  the  stnaetion  of  a  s[)ecial  glaiul 
known  us  tlm  prostate. 

Th«^  majority  of  the  accessoiy  structures  connected  with 
the  nnde  ducts  are  concerntMl  in  the  formation  of  cases  or 
uptU'inatophoreH  in  which  a  number  of  spermatozoa  are  eu- 


i 

if 


356 


IN VEUTEBUA  TE  MOliPUOLOQ  T. 


closed.  Such  cases  are  cylindrical  structures  with  a  double 
wall,  and  are  provided  at  one  extremity  with  a  somewhat 
complicated  apparatus  lor  the  ejection  of  the  spermatozoa. 
The  exact  method  of  their  formation  is  not  understood,  but 
ajipareutly  the  seminal  vesicles  and  the  prostate  play  an  im- 
portant part  in  the  process,  the  Needham's  pouches  being  a 
reservoir  in  which  they  may  be  stored  up  until  required  for 
fertilization. 

Since  the  genital  capsule  is  a  portion  of  the  viscero-pericardial  cavity, 
and  the  reproductive  ducts  are  continuations  of  its  walls,  these  structures 
must  also  be  regarded  as  prolongations  of  the  enteroccel;  and  indeed  second- 
ary communications  may  exist  between  them  and  the  viscero-pericardial 
cavity  proper.  Tiie  genital  capsule  is  not  completely  separated  off  fn.uu 
the  rest  of  the  enterocaM,  so  that  it  might  be  possible  for  the  reproductive 
elements  to  pass  from  its  cavity  into  the  viscero-pericardial  cavity  proper, 
and  so  to  the  exterior  through  the  nephridia,  though  this  method  of  exit 
does  not  seem  to  be  made  use  of. 

A  remarkable  moditicutiou  of  ^ne  of  the  armlike  processes  of  the  foot 
occurs  in  the  males  of  certain  species  in  connection  with  reproduction. 
The  arm— in  I'remoctopus  and  f'/iilonexis  the  third  arm  of  the  right  side 
of  the  body  counting  from  the  anterior  mid  line,  in  Argonauta  (Fig.  158) 


A  B 

Fig.  158.  — Malk  ok  Arf/onnuta  wirn  FlECTocoTYMZEn  Arm 

(rtllcr  H.  Mii.LKii  from  Hathciikk). 
A  =  arm  still  euclo-st-d  within  u  membranous  sac. 
B  =  arm  freed  from  the  sue. 

the  third  of  the  left  side — is  at  first  enclosed  within  a  sac.  by  the 
biu'sting  of  which  it  becomes  free,  the  walls  of  the  sac  being  refiected 
back  so  as  to  form  a  pouch  which  in  some  unexplained  manner  receives  a 


I 


i» 


TYPE  MOLLUSC  A. 


357 


!i 


speriualophore.  Tlio  turmiiial  portion  of  the  arm,  whicli  is  traversoil 
throughout  its  entire  length  by  a  canrtl,  is  developed  into  a  long  terminal 
filament  through  which  the  spermatozoa  may  pass,  huriiig  copulation 
the  arm  is  probably  thrown  otf  and  passes  into  the  mantle-cavity  of  the 
female,  the  manner  in  which  the  spermatozoa  reach  the  ova  being,  however, 
not  yet  understood.  When  first  discovered  in  the  mantle-cavity  of  a  female 
the  arm  was  regarded  as  a  parasitic  worm,  and  t  he  name  Hevtocotylus  was 
applied  to  it— a  term  which  is  still  retained  on  account  of  its  convenience. 
In  other  genera  of  Cephalopods  oqe  arm  is  generally  peculiarly  modified  in 
the  male — in  the  Decapoda  usually  the  fourth  of  tlie  left  side  and  in  the 
Octopoda  usually  the  third  of  the  right  side,  though  frequent  exceptions  are 
found.  This  arm  is  termed  the  hectocotylized  arm,  though  it  is  doubtful 
whether  it  takes  any  part  in  copulatiou. 

As  will  be  seen  from  the  above  descriptiou  the  genus 
Nautilus  differs  in  many  important  particulars  from  the  re- 
maining genera  of  Cephalopods,  and  the  class  is  therefore 
divided  into  two  orders. 

1.  Order  Tetrabranchia. 

This  order,  of  which  the  genus  Nmitilm  (Fig.  159)  is  the 
sole  living  representative,  was  in  former  periods  of  the  earth's 
history  the  dominant  group  of  the  Cephalopods — the  Ortho- 
cerites  of  the  PalaBozoic  and  the  Ammonites  of  the  Mesozoic 
being  extinct  members  of  it.  It  is  characterized  by  its  mem- 
bers possessing  four  cteuidia,  four  auricles  to  the  heart,  and  four 
nephridia;  and  in  addition  there  maybe  mentioned,  as  further 
peculiarities,  the  presence  of  paired  reproductive  ducts,  of 
which  the  right  one  alone  is  functional,  and  also  of  direct 
communication  of  the  viscero-perieardiul  cavity  with  the  ex- 
terior by  two  pores,  and  by  the  occurrence  of  a  single  pair  of 
osphradia.  For  a  more  detailed  a  !Couut  of  the  peculiarities 
of  Nauiihis  the  preceding  general  description  may  be  con- 
sulted. It  renuiins  to  discuss  here  the  shell  and  the  structure 
of  the  foot-lobes — structures  whicli,  with  tlie  other  characters 
mentioned,  serve  to  distinguish  Nautilus  from  all  its  living 
congeners. 

The  slu'll  is  voluminous,  coiled,  and  calcareous,  its  cavity 
being  dividt'd  by  a  series  of  transverse  partitions  into  a  num- 
ber t)f  c]iaml)ers,  in  the  last — that  is  to  saj",  the  youngest — of 
whieii  the  animal  lives,  while  the  remaining  ones  are  tilled  with 


% 


358 


INVEHTEBRATE  MORFUOLOGY. 


gas.  The  centre  of  each  partitiuu  is  perforated,  aud  through 
the  opening  there  extends  to  the  tip  of  the  shell  a  prolongation 
of  the  body  of  the  animal,  termed  the  sipuucle. 

The  foot  of  Nautiliis,  or  at  least  that  portion  of  it  which 
fuses  with  the  head,  has  already  been  described  as  forming  a 
number  of  tentaculiferous  lobes.  These  lobes  are  arranged 
in  the  female  in  two  series — one  ventral,  consisting  of  three 


Fig.  159. — Nautilus  pompUtu8,—FKM  \t,k.  wiTnTHK  Shell  sectioned  Lonqi- 

TUDINALIiY  TO  SHOW   ITS  InTEUNAI-  ttTltUCTUUE  (after  Lkunis  from  Hkrtwio). 

1  =  iiiiuitle.  7  =  tiidiiineutal  glaud. 

2  =  dorsitl  lobe  of  mantle.  8  =  .sliell-muacle. 

3  =  leiiiacles.  9  =  termiiml  chamber  of  shell. 

4  =  hciulciip.  10  =  partillous    between   the    various 

5  =  eye.  fhainbors. 

6  -  fiiDuc'l.  11  =  sipunclu. 

lobos  \vlii(!h  immediately  abut  upon  tlie  mouth,  and  a  more 
dorsal  ringlike  lobe  the  anterior  portion  of  which  is  de- 
vel( >})«(!  into  a  hood  (4)  which  arches  over  niid  protects  the  re- 
tracted toiitiU'hjs.  Around  the  margins  of  both  the  ventral 
and  dorsal  lobes  are  arranged  tlie  tentacles,  each  of  which 
is  filiform  and  capable  of  being  witlidrawn  into  the  basal  por- 
tion, which  thus  serves  as  a  siieatli.  In  addition  to  these 
tentacles  two  other  tentacles  are  found  in  close  proximity  to 
the  oyo,  one  being  on  its  ventral  side  and  the  other  on  its 
dcn-sal.  In  the  male  the  arrangement  ib  very  similar,  except 
that  the  median  lobe  of  the  ventral  series  is  transformed  into 
a  lamellated  structure  and  does  not  bear  tentacles,  while  a 
portion  of  each  of  the  lateral  lobes  of  the  inner  series  is  sepa- 


TYPK  M0LLU8CA. 


359 


rated  from  the  rest  of  the  h)be — that  of  the  left  side  becoraiug 
modilied  into  a  couical  structure,  himelhited  at  the  extremity 
and  destitute  of  tentacles,  formin«^  what  is  termed  the  spadix, 
])rol)al)ly  homolojjjous  with  the  hectocotyli/ed  arm  of  the 
male  Octopods  and  Decapods. 


ii.  Orclor  Dibranchia. 

The  members  of  this  order,  which  includes  the  majoiitv 
of  living  Ceplialopods,  ])ossess  but  a  sinj^lo  pair  of  ctenidia, 
ne|)liri(lia,  and  aurich^s,  and  lack 
tlie  direi't  communication  of  the 
viscero-pericaidial  cavity  with  the 
(ixterior  as  well  as  tlu^  os[)liradia 
which  occur  in  N<n>tlhis.  'J'Ikj 
])ortion  'ot  the  foot  which  is  fused 
with  the  luiad  is  drawn  out  into 
a  uund)er  of  arms  provided  with 
sucktjrs,  which  seem  to  n^pri'sent 
the  tcntairhis  and  tiuMr  sheaths 
found  in  NunfUufi.  Tiie  suckers 
are  very  nuuKjrous  and  may  be 
arrun^^ed  in  from  one  to  four  fows 
on  ilu)  ventral  surface  of  the 
arms,  the;  marj;in  of  each  sucker 
bein<j;  in  sonu;  forms  strengthened 
by  a  horny  rinj^,  which  may  be 
toothed.  The  number  of  tln^ 
arms  varies,  bein<.>-  .iflufr  eii^ht 
or  ten  ;  and,  since,  other  struc- 
tural ditlereiices  are  associated 
with  this  ditlerenee,  the  ord.M- 
may  be  divided  into  twosubordcrs  Fkj.  \m.—TM^p„ll,;t„.  Doupai, 
— the  ()ctoj)odu  with  eif;ht  arms, 
im^ludin}^  the  j^'en 
(Fi^'.  158),  and  the    Decapoda  wii'li   t 


VmCW  uifl.T  Kmi 


liioN  t'i'iitn  Viiiiiii 


era    Octnpu.s,    I'miuH'topm,   and    Anjonania 

en  arms,  the  f^ouera  Spi- 
rnlu,  OnimastirphH,  Sepia,  and  /mIhjo  {V\y^.  KiO;  belonging'  to 
this 


uroni). 


In  tlit!  Decapoda  the  teu  arms  are  not  of  etiual  size 


QUO 


360 


INVEliTEDRATE  MOliPlIOLOGY. 


on  each  side  of  the  liesul,  the  fourth,  counting  from  the  ante- 
rior mid-liue,  beiiij^  loii^tn-  than  the  rest,  iisiuilly  destitute  of 
suckers  excei)t  ttnvurds  the  tip,  aiid  iu  most  species  kept 
retracted  within  a  groove  ou  each  side  of  the  head  except 
when  required  for  prehension.  They  are  all  good  swimmers, 
and  the  body  is  ehmgated  and  provided  Avith  lateral  tins  of 
greater  or  less  extent. 

A  shell  is  present  in  the  Decapoda,  but  shows  a  great 
reduction  iu  size  and  complexity  from  that  of  the  Tetra- 
branchiates  ;  and  in  order  to  understand  its  homologies  in  the 
different  genera  it  will  be  necessary  to  obtain  an  idea  of  its 
form  in  the  fossil  members  of  the  group  which  occur  in  the 
Mesozoic  rocks  forming  the  family  Belemnitidio.  In  the 
genus  Jlelevmites  (Fig.  IGl,  /?),  for  instance,  the  shell  consisted 
of  a  terminal  conical  solid  portion,  termed  the  rostrum  (r), 
the  base  of  which  was  hollow  and  contained  a  chambered 
shell,  the  phragmacone  {ph\  corresponding  to  the  AautUvs 
shell,  the  anterior  portion  of  the  last  chamber  of  this  being 
elongated  into  a  broad  flat  process  termed  the  procistracon 
{pr).  By  various  modifications  of  this  structure  the  shells  of 
the  different  living  Decajjods  have  been  developed.  In  Spirula 
the  shell  is  coiled  into  a  spiral  and  is  partly  enclosed  by  the 
mantle,  the  rostral  and  proostracal  portions  having  disap- 
peared. In  all  other  forms  the  shell  has  a  more  or  less  flat- 
tened form  and  boconjes  completely  enclosed  within  the 
ni.intle,  folds  of  which  grow  up  around  it.  In  Scpid  (Fig. 
101,  A)  the  proostrjicon  becomes  almost  obliterated  and  the 
rostrum  (r)  is  exceedingly  small,  th(j  i)hragmacone  (/)//)  form- 
ing the  j)riiuM]>.il  bulk  of  the  shell.  Tiiis,  however,  has  become 
very  niucli  modified — tiiat  ])ortion  of  it  which  lies  ])osteri()r  to 
the  sipuncle  (.s)  ceasing  to  develop,  or  rather  becoming  exceed- 
ingly conii)act  by  tlie  v;iiif)iis  i)iivtitions  lying  in  close  con- 
tact with  one  aiiother  without  any  intervening  .lir-chiunlxMs. 
These  cliambeis  ;ue,  however,  developed  in  the  portion  ant*'- 
rior  t<»  th«'  sipuncle,  but  are  conjparatively  flat  autl  trav»?rsed 
by  calcareous  s])icules,  so  tiiat  the  shell  Inis  a  somewhat 
8])ongy  appearance  and  is  excetulingly  light.  In  otln^r  forms, 
however,  the  proiistracon  is  th(^  ]>ortion  that  ))ersists,  the 
rostrum  and  i)hragmac»)ne  both  disnppearing  (Fig.  1(51,  C),  so 


TYPE  MOLLUSC  A. 


361 


that  fiually  ii()tliinf»  is  left  but  a  plate  imbeddeil  iii  the  mau- 
tie,  formed  entirely  of  chitiuous  material  aud  destitute  of  auy 
calcareous  substauco.  This  forms  the  so-called  "  peu  '*  of 
such  forms  as  Loliijo  aud  Ommaat replies  (Fi^.  101,  JJ),  a  slight 


r  -^ 


Pig.  161  — Diagrams  of  Siikm-  ok  A,  Sepia;  B.  JielemniUH  (fossil);  C,  Os- 
IrncottuUiiit  (fossil);  iiiul  D,  ()juiniistrf'fihti»tiitifv  LAtio). 
ph  =  plimiriiHM'onc.  ?•  =  rostnim. 

pr  =  proostracon.  x  =  sipuiiclo. 

thickeiiiii)^  f)f  the  dorsal  eud  of  it  iu  soim^  forms  represeutmg 
the  rcniiiiiis  of  the  phni<^mac()ue. 

In  tiui  Oftopotia  the  ei^dit  ;irnis  are  practically  t-tpiul  iu 
h'Pi^th,  and  the  body  is  nioro  massive  tluiu  in  th^*  Dec.ipod- 
.iiid  K'ss  suited  for  activt-  svvinuiiin<i.  Tlie  visceral  hump  is  .i 
more  or  less  p;lobular  mass,  destitute  us  a  nmle  of  laterui  tiur., 
and  in  ail  forms  u  shell  (*i>iii)iii,nil)le  to  tluiss  of  tbi^  Decapods 
is  wantinj.?.  In  the  feinules  of  Anjoiututu,  '.M»i«ev4»r,  a  non- 
chambered  calcareous  shell  is  preseut,  to  w -.<  t!i  body  <jf 
the  animal  does  not  (closely  adhere,  but  wiau*  -  li^-id  iu  p«ntt- 
tiou  by  the  broad  plate-like  anterior  arnus  wkteh  embrace  it 


I 


362 


J  A'  \  EUTELUA  TE  MOIWIIOLOO  Y. 


It  seems  to  be  a  secretiou  of  the  ectotlorm  of  the  inniitle  and 
visceral  hump,  the  auteriur  arms  contributing  only  a  thiu 
external  layer. 

Dacelopmeitt  and  AJJ'uiities  of  the  Cephalnpoifs. — Tlio  ova  of  those 
CVplialoiHxls  which  have  boeii  .stiidifd  art'  riclily  provided  witii  yolk,  iti 
con.se(|iieiic'(!  of  wliicli  liir  devclopnit'iif  bt'coincs  (,'oiisi(hM'ai)ly  inodilicd, 
dufiiiito  traces  of  llie  \'eli;;er  coiidiliuii  Iwiiig  eiilircly  lost.  It  seems  clear, 
however,  from  the  mari\ed  development  of  the  head,  tlio  proseiico  of  a 
vadula,  and  tlie  },'eneral  arra!ij,'ement  of  tiie  viscera,  that  tlie  ancestry  of  llie 
group  is  to  l»e  sought  for  among  llie  primitive  Gastcropods,  hut  in  forms 
more  primitive  tiian  any  existing  forms.  The  symmetrical  shape  t)f  the 
body  and  the  character  of  the  viscero-pericardial  cavity  suggests  forms 
intermediate  indevcjlopruent  between  the  Ampliineuraand  the  IHotocardlate 
Prosobranchs. 

So  far  as  the  vari(»us  groups  are  concerned  there  can  be  little  doid)t  but 
that  tlu'  Tetraltranelis  are  the  mt»re  primitive  forms,  sliowing  as  they  do 
less  specialization  of  the  foot,  what  must  l)e  considered  a  more  primitive 
sliell,  and  a  more  general  tetidency  towards  a  paired  arrangement  of  the 
organs  than  is  found  in  the  I>il)ranchs.  The  duplication  of  the  ctenidia 
and  nephridia  must,  however,  i)e  considered  a  .secoudary  accjuisition.  The 
Decapods,  again,  seem  to  be  on  the  whole  more  primitive  than  the  Octo- 
pods.  the  character  of  the  cu'lom  and  the  presence  of  a  .shell  in  the  former 
being  points  to  which  attention  may  b«'  called  in  this  connection. 

The  AJIinHhs  nf  the  Mitllnsiui.  —  Attention  has  already  been  called  to 
the  siinilariiy  of  the  typical  (Jasteropod  and  I'elecypod  larva'  to  the  An- 
nelid Trochophore,  and  the  evident  conclusion  h.is  been  ])ointed  out  that 
the  Annelids  aiui  Mollusca  are  to  be  traced  back  to  a  common  ancestor 
represeiUcd  by  the  Trociiopliore.  it  is  dillicult  oiherwise  to  understand 
the  remaikable  similarity  which  exists  between  the  two  larva'  — similarities, 
iiu'ludiug  no!  only  tlie  general  arrangenu'Ht  of  llu^  locomotor  cilia,  but  ex- 
tending as  well  to  internal  organs,  such  as  the  nephridia.  In  two  respects, 
liowever.  the  Molluscan  V'eliger  dilfers  from  the  Annelid  Trochophore  ;  it 
l>os.ses.sos  a  shell  and  a  fool.  These  features  are.  lii>wever,  readily  ex- 
plicable as  a  throwing  back  in  the  ontogeny  of  important  structures  origi- 
nally developing  at  a  much  later  period  in  the  life  of  the  animal — a  pluv 
nomenon  of  by  no  means  unfrei|ueut  <»ccurrenc«'.  It  must  be  admitted, 
however,  that  freciuent  moflilicatioiis  of  the  Trochophore  arrangement  are 
to  lie  found,  as  has  l)een  indicated  in  the  descriptions  of  tlu;  Amphiiu'iira, 
iiiul  these  become  especially  interesting  from  the  fact  that  in  the  foriru'r  a 
jjiimitivo  arrangenu.'iit  of  the  parts  of  the  body  nnist  be  recognized.  If 
the  Trochophore  represents  an  ancestor,  then  it  nught  be  ex|)ected  that  it 
would  be  found  more  perfectly  represented  in  the  Amphineura  than  in  the 
highly  specialized  (Jasle-opods,  or  even  than  in  the  l*elecyi)ods. 

It  is  imi)ortant,  then,  that  the  po.ssibility  of  some  of  the  similar  struct- 
ures of  the  Trochoi)hort  and  Vuliger  having  been  indepeiulcntly  acquired 


TYPK  MOLLUSC  A. 


3fl;j 


tshouhl  be  kept  in  iniii<l,  especially  in  view  of  another  idini  as  to  the  gene- 
alogy  of  liie  Moiln.sca  wiiieh  has  been  advoeatcd  by  ecrtain  authors.  Ac- 
cording to  this  liicory  I  hoy  iiave  sprung  from  Turbellarian-like  ancestors,  I  lie 
creeping  surface  of  tlie  worm  having  iHseonie  more  muscnhir,  and  so  iiaving 
giv(Mi  rise  to  the  foot  of  lh»?  Moilusk,  the  dorsal  region  «)f  the  body  eU'val- 
ing  into  the  visceral  hump.  The  nervous  system  of  the  Ainphineura,  with 
its  ladder-like  arrangement,  might  readily  Im(  deduced  from  the  arrange- 
]iient  found  in  tlu;  I'latyhelminths,  and  thus  many  points  on  which  the  Tro- 
chophore  theory  throws  no  light  become  intelligil»h!. 

This  theory  concerns  itself  maiidy  with  the  adult  forms,  yet  it  is  not 
imj»ossil)le  that  a  reconciliation  between  it  and  tlusTrochophon!  theory  may 
be  po.ssible.  It  has  already  been  pointed  out  that  the  Trochophore  may 
possibly  Ije  the  representative  of  a  Turbellarian  larva,  and  the  .same  idea 
may  be  applied  to  the  Veligcr.  In  other  words,  it  is  possible  that  the 
Mollusca  may  have  iM'en  derived  from  the  Turbellaria,  and  that  the  ances- 
tral worms  pos.se.s.scd  in  their  life-history  a  larva  which,  independently  of 
the  adults,  underwent  a. series  of  moditications  leading  to  the  V»;ligcr.  Tiie 
Veligcr  would  then  be  the  descendant  of  a  Turbellarian  larva,  while  tlm 
adult  Moilusk  would  Ije  directly  descended  from  the  Turbellarian.  This 
view  nuiy  be  contrasted  with  that  which  regards  the  Trochophore  (includ- 
ing the  Veligcr  under  this  term  for  convenience)  as  the  ancestor  by  means 
of  the  following  scheme: 


I 


TURBELLARIAN   THEORY. 


Turbellarian  larva==Turbellarian 
(ancestoi'  \ 


Trochophore 


A.nnclid 


Moilusk 


re 

a 
llf 
lit 


TROCHOPHORE  THEORY. 
^Turbellarian 


Turbellarian  larva 
(ancestor) 


Trochophore 


Annelid 


Moilusk 


SUBKINGDOM   METAZOA. 
TYPK  MOLLUSCA. 

I.  Class  Amphineura. — Visceral  hump  not  developed;  bilaterally  sym- 
metrical; shell  represented  by  scattered  spicules  or  by  a  series 
of  calcareous  plates;  anus  terminal. 


364 


INVERT EBliA TE  MOHPIIOLOG  Y. 


1.  Order  Ni>h'noi/(ist res. — Shell   roprosoiitt^d   by  scattered  calcareous 

spiciiU.'.s.     uXeometiHi,  Proneumenfu,  (HHttiuh-nnn,  JJondtrst'a. 

2.  (Jrder  I'nhjiddcitphora. — Slicll   formed   by  eij?lit  plates  on  dorsal 

surface  of  body.     Vliituii,  Trw/iiji/t  rnioN,  ChilontUuH. 
II.  Class  Gastkuoi'oda. — Visceral    hump   usually  well    developed;    body 
asymmetrical;  shell  uuivalved  and  usually  spirally  coiled,  souie- 
times  absent;  anus  not  terminal. 

1.  Order  l*rim)hi(uichiii.-\]Un\\([\A  present,  situated  in  front  of  the 

heart;  auricle  in  front  of  ventricle;  munlle  edge  not  fused  with 
body. 
Heart  with  two  auricles;  two  nephridia  {IHutomnUu).  Huiiotis, 
Turbo,  Tror/uifi,  Neritinu,  I'leurotomarid,  blssurella,  Pitttlla, 
Av.maa. 
Heart  with  a  single  auricle  and  a  s..igle  neiihridium  {Monotocar- 
dia).  C 

Dentition  ta'nioglosaate. 
With  (M'e«'ping  habit,     Cj/prini,  Pa/ml  inn,  Natica,  Ampul- 

htrht,  JJ/torhm,  Ci/v/osfoiiia,  Calt/ptrau,  Struinbus. 
Willi  pelagic   habit  {/leterojmda).      Atutdida,  Varinaria^ 
Ptfrotradu'ti. 
Dentition  plenoglossate.     Idnffifna,  Srahiria,  /Solarium. 
Dentition   rachiglossate.     Fusus,   Bncciuum,  JVu.sfia,  Murex, 

Purpura,  Olira,  Maryhulla. 
Dentition  to.xiglos.sate.     Terebra,  Conus,  Pleurotuma. 

2.  Order  (t/ifsfliohrandiia. — Ctenidia  frequently  absent,  when  pres- 

ent  behind   the  heart;  auricle  behind  ventricle;  mantle  when 

presiMit   not  fused  by  its  edges  to  iMidy-wall;  shell  freqiieutly 

absent. 
Mantle  present  {TiHibrauchia). 

Foot  with  broad  flat  sole;  with  creeping  habit.     Bulla,  Janus, 
A p/i/sia,  Pleu robra iic/tus. 

Foot  with  winglike  parapodia.  pelagic  (Plero/iodu). 

W'WhshvW  {Tltero.somala).     Lhna>.'h,   ,  JStijNola,  Ci/mbulinpsis^ 
Without  shell  {iiipnuosinnata).     J'lieuniodcrma,  Cliouf. 
Mantle   not   developed    ( Xudibntwhiu).     Pliurophi/llidia,  P/iyl- 

lirhtH',  Limapnulia,  Horis,  JKolis,  Faccl/iua. 

3.  Order  i^/////<o//f</a.— Ctenidia  wanting;  mantle  fused  by  its  edges 

to  body-wall;  terrestrial  or  aquatic. 
Eyes  at  base  of  tentacles  {Basounnatophora).     Limnita,  Physa, 

Planorbis. 
Eyes  at  tip  of  tentacles  {Styhm uudnphora).    He'ir,  Limax,  Arion, 

Vaninula.  Daudebardia,  (tmhidium. 
III.  Class  ScArndPODA. — Visceral  hump  developed;  bilaterally  symmetrical; 

shell  cylindrical,  open  at  both  ends.     Dentalium,  Siphodenta- 

littm,  Cadulus. 


TYI'A'  MULLUaCA. 


365 


IV.  Class  PKLKrvPODA —Visceral  liiiiiip  not  dcvcloptMl;  bilaf orally  sym- 
iiu>ti'i<-.'il;  luautlc  forms  two  IiUcrai  t'olds;  slirll  bivalvcd;  anus 
tfi-iiiiiiai. 

1.  Order  J'rofohraiiv/tiii. — Gill  a  true  etenidhim;  pleural  Kanj,'lia  not 

unile<l  lo  eerebral.     Niicititt,  Yoldia. 

2.  (Jrder  Filihraiuliia. — (Jill-filaint'iits  eioii^^aled  and  Itent  upwards 

at  ends;  t'ereljral  and  pleural  ganylia  fused.     Aiiomiti,  Mi/fi/us, 
Mniili>laria,  Amt. 

3.  Order  I'sfuifi>f(Hiir/lihrun(itiii.—G\\\-t\h\uu'\\ts  turned    up  at  ends 

and  with  iuterlainellar  jiinetions;  cerebral  and  pleural  ^Mn;;lia 
united.     I'lrftn,  Ustntt. 

4.  Order  l''n/inni'//ihtiitifliiii.  -(Jill  filaineiits  united  to  form  a  plato- 

likc  j^ill;  cerel)ral  and   pleural   gan;;lia   united.      Vinn.s,  Mi/a, 
Eiisdtdlit,  Pliohts,   Teriifo,   I'llin,  Aiini/mt,  CijrfiiH. 

5.  Order  Si ptihnittrhid.     (WW  reduced  to  a  iniiseular  perforated  sep- 

tum between  the  mantle  and  suprabrancliial  chambers.  lUis- 
phUiria. 
V.  Class  ("Ki'ii.\i,(»i'(>i),\  — Vi.s<'eral  hump  developed;  bilaterally  .synunetri- 
cal;  mantle  a  eircidar  fold;  foot  ( jtrd/mi/iinn  -.[ud  iinso/ioi/ium) 
forming,'  arm-like  structures  provided  with  suckers  and  sur- 
rounding the  motitii. 

1.  Order    Ti'tradnuu'hia. — With   four    ctetiidiii    and   with    external 

chamljered  shell.     Nttiitilufi. 

2.  Order   Dihniuchia. —W'ilh   two    ctenidia ;    shell   if  external  not 

chambered,  usually  internal. 
With  eight  arms  to  foot  ((Hnpoda).     Arynuauta,  (fctn/ms,  Tiiin- 

octopus,  Ph ilotuxifi. 
With   t«'n   arms  to   foot   {Derajmla).       Sjnrnla,   Sepia,   TmUijo, 

Umma.strephes. 

LITKUATrUE. 

OKNKU.VL. 

Gould  and  W.  G.  Binney.     The  Inn  rtibrdtn  of  North  Amenea.     Boston,  1870. 
G.  W.  Tryon  and  H.  A.  Pilsbry.     Miiunal  of  Coucholom/.     In  course  of  publi- 

catiDii.     IMiiladelpliia. 
8   P   Woodward.     .1  .\fiiiiiiiil  (f  the  MoUuhch.     !M  ed.     Loiidoii,  187H. 
£    R.   Lankester.      Artidv  "  MoUnnca."     Kncy<'lopietliH   Britaniiioii,    IHli    Kd. 

London,  1HS8. 
W.  Patten.     T/k  Kijcs  of  Mollnnka  tiiid  Arthropods.     Mitth.  n.  d.  Zool.  Station 

Neapel,  VI,  1886. 


AMPniNKi;u.\. 


B.  Haller.     Die  Organisntion  der  Chitonen  der  Adnn.     Arbeiten  n.  d.  Zoolojj. 

Institut  Wlen,  iv.  1882;  v,  188a. 
H.  N.  Moseley.     On  the  Presence  of  Eyes  in  the  Shells  of  certain  Chitonidm  and 


IMAGE  EVALUATION 
TEST  TARGET  (MT-3) 


1.0     ^1^  US 


1.1 


11.25 


■i>li2 

lift 

■40 


■  2.2 

US 

u 

•a    .. 

•UUU 


1^ 


I 


.Sciences 
CorporatiGn 


23  WIST  MAIN  STRUT 

WnSTIR,N.Y.  USM 

(716)l7a-4S03 


366 


INVERTEBRATE  MORPHOLOGY. 


on  the  Structure  of  these  Organs,  Quarterly  Journ.  Microscop.  Science, 
XXV,  1885. 

H.  J.  Pnivot.  L'organtsation  de  quelquea  Neomeniens  dea  COtea  de  France.  Ar- 
chives de  Zool.  exper.,  2me  Ser.,  ix,  1891. 

S.  Wiren.  Studien  iiber  die  Soienogastrea.  I.  Monographie  dea  Ghcetoderma 
nitidulum.     Svensk  Vetenskap.  Akad.  Handl.,  xxiv,  1892. 

GASTEKOPODA. 

Alder  and  Hanoook.    A  Monograph  of  the  Britiah  Nudibranchiate  Molluaca. 

London,  1850-51. 
J.   W.  Spengel.    Die  Qeriichaorgane  und  daa  Nervenaystem  der  MoUuaken. 

ZeitBcbr.  fUr  wissenscb.  Zoologie,  xxxv,  1881. 
H.  L.  Osborn.     On  the  QUI  in  aome  Forma  of  Proaobranchiate  Molluaca.    Studies 

from  the  Biolog.  Labor.  Johns  Hopkins  Univ.,  iii,  1884. 
B.  Haller.    Beitrdge  zur  Kenntniaa  der  Niere  der  Proaobranchier.    Morpholpg. 

Jahrbuch,  xi,  1885,  ' 

H.  Simroth.     Verauch  einer  Naturgeachichte  der  deutachen  Nacktachnecken  urid 

ihrer  europdiachen  Verwandten.    Zeitschr.  fUr  wissensch.  Zoologie,  xlii, 

1885. 
W.  G.  Binney.    A  Manual  of  American  Land-aheUa.    Bulletin  U.  S.  National 

Museum,  No.  28.  1885. 
Bela  Haller.     Unterauchungenuher  maHnenRhipidoghaaen.    Morpholog.  Jahr- 
buch, IX,  1883  ;  XI,  1886. 
E.  L.  Bouvier.     Syat^me  nerveux,  morphologie  generate,  et  cla^aification  dea  Oaa- 

teropodea  proaobranchea.    Annales  Sciences  Nat.,  Zool.,  Ser.  7me,  iii,  1887. 
B.  Haller.    Die  Morphologie  der  Proabranchier  geaammelt  auf  einer  Erdumae- 

gelung  durch  die  konigl.  ital.  Corvette  "  Vettor  Piaani."    Morpholog.  Jahr- 
buch, XIV.  1888 ;  XVI,  1890 ;  xviii,  1892. 
2.  Pelseneer.     Report  on  the  Pteropoda.    Scient.  Besults  of  the  Voyage  of 

H.  M.  S.  Challenger.  Zoology.  LViii.  1887 ;  Lix,  1888  ;  Lxv,  1888. 
J.  I.  Peek.     Ou  the  Anatomy  and  Hiatology  of  Cynibuliopaia  caleeola.    Studies 

from  the  Biol.  Labor.  Johns  Hopkins  Univ.,  iv,  1889. 

A.  Lang.     Veranch  einer  Erkldrung  der  Aaymmetrie  der  Oaateropoda.   Viertel- 

jabresschr.  Naturf.  Qesellsch.  ZQrich,  xxxvi,  1891. 

B.  von  Erlanger.     Zur  Enttoickelung  von  Paludina  vivipara.    Morpholog.  Jahr- 

buch, XVII,  1891. 
H.  von  Ihering.    Morphologie  und  Syatematik  dea  Oenitalapparatea  von  Helix. 
Zeitschr.  fUr  wissensch.  Zoologie,  i.iv,  1892. 


8CAPH0P0DA. 


H.  de  Lacaie-Dathiera.   Hidoire  de  V organisation  et  dti  developpement  du  Dentale. 

Annales  des  Sci.  Nat.,  Zool.,  4me  Ser.,  vi,  1856  ;  vii,  1857  ;  viii,  1858. 
L.  H.  Plate.     Ueber  den  Bau  und  die  VertcandtaeJiaftabezielmngen  der  Sokno- 

eonchen.    Zoolog.  Jahrbllcher,  Anat.  Abth.,  v,  1892. 


PELECYPODA. 

B.  H.  Peek.     The  Structure  of  the  LameUibranch  QUI. 
croscop.  Science,  xxvi,  1876. 


Quarterly  Journ.  Mi- 


?JWM' 


msam 


TYPE  MOLLUSC  A. 


367 


C.  Grobben.     Ueher  die  Pericardialdruse  der  Lamellibramhiaten.    Mil  Beitrag 
zur  Kenntniss  der  Anatomie  dieser  MoUuskenklasse.    Arbeiten  a  d.  Zool 
Institut  Wien,  vii,  1888. 

W.  M.  Bankin.     Ueber  das  Bojanus'schen  Organ  der  Teichmuschel  {Anodonta 
Cygnea,  Lam.).     Jenaiscbe  Zeitschr.,  xxiv,  1890. 

B.  Bawitz.     Der  Mantelrnnd  der  AcepJialen.    JenaiscLe  Zeitschr    xxir  1888  • 

XXIV,  1890  ;  xxvri,  1893.  

P.  Pelaeneer.     Gontributior    d  I'etude  des  LamellibrancJiea.    Archives  de  Bio- 
logie,  XI,  1891. 

CEPHALOPODA. 

E.  Owen.     Article  "Cephalopoda."    Todd's  Cyclopedia  of  Anatomy  and  Phy- 
siology,  I.    London,  1836. 

J.  Brook.    Zur  Anatomie  und  Systematik  der  CepJialopoden.    Zeitschr.  f  Ur  wis- 
sensch.  Zoologie,  xxxvi,  1883. 

C.  Grobben.     Mm-phologische  Studien  iiber  den  Earn,  und  Geachlechtaapparat 

some  die  Leibeahohle  der  Cephalopoden.  Arbeiten  a.  d.  Zooloe  Inst'tat  Wif^n 
V,  1884.  ^' 

P.  Pelaeneer.     Sur  la  valeur  morphologiqtie  des  bras  et  la  composition  da  syst^me 
nervettx  central  des  Cephaiopodea.    Archives  de  Biologie,  viii,  1888. 


368 


INVERTEBRATE  MORPHOLOGY. 


CHAPTER    XIII. 


TYPE  CRUSTACEA. 


The  group  Crustacea  includes  a  very  large  number  of 
forms,  most  of  which  are  mariue,  though  mauj  are  found  in 
fresh  water  and  a  few  are  even  terrestrial.  A  great  diversity 
of  form  is  found  in  the  various  members  of  the  group,  but  at 
the  same  time  the  general  structure,  except  in.  forms  degen- 
erated by  parasitism,  shows  comparatively  close  similarity 
throughout. 

The  body  is  enclosed  in  a  thick  chitinous  cuticle  which 
not  infrequently  becomes  hardened  by  the  deposition  of 
calcareous  matter  in  it,  producing  what  may  almost  be  con- 
sidered a  shell  and  giving  origin  to  the  name  applied  to  the 
group.  This  covering  serves  not  only  for  protection,  but  also 
as  a  point  d'appui  for  the  insertion  and  origin  of  muscles. 
Where  it  reaches  a  considerable  thickness  it  becomes  more  or 
less  regularly  divided  into  segments,  separated  by  intervals  in 
which  the  cuticle  remains  thin,  so  that  movement  of  the 
various  segments  upon  each  other  are  possible. 

As  a  rule  there  is  attached  to  the  sides  of  each  of  these 
segments  an  appendage,  also  inclosed  in  a  more  or  less  thick 
cuticle  and  jointed,  this  jointed  character  having  suggested 
the  reference  of  the  Crustacea  together  with  the  Araclmida 
and  Tracheata  to  a  single  group  termed  the  Arthropoda.  An 
examination  of  the  internal  parts,  especially  of  the  nervous 
system,  shows  that  these  various  body -segments  are  in  reality 
metameres,  and  that  the  Crustacean  is,  like  the  Annelid,  a 
metameric  organism.  A  characteristic  of  the  Crustacea,  how- 
ever, is  a  tendency  towards  a  greater  diflferentation  and  con- 
solidation of  the  metameres  than  is  found  in  the  Annelida,  a 
tendency  especially  well  marked  in  the  anterior  region  of  the 
body,  where  a  varying  number  of  the  metameres  fuse  more  or 
less  perfectly  together  to  form  a  distinct  head,  bearing  the 


■"^^fgna 


TYPE  CRUSTACEA. 


369 


principal  sense-orgaus  aud  the  organs  of  mastication  (Tig. 
162),  there  being  behind  this  head,  more  or  less  perfectly  dis- 
tinguishable, a  thorax  and  an  abdomen.  Judging  from  the 
number  of  pairs  of  appendages  arising  from  this  head  region 
it  seems  that  the  typical  number  of  metameres  consolidated 
to  form  it  is  five,  but  to  these  there  must  be  added  an  anterior 
segment  which  does  not  bear  appendages  but  upon  which  the 
eyes  are  developed.  To  these  six  somites  there  are  added, 
especially  in  the  more  highly-differentiated  forms,  a  number 
of  additional  metameres  which  properly  belong  to  the  thorax, 
the  apparent  extent  of  the  head  region  being  thus  increased. 


Fig.  163. — A  Decapod  CkustacIian,  Cambarua. 

There  is  indeed  throughout  the  Crustacea  a  tendency  towards 
what  has  been  called  "  cephalization,"  i.e.,  a  condensation  of 
the  anterior  metameres,  and  as  a  rule  the  higher  the  form  the 
greater  is  this  cond'^nsation  and  the  greater  the  apparent  ex- 
tent of  the  head  region.  The  number  of  segments  composing 
the  thorax  and  abdomen  is  exceedingly  variable  in  the  lower 
forms,  but  in  the  higher  there  are  constantly  eight  thoracic 
and  seven  abdominal  segments,  the  posterior  one,  termed  the 
telson,  being  alone  destitute  of  appendages.  Frequently, 
especially  in  the  higher  forms  (Fig.  162),  the  thoracic  seg- 
ments consolidate  to  a  greater  or  less  extent,  the  segmentation 
of  this  region  of  the  body  being  indicated  in  some  forma  only 
by  the  appendages  and  the  nerve-ganglia,  and  furthermore 
lateral  folds  of  the  body-wall  may  project  backwar-^^  from  the 
sides  and  dorsum  of  the  head  or  anterior  thoracic  regions, 
enclosing  the  thorax  or  even  the  entire  body  in  a  firm  cara- 
pace or  else  in  a  bivalved  shell,  sometimes  provided  with 
adductor  muscles. 


370 


INVERTEBBA TE  MORPHOLOO  T. 


The  study  of  the  embryology  of  some  of  the  higher  Crustacea  has 
brought  out  the  fact  that  in  these  there  are  indications  of  a  segment  desti- 
tute of  appendages  but  represented  by  a  pair  of  nerve-ganglia,  immedi- 
ately succeeding  the  eye-bearing  anterior  segment.  In  these  cases,  then, 
the  head  really  consists  altogether  of  seven  segments.  Whether  this  seg- 
ment represents  the  first  appendage-bearing  segment  of  the  lower  forms 


Pig.  168.— Crustacean  Appendages. 
A,  antennule  of  Crayfish,  Cambarus;  B,  antennulu  of  Copepod,  Oithona  (after 
GiKSBBEOHT);  C,  autcnua  of  Cambarus;  D,  anteDua  of  Phyllopod,  Eulimnadia 
(after  Packard);  as,  sensory  hairs. 

or  whether  in  these  also  it  exists  in  a  degenerate  condition  has  not  yet  been 
determined ;  for  convenience  at  present  the  six  fully-developed  head- 
segments  may  be  considered  homologous  throughout  the  group. 

The  appendages  vary  much  in  form  in  different  parts  of 
the  body  and  in  different  forms.     Those  of  the  head  region 


TYPE  CRUSTACEA. 


371 


are  modified  to  serve  as  sensory  organs  and  organs  of  masti- 
cation. The  first  pair,  termed  the  first  antenmc  or  antennnJes, 
are  usually  sensory  in  function,  though  occasional!}'  also  loco- 
motor (Ostracoda  and  some  Copepoda),  and  are  freijuently 
supplied  with  peculiar  sette  supposed  to  be  olfactory  in  addi- 
tion to  others  probably  tactile  in  function  (Fig.  1613,  />').  They 
consist  in  their  typical  form  of  a  basal  portion  composed  of 
three  or  four  joints,  the  terminal  one  bearing  one  (Fig.  103,  li) 
or  two  (Fig.  163,  A)  many-jointed  flagella.  The  second  pair,  the 
second  antenmv  termed  also  simply  antenna^  (Fig,  163,  C  and  />), 
are  also  principally  sensory  and  consist  typically  of  a  two-joint- 
ed basal  portion,  bearing  two  many-jointed  branches.  One  of 
these,  that  upon  the  outer  side,  frequently  becomes  reduced  to 
a  scalelike  rudiment  (Fig.  163,  C),  the  inner  branch  persisting  as 
the  fiagellum.  The  third  pair,  the  mandihles,  serve  as  mas- 
ticatory organs  and  are  generally  much  modified  in  correspon- 
dence with  this  function.  Typically  (Fig.  164,  A)  they  consist 
of  a  two-jointed  basal  portion  bearing  two  branches.  The 
proximal  joint  of  the  basal  portion,  however,  becomes  much 
indurated  by  the  thickening  of  the  chitinous  cuticle  and  also 
toothed,  forming  the  mandible  proper,  while  the  remaining  joint 
and  the  two  branches  undergo  reduction  even  to  disappear- 
ance, being  known  when  present  as  the  mandilmJar  palps 
(Fig.  164,  C,  mp).  The  fourth  pair  are  the  first  maxilla'  (Fig. 
164,  H  and  D)  and  serve  like  the  mandibles  for  mastication,  un- 
dergoing a  somewhat  similar  modification.  They  do  not,  liow- 
ever,  become  so  indurated,  though  one  or  both  of  the  basal 
joints  may  be  provided  with  stift'  setse  and  serve  as  a  jaw,  and 
the  two  branches  more  frequently  persist  than  in  the  mandi- 
bles. The  fifth  pair,  the  second  maxilUv,  are  also  masticatory 
and  resemble  the  first  in  the  modifications  which  they 
undergo. 

The  thoracic  and  abdominal  appendages  in  all  but  the 
lowest  forms  can  be  reduced  to  a  typical  appendage  consisting 
of  a  two-jointed  basal  portion  tipped  by  two  branches  also 
jointed.  In  appendages  employed  for  swimming  both 
branches  persist  (Fig.  165,  A),  and  may  possess  a  broad 
platelike  form,  but  when  modified  for  walking  the  outer 
branch    disappears.       From   limbs   modified  iu   this   latter 


IU; 


372 


INVERTEBBATE  MOBPUOLOGT. 


V  ■ 


Fig.  164.— Crustacean  Appendages. 
A,  mandible  of  Copepod,  NotodelpJtya  (from  Bronn);  B,  first  maxilla  of  Noto- 
delphya  (from  Bbonn);  C,  mandible  of  Cambarus;  D,  first  maxilla  of  Carti' 
barua. 
en  =  endopodite.  ex  =  exopodite.  mp  =  mandibular  palp. 


Pig.  165. — Ckustacean  Appendages. 
A,  second  thoracic  appendage  of  Mysia  (after  Saks);  B,  second  thoracic  appen- 
dage of  an  Amphipod. 


f 


TYPE  CRUSTACEA. 


373 


manner  the  grasping  claws  (Fig.  105,  A*)  or  chelae  are  devel- 
oped by  the  flexion  of  the  terminal  joint  on  the  subterminal  or 
by  the  elongation  of  the  angles  of  the  latter  into  a  more  or 
less  strong  process  against  which  the  terminal  joint  may  be 
approximated. 

The  description  given  above  of  the  various  appendages  is 
of  course  general,  the  modifications  found  in  the  various 
forms  being  almost  endless.  Indeed  in  parasitic  forms  the  ap- 
pendages, except  those  concerned  in  mastication,  may  entirely 
disappear,  all  gradations  between  fully-developed  append- 
ages and  the  merest  rudiments  being  found  in  various  forms. 


Fig.  166.— Sixth  (^1)  and  Second  (B)  Tiiouacic  Appendages  of  Bkancuiu- 

POD,  Apus  (after  Zaddach  from  Biionn). 

br  =  bract.  fl  =  tlabellum.  1-6  =  inuci  lobes. 

From  what  has  been  said,  however,  it  may  be  seen  that  typi- 
cally the  Cmstace.in  appendage  may  be  considered  a  biramous 
structure,  consisting  of  a  two-jointed  basal  portion  termed 
the  protopodite  and  two  jointed  branches  termed  the  exopodite 
and  endopodite  (Fig.  165,  ex,  en)  according  to  their  relation  to 
the  median  axis  of  the  body.  Additional  rami  are  frequently 
developed  upon  the  protopodite — such,  for  example,  as  that 
termed  the  epipodite  (Fig.  167,  ep)  and  the  branchia  {br).  How- 
ever, although  such  a  limb  may  be  considered  topical,  it  is 
not  necessarily  also  the  most  primitive.  Indeed  when  the 
simplest  forms,  such  as  the  Phyllopoda,  are  examined  it  will 
be  found  that  the  more  posterior  appendages  have  a  very 
different  composition.  Thus  in  the  genus  Aptis  the  sixth 
thoracic  appendage  (Fig.  166,  A)  consists  of  a  central  two- 


374 


INVERTEBRATE  MORPHOLOGY. 


joiuted  axis  ending  in  a  rounded  lobe  and  bearing  upon  its 
inner  edge  six  lobes  (1-6),  some  of  which  are  united  to  the 
axis  by  a  joint.  On  the  outer  side  are  two  large  lobes,  the 
distal  one  being  termed  the  tiabellum  {Ji),  while  the  proximal 
one  is  the  bract  {hr)  and  serves  respiratory  purposes.  The 
entire  appendage  has  thus  a  leaflike  form.  In  one  of  the 
more  anterior  appendages,  however,  an  interesting  modilica- 
tion  of  this  will  be  found.  Thus  in  the  second  thoracic  ap- 
pendage (Fig.  100,  B)  the  axis 
will  be  found  to  be  more  dis- 
tinctly divided  into  two  joints, 
each  bearing  two  of  the  internal 
lobes  somewhat  reduced  in 
size,  while  the  terminal  one  in 
addition  carries  two  other  lobes, 
the  fifth  and  sixth,  which  have 
become  somewhat  elongated. 
The  flabellura  and  bract  remain 
nearly  the  same  as  in  the 
first  post  -  genital  appendage. 
If  now  such  an  appendage  be 
compared  with  the  second  tho- 
racic appendage  of  the  Shrimp 
Palcemonetes  (Fig.  167),  a  direct 
Fig.  167.— The  Second  IMaxilli-  homology  of  the  parts  may  be 
FED  OF  Palcemonetes.  discovered.      The   axis  of  the 

Phyllopod  limb  is  represented 
by  the  protopodite  while  the 
exopodite  (ex)  and  endopodite 
{en)  represent  the  two  terminal 
inner  lobes,  the  others  having  disappeared;  the  flabel- 
lum  is  represented  by  the  epipodite  (ep)  and  the  bract  per- 
haps by  the  branchia  (6r),  attached  to  the  epipodite  in  this  par- 
ticular limb,  but  free  on  the  more  posterior  ones. 

It  would  appear  probable  from  these  facts  that  the  bira- 
mous  limb  is  really  a  derivative  from  the  more  complicated 
foliate  appendage  possessed  by  the  Phyllopods ;  the  foliate 
condition,  however,  has  given  place  to  such  a  great  extent  to 


dr  =  biaiicbia. 
en  =  emlopodite. 
ep  =  epipodite. 
ex  =  exopodite. 


TYPE  CRUSTACEA. 


376 


the  biramous  that  it  is  most  couveuieut  to  rej^ard  the  latter  as 
the  typical  coutUtioii  in  the  Crustacea. 

Respiratory  organs  are  not  always  present,  but  when  they 
are  they  take  the  form  ot  thin-walled  outgrowths  of  the  body- 
wall.  In  some  forms  in  which  the  surface  of  the  body-wall  is 
increased  by  the  development  of  a  bivalved  shell  or  carapace 
the  lining-surface  of  the  fold  serves  for  respiration,  and  may 
be  thrown  into  a  number  of  folds  so  as  to  increase  the  extent 
of  surface,  as  in  the  Gasteropod  Patella.  In  the  majority  of 
cases,  however,  more  or  less  branched  hollow  processes  are 
seated  upon  the  sides  t)f  the  body  or  on  a  greater  or  less 
number  of  the  appendages,  their  cavities  communicating  with 
the  lacunar  spaces  of  the  boiiy,  so  that  the  blood  can  circulate 
through  them  and  receive  aeration  through  their  thin  walls. 
In  the  Isopoda  a  certain  number  of  the  appendages  are  de- 
voted to  the  respiratory  function,  both  the  exopodite  and 
endopodite  being  lamellar  and  thin-walled,  or  else  the  endo- 
podite  alone  may  have  this  function,  the  exopodite  serving  as 
a  covering-plate  for  the  protection  of  the  inner  respiratory 
ramus. 

As  already  stated,  the  body  is  covered  by  a  chitiuous  or 
more  or  less  calcareous  cuticle.  This  is  secreted  bj'  the  cells 
of  the  hypodermis,  as  it  is  termed,  which  correspond  to  the 
ectoderm  of  other  forms  and  rest  below  on  a  more  or  less 
well-developed  layer  of  connective  tissue.  A  dermal  muscu- 
lar system  is  entirely  unrepresented  in  the  Crustacea,  owing 
no  doubt  to  the  development  of  the  thick  cuticula ;  but  never- 
theless muscles  are  well-developed.  These  take  the  form  in 
the  body  of  four  longitudinal  bands,  two  situated  dorsally  and 
two  veutrally,  giving  off  slips  to  be  inserted  into  the  cuticle 
of  each  metamere,  flexion  and  extension  of  the  various  meta- 
meres  upon  one  another  being  thus  permitted.  In  addition 
muscles  extend  from  the  body- wall  to  the  various  appendages 
and  between  the  various  joints  of  these  structures,  being  in 
all  cases,  it  is  needless  to  say,  situated  within  the  body  and 
the  appendages.  In  some  cases,  more  especially  in  those 
forms  in  which  the  appendages  are  adapted  for  walking, 
special  chitinous  plates  or  processes  project  into  the  bodj- 
cavity  from  the  ventral  surface  forming  the  endophragmal  syS' 


ft' 


876 


INVEHTEDltATE  MORPHOLOGY. 


tern  and  serving  for  the  attachment  of  the  muscles  passing  to 
the  appendages.  In  forms  furnished  with  a  bivalved  shell 
special  adductor  muscles  for  its  closure  are  frequently  devel- 
oped ;  and  in  the  higher  Crustacea,  in  which  the  so-called 
stomach  is  usually  provided  with  a  series  of  chitinous  teeth, 
special  muscles  are  developed  for  their  movement. 

The  coelom  of  the  Crustacea  consists  for  the  most  part  of 
a  series  of  cavities,  without  definite  walls,  between  the  viscera 
and  the  muscle-bundles  and  extending  out  into  the  append- 
ages and  the  brauchiro.  One  of  these  occupies  the  mid-line 
below  the  dorsal  surface  of  the  body,  and  contains  the  heart, 
whence  it  is  known  as  the  pericardial  sinus.  It  is  bounded 
below  by  a  distinct  partition,  the  pericardial  septum,  but 
seems  to  be  a  schizocoelic  space,  since  it  contains  blood,  and  is 
therefore  not  comparable  to  the  pericardial  cavity  of  the 
Mollusca.  A  true  enterocoel  does  exist,  however,  in  some  of 
the  higher  forms  (e.g.,  Palc^monetes),  consisting  of  a  sac  lying 
in  the  anterior  thoracic  region.  It  surrounds  the  anterior 
aorta  as  a  narrow  cavity  and  behind  expands  so  as  to  cover 
the  anterior  portion  of  the  reproductive  organs,  and  then 
passes  ventrally  into  the  schizocoelic  cavity  which  surrounds 
the  intestine.  It  is  a  perfectly  closed  sac,  having  no  com- 
munication with  the  pericardial  sinus  beneath  which  it  lies, 
and  contains  a  coagulable  fluid  in  which  no  corpuscles  have 
been  observed. 

The  saclike  cavity  into  whicli  tlie  antennary  gland,  to  be  described 
later,  opens  is  also  to  be  regarded  as  a  true  enterocoel;  but  attention  must 
again  be  called  to  the  inadvisability  of  maintaining  a  wide  distinction  be- 
tween schizocojlic  and  enterocoelic  spaces,     (See  p.  231.) 

The  circulatory  system  is  comparatively  simple.  In  many 
forms  a  heart  and  distinct  blood-vessels  are  entirely  wanting, 
the  blood  circulating  through  the  lacunar  coelom  by  the 
movements  of  the  appendages.  In  the  majority  of  forms, 
however,  a  pulsatile  heart  (Fig.  168,  h)  is  present,  Ij'ing  near 
the  dorsal  surface  of  the  body  in  the  pericardial  sinus,  ex- 
tending in  some  forms  throughout  the  entire  thoracic  and 
abdominal  regions  of  the  bod3\  More  usually,  however,  the 
heart  is  limited  to  the  thoracic  region,  or  occasionally  is 
almost  entirely  confined  to  the   abdomen,  its   anterior  ex- 


n 


TYPE  CRUSTACEA. 


377 


tremity  encroacliiug  but  slightly  upou  the  thorax  (iHopoda). 
It  is  provided  with  a  vai'yiufj;  number  of  opeuiugs  along  its 
sides,  through  which  the  blood  gains  entrance  to  its  cavity 
from  the  pericardial  sinus — these  openings,  termed  otitioy 
being  guarded  by  valves  opening  iu  wards  and  preventing 
regurgitation  of  the  blood  during  systole.  From  either  end 
of  the  heart  arteries  arise  which,  after  a  longer  or  shorter 
course  and  many  or  few  branchings,  open  widely  into  the 
lacunar  spaces.  From  these  the  blood  passes  in  some  forms 
into  a  venous  sinus  situated  on  the  ventral  surface  of  the 


Hd  vn 


Pig.  168.— Diagram  of  Structurb  of  Crustacean  (Oambanu). 


an  =  unus. 

ea  =  carupuce. 

ee  =  ctiiebrul  guogliou. 

h  =  heart. 

1  =  iutesliue. 

I  =  digestive  glaud. 
m  —  luoutb. 
mp  =  opeuing  uf  vas  deferena. 


ne  =  nephridium. 

8  =  stomach. 

9a  =  sternal  artery. 

te  =  testis. 

tl  =  telson. 

vd  =  viis  deferens. 

vn  =  ventral  uerve. 

1-6  =  ubdoiuiual  segments. 


body,  and  thence  is  distributed  to  the  branchiae,  passing  from 
them  back  to  the  pericardial  sinus,  and  so  to  the  heart  again. 
The  blood  is  usually  colorless,  though  occasionally  greenish, 
in  which  case  it  contains  a  respiratory  copper-containing  pig- 
ment termed  hsBmocyanin,  or  reddish,  in  which  case  the  pig- 
ment is  hasmoglobin.  It  consists  of  a  plasma  in  which  float 
amoeboid  nucleated  corpuscles. 

The  digestive  system  consists  of  an  almost  straight  tube 
extending  from  mouth  (Fig.  168,  m)  to  anus  (an)  and  divisible 
into  three  re:'ions.  The  mouth  is  bounded  in  front  ^^y  an 
overhanging  lip,  and  behind  by  a  lower  lip  which  arises  as 
two  separate  parts,  which  by  some  writers  have  been  regarded 


378 


INVERTEBRATE  MORPHOLOGY. 


as  appendages,  though  the  absence  of  a  corresponding  nerve- 
gauglion  tells  very  strongly  against  such  an  idea.  The  ante- 
rior portion  of  the  digestive  tract  arises  in  the  embryo  as  an 
ectodermal  invagination,  and  is  frequently  lined  throughout 
by  a  c'iitinous  cuticle.  In  the  higher  forms  (Malacostraca) 
the  posterior  portion  of  this  foregut  is  enlarged  to  form  a  so- 
called  stomach  (Fig.  168,  «),  in  which  the  chitinous  lining 
thickens  to  form  a  complicated  arrangement  of  teeth,  which, 
moved  by  special  muscles  extending  from  the  stomach  to  the 
walls  of  the  body,  serve  for  the  comminution  of  the  food. 
No  salivary  glands  occur.  The  midgut  is  frequently  of  very 
small  extent,  and  has  usually  connected  with  it  a  digeistive 
gland  (Fig.  168, 1)  consisting  either  of  from  one  to  four  pairs 
of  simple  or  but  slightly  branched  coecal  tubes,  or  else  of  a 
much-branched  compact  gland  opening  into  the  intestine  by 
two  or  more  ducts.  The  hindgut  (*),  like  the  foregut,  arises 
as  an  ectodermal  invagination,  and  is  usually  lined  with  chitin 
and  unprovided  with  special  glands. 

The  nervous  system  presents  a  typically  metameric  condi- 
tion throughout  the  greater  portion  of  the  body,  a  pair  of 
ganglia  occurring  in  each  segment,  united  by  paired  connec- 
tives with  the  ganglia  of  the  preceding  and  succeeding  meta- 
meres  (Fig.  168,  vn).  In  the  anterior  portion  of  the  body, 
however,  as  well  as  posteriorly,  a  certain  amount  of  concen- 
tration and  fusion  of  the  various  ganglia  occurs.  An  ideal 
condition  in  Avliich  no  fusion  has  taken  place  would  show  a 
pair  of  cerebral  ganglia  (Fig.  169,  ce)  with  which  more  or  less 
complicated  optic  ganglia  are  connected.  From  the  cerebral 
ganjjlia  connectives  pass  backward  and  unite  with  a  pair  of 
ganglia  ({/'),  clearly  indicated  in  the  embryos  of  many  of  the 
higher  forms,  but  not  yet  definitely  known  in  the  Eutonio- 
straca,  though  it  seems  probable  that  they  occur  in  these 
also.  The  metamere  and  appendages  which  should  properly 
be  associated  with  them  seem  to  have  disappeared ;  that  is 
to  say,  they  are  the  sole  representatives  of  a  metamere  inter- 
vening between  the  cerebral  and  antennulary  segments.  These 
ganglia  are  united  by  a  pair  of  connectives  with  a  third  pair 
sending  nerves  to  the  antennules  (</"),  and  these  again  with  a 
fourth  pair  belonging  to  the  antennary  metamere  {g*),  and  so 


:.^»iS**^*«ai**K*«^^ 


TYPE  CRUSTACEA. 


379 


on,  a  pair  of  ganglia  occurring  in  each  metamere  throughout 
the  body.  Such  a  condition  as  this  is  found  only  in  em- 
bryonic  stages,  and  even  there  not  always  perfectly.  The 
ganglia  representing  the  preantennulary  metamere  fuse  with 
the  cerebral,  as  do  also  the  antennulary,  and  in  higher  forms 
the  antennary  ganglia,  there  being  thus  formed  a  complex 


ill'.... 


Fig.  169.— DrAoKAM  of  Neuvous 
System  op  Crustacean. 
ce  =  cerebral  guuglion. 
g^  =  second  " 

g^  =  autenmiliiry  " 
(f  =  anteunury      " 
?n/i'  =  mandibular   " 
wia',  mx^  =  nm.xlllary  ganglia, 
(jj  =  oDsopbagus. 
W  =  liist  thoracic  ganglion. 


Pig.  170.— Nebvous  System  op 

(A)  AN  Isoi'oi),  Aaellns,  and 

(B)  A  Buaciiyuhan  Decapod, 
Maja  (after  Milnb-Kuwakds). 


cerebrum,  which,  in  contrast  to  the  simple  cerebrum  {arcliu 
cerehrum)  of  the  Annelida,  may  be  known  as  a  syncerehrum. 
The  remaining  ganglia  may  remain  perfectly  separate,  the 
connectives  joining  the  more  anterior  ones  usually  being 
much  shortened,  or  a  greater  or  less  number  of  them  may 
fuse.    Thus  in  the  Crayfish  the  ganglia  of  the  three  posterior 


ii  i » 


I!  : 


380 


INVERTEBRATE  MORPHOLOGY. 


head-metameres  unite  with  those  of  the  two  anterior  tlioracic 
segments  to  form  a  single  ganglionic  mass  lying  behind  the 
oesophagus  and  sending  nerves  to  the  appendages  of  the 
somites  represented  in  the  fusion.  Similarly,  in  the  posterior 
region  of  the  body  of  the  Isopoda  all  the  ganglia  of  the 
abdominal  region  may  fuse  to  a  more  or  less  simple  mass 
(Fig.  170,  A,  ab),  and  an  extreme  condition  of  fusion  is  to  be 
found  in  some  Crabs  (Fig.  170,  B),  in  which  all  the  ganglia 
behx^id  the  antennary  segment  fuse  to  a  single  mass  [tab), 
lying  in  the  thorax — a  condition  standing  in  relation  to  the 
reduction  of  the  abdomen  and  the  extensive  concentration  of 
the  head  and  thoracic  regions  which  are  characteristic  of 
these  forms.  ) 

A  sympathetic  nervous  system  seems  to  be  generally 
present,  consisting  in  its  most  complete  condition  of  an  un- 
paired nerve  arising  from  the  syncerebrum  and  passing  back- 
wards to  be  distributed  to  the  stomach,  and  of  a  median 
nerve  (Fig.  170,  A,  m)  extending  from  one  pair  of  postoesoph- 
ageal  ganglia  to  the  other,  lying  between  the  two  connectives. 

Sense-organs  reach  a  high  degree  of  development  in  the 
group.  Hairs  occur  in  abundance  on  the  appendages  and 
body,  the  majority  no  doubt  having  merely  a  mechanical 
function ;  but  among  them  will  be  found  some  beneath  which 
lie  one  or  more  ganglion-cells,  giving  rise  to  a  nerve  which 
passes  into  the  hair.  These  hairs  are  supposed  to  be  tactile 
in  function.  On  the  anteuuules  of  many  forms  and  more 
rarely  upon  the  antennae,  hairs  of  special  forms  occur,  usually 
in  bunches  or  in  rows.  They  may  be  club-shaped  or  cylin- 
drical, and  each  has  a  nerve-fibre  extending  into  it  without 
dilating  into  a  ganglion-cell  beneath  its  base.  To  these  hairs 
an  olfactory  function  has  been  assigned,  and  it  is  noticeable 
that  they  are  usually  more  abundant  upon  the  antennules  of 
the  males  than  on  those  of  the  females — an  arrangement 
which  suggests  a  probable  service  as  guides  in  finding  the 
latter. 

Eyes  are  very  generally  present  in  the  Crustacea,  and 
reach  usually  a  high  degree  of  efiiciency.  Two  forms  of  eye 
are  known — a  median  unpaired  one,  frequently  spoken  of  as 
the  simple  eye,  and  the  lateral  or  compound  eyes.     The  un- 


,s;»i&*si^i^i«*«Kfc'» 


TYPE  CRUSTACEA. 


381 


paired  eye  is  present  in  the  larval  stages  of  probably  all 
Crustacea,  and  persists  in  a  more  or  less  perfect  form  in  the 
adults  of  most  Entomostraca, — a  group  which  contains  the 
more  primitive  forms, — and  has  even  been  detected  in  those 
•of  some  of  the  higher  forms  (e.g.  Crangon).  It  consists  when 
well  developed  of  three  patches  of  pigment,  forming  cups  in 
each  of  which  lies  a  group  of  clear  cells  from  which  nerve- 
fibres  arise  passing  to  the  optic  nerve. 

The  lateral  eyes  are  composed  of  a  number  of  units  each 
of  which  possesses  all  the  parts  of  a  visual  organ  and  is 
termed  an  ommatidium,  and  consequent- 
ly these  eyes  have  been  regarded  as  an 
aggregation  of  a  number  of  individual 
eyes,  whence  the  term  compound  usually 
applied  to  them.  Each  ommatidium  is 
a  complicated  structure  consisting  of 
several  parts  (Fig.  171).  The  outermost 
layer  of  each  is  a  transparent  cornea 
which  is  continuous  with  the  general 
cuticle  of  the  body,  and  in  some  forms 
is  only  distinguished  from  this  by  its 
transparency.  More  frequently,  how- 
ever, this  cuticle  becomes  more  or  less 
perfectly  divided  into  a  series  of  corneas 
of  an  hexagonal  or  tetragonal  shape, 
one  corresponding  to  each  ommatidium, 
the  surface  of  the  eye  thus  acquiring 
a  faceted  appearance.  Below  the  cuti- 
cle   come   the    hypodermal  cells    {Oil)  Clff  =  corneal  hypodennis. 

which  secrete   it,   arranged   irregularly  ^1^  ^  crystivlline  cone. 

'L^       A.       e  i     i.1  i'T      •     -OK  =  <listal  retinula. 

Without  reference  to  the  omniatidia  in  pj^  ^  j,,.„^i,„.,i  ,.^.^j^^,^ 

the  simpler  non-faceted  eyes,  but  in  Eh  =  iliabdom. 
the  faceted  eyes  with  two  hypodermal  cells  lying  beneath 
each  cornea  and  constituting  the  corneal  hypoderinis. 
Below  these  come  the  cone-cells  (6^),  two  to  four  in  number 
as  a  rule ;  these  are  elongated  cells  a  portion  of  whose  pro- 
toplasm becomes  converted  into  a  refractive  translucent  body, 
the  crystalline  cone  (  CG\  composed  of  as  many  segments  as 
there  are  cone-cells  taking  part  in  its  formation,  and  sur- 


FiG.  171.— Diagram  of 
CiiusTACEAN  Ommati- 
dium. 

C  =  cone-cell. 


H 


>'i  ,'j 


382 


INVERTEBliATE  MORPHOLOGY. 


f 


rounded  upou  the  outside  by  a  delicate  layer  of  protoplasm 
placing  the  part  of  each  cell  above  the  cone  in  continuity  with 
the  part  lying  below  it.  In  the  higher  forms  there  occur, 
partially  surrounding  the  cone-cells,  two  pigmented  cells 
which  seem  to  be  sensory  in  function  and  are  termed  the 
distal  retinular  cells  {DR).  They  are,  however,  unrepresented 
in  the  lower  forms,  in  which  the  sensory  portion  or  retinula  is 
represented  by  a  single  circle  of  usually  5  cells  {Pli)  lying 
proximally  to  the  cone-cells  and  surrounding  a  chitinous  rod 
Avhich  is  manufactured  as  a  secretion  from  their  approximated 
surfaces,  and  is  termed  the  rhabdom  {llh).  These  cells  are 
also  pigmented  and  are  prolonged  below  into  nerve-iibres, 
which,  piercing  the  basement-membrane  upon  which  the^  om- 
matidia  rest,  pass  to  the  optic  ganglia.  In  the  higher  Crus- 
tacea, in  which  a  distal  retinula  is  present,  the  rhabdom  is 
formed  by  a  circle  of  eight  cells  (one  of  which  is  almost 
aborted,  so  that  there  appear  to  be  only  seven).  These  con- 
stitute the  proximal  retinula,  and  appear  to  correspond  to  the 
single  retinula  of  the  simpler  forms.  Finally,  a  number  of 
accessory  cells,  usually  pigmented,  may  surround  each  om- 
matidium,  separating  it  from  its  neighbors,  but  not  appearing 
to  be  essential  constituents  of  the  eye. 

Tlie  view  according  to  which  these  lateral  eyes  are  regarded  as  an  ag- 
gregation of  a  number  of  independent  eyes  has  already  been  referred  to. 
It  seems  questionable,  however,  if  this  be  the  correct  interpretation  of 
them  in  view  of  the  occurrence  of  so-called  compound  eyes  in  the  Moliusca 
{Area)  and  the  Polycluetous  Annelida.  It  seems  more  probable  that,  as 
in  these  forms,  the  Crustacean  eye  is  to  be  regarded  as  a  separation  into  a 
number  of  more  or  less  isolated  parts  of  an  originally  continuous  retina, 
a  corresponding  division  of  the  originally  simple  refractive  apparatus 
also  taking  place.  This  view  seems  to  harmonize  most  satisfactorily  with 
tlie  facts  of  development. 

Occasional  departures  from  the  usual  arrangement  of  the 
eyes  are  to  be  found — as  for  instance  in  Phronima,  one  of  the 
AmpLipoda,  in  which  two  pairs  of  compound  eyes  occur  on 
the  head.  Mention  may  also  be  made  here  of  the  peculiar 
eyelike  structures  occurring  in  Etiphaitsia,  one  of  the  Schizo- 
poda.  They  occur  on  the  basal  joints  of  the  appendages  of 
certain  of  the  thoracic  metaraeres,  as  well  as  upon  the  ventral 


TYPE  CRUSTACEA. 


383 


surface  of  the  abdomen,  and  appear  to  be  rather  phospho- 
rescent than  optic  organs. 

Otocysts  occur  throughout  the  group  Decapoda,  to  which 
the  Crayfish,  Lobster,  and  Crab  belong,  and  consist  of  sacs 
lined  by  sensory  sett©  and  containing  otoliths.  They  are 
situated  on  the  basal  joint  of  each  of  the  antennules  and  in 
some  forms  are  completely  closed,  though  usually  their  cavity 
communicates  with  the  exterior,  being  guarded  by  a  number 
of  closely-approximated  bristles.  In  the  Schizopoda  similar 
otocysts  occur  in  the  endopodite  of  the  last  pair  of  abdominal 
appendages,  and  in  the  Amphipod  OxycepTialus  two  lie  above 
the  syncerebrum.  These  structures,  which  are  usually  spoken 
of  as  auditory  organs,  seem  to  be  rather  sense-organs  of  equi- 
librium. 

In  the  larvae  of  many  forms  and  in  the  adults  of  some 
Entomostraca  one  or  two  papilla-like  processes  project  from 
the  anterior  surface  of  the  head  and  are  supposed  to  be  sen- 
sory in  function,  though  what  purpose  they  may  subserve  is 
unknown.  Strong  nerves  pass  to  these  frontal  sense-organs 
which  appear  to  be  of  considerable  importance. 

The  excretory  system  consists  of  two  pairs  of  nephridia, 
one  or  other  of  which  may  be  absent  in  many  forms.  One  of 
these  develops  in  connection  with  the  antennary  segment  and 
opens  to  the  exterior  on  the  basal  joint  of  the  antenna)  (Fig. 
168,  »ie),  whence  it  is  known  as  the  antennary  gland,  some- 
times, however,  receiving  the  name  of  the  green  gland.  It 
reaches  its  highest  development  among  the  Malacostraca, 
occurring  in  many  Entomostraca  only  in  larval  stages,  later 
on  degenerating.  In  its  simplest  condition  it  consists  of  a 
coiled  tube  whose  lumen  appears  in  some  cases  to  be  intra- 
cellular, though  in  others  it  is  undoubtedly  intercellular,  and 
which  terminates  internally  in  a  saclike  dilatation  whose 
wall  is  richly  supplied  with  blood-lacume.  In  the  higher 
forms  (Fig.  172,  A)  a  great  complexity  is  brought  about  by 
the  development  of  lateral  branches  from  the  tubular  portion, 
and  the  terminal  sac  (s)  may  enlarge  and  fuse  with  that  of 
the  opposite  side  to  form  a  cavity  of  considerable  size  lying 
in  the  anterior  portion  of  the  thorax  and  termed  the  nephro- 
peritoneal  sac. 


ai: 


^1 


i;i 


11 


384 


INVERTEBRATE  MORPHOLOOY. 


I 


The  second  nephritlium  (Fig.  172,  B)  develops  in  connec- 
tion with  the  second  maxillary  segment,  and  opens  usually 
upon  the  appendage  of  that  segment.  It  is  especially  devel- 
oped in  the  Entomostraca,  in  which  it  may  lie  in  the  folds  of 
the  body-wall  which  form  the  shell,  and  hence  is  usually 
known  as  the  shell-gland.  It  occurs  also  in  the  larval  stages 
of  many  Malacostraca,  and  may  possibly  persist  in  a  degen- 
erated condition  in  the  adults  of  some  forms.     In  structure  it 


Fig.  173. — A,  Diagram  of  Nephridium  (Green-gland)  of  Aataeua  (after 

Marchal);  B,  Shell-gland  of  Eulimnadia. 

a  =  terminal  sac.  aa  =  saccule. 

resembles  closely  the  antennary  gland,  but  does  not  present 
the  complexity  frequently  found  in  that  gland. 

The  majority  of  the  Crustacea  are  bisexual,  hermaphro- 
ditism occurring  only  in  forms  which  have  a  parasitic  habit 
and  in  some  which  are  sessile  in  adult  life  (Cirrhipedia).  The 
ovaries  or  testes  (Figs.  173,  A  and  B)  are  paired  organs  lying 
alongside  of  the  intestine  or  slightly  dorsal  to  that  organ, 
transverse  connecting  bars  in  some  cases  passing  from  the 
organ  of  one  side  to  that  of  the  other.  Each  organ  may  be 
regarded  as  a  tube,  sometimes  simple,  sometimes  branched, 
and  lined  on  its  interior  by  an  epithelium  which  gives  rise 
to  the  germ-cells.  Special  germ-producing  regions  are  fre- 
quently developed,  as,  for  instance,  at  the  extremities  of  the 
tubes  or  along  one  side  (Isopoda),  the  cells  in  other  regions 
ceasing  to  give  rise  to  ova  or  spermatozoa.  The  reproductive 
elements  pass  to  the  exterior  by  special  ducts,  oviducts  {od) 


^awito'--"*-'*'-^**"-*''^ 


TYPE  CRUSTACEA. 


386 


or  vasa  deferentia  {vd)y  connected  with  each  organ,  and  open- 
ing usually  upon  the  ventral  surface  of  the  body  at  or  near 
the  junction  of  the  thoracic  and  abdominal  regions.  The 
origin  of  these  ducts  has  not  yet  been  discovered,  but  it  has 
been  suggested  that  they  may  represent  a  third  pair  of  ue- 
phridia.  Accessory  structures,  such  as  receptacula  semiuis 
and  cement-glands,  for  the  attachment  of  the  ova  in  the  fe- 
males, and  spermatophore-sacs,  in  which  the  spermatozoa  are 
encapsuled  in  spermatophores  in  the  males,  are  frequently  de- 


'--OV 


Fig.  17;}.— Ovaky  and  Testis  op  Jlfysis  (after  Sarb). 
od  =  oviduct.  tb  =  transverse  bar  of  ovary. 

ov  =  ovary.  ie  =  testis. 

td  =  vas  deferens. 

veloped  in  connection  with  the  ducts,  and  in  the  Malacostraca 
certain  of  the  appendages  in  the  neighborhood  of  the  genital 
openings  are,  especially  in  the  males,  modified  so  as  to  serve 
as  copulatory  organs. 

Owing  to  the  great  variety  of  form  and  structure  met 
with  in  the  various  species  of  Crustacea  the  group  is  separa- 
ble into  a  large  number  of  subdivisions.  Two  principal  classes 
are,  however,  readily  discernible,  of  which  the  first  is 

I.  Class  Entoniostraca. 

In  this  class  the  number  of  segments  of  which  the  body  is 
composed  varies  greatly  in  the  various  groups  and  even  in 
closely-related  genera.  The  abdominal  region  is  in  some 
forms  very  much  abreviated  and  is  destitute  of  appendages,  a 
ml©  which,  however,  finds  exception  in  certain  Phyllopods  in 


386 


INVERTEBRATE  MOliPUOLOQY. 


which  some  of  the  segmeuts  behiud  the  genital  openings, 
which  may  be  taken  as  indicating  the  line  of  separation  be- 
tween the  two  regions,  are  provided  with  appendages.  Folds 
arising  from  the  head  region  and  forming  either  a  carapace 
or  a  bivalved  shell  are  frequently  present  and  the  animals  are 
for  the  most  part  small,  the  largest  reaching  a  length  of  about 
eight  centimetres,  while  the  majority  measure  less  than  a  milli- 
metre. The  unpaired  eye  usually  persists  in  the  adult,  as 
does  also  the  shell-gland,  the  antennary  gland,  on  the  other 
hand,  being  usually  rudimentary  or  absent.  A  masticatory 
stomach  is  never  present,  and  a  further  characteristic  is  found 
in  the  fact  that  the  larva  which  hatches  from  the  egg  is  almost 
invariably  a  Nauplius  (see  p.  417).  "-n, 

1.  Order  Fhyllopoda. 

The  Phyllopoda  are  principally  confined  to  fresh  water, 
the  genus  Artemia,  however,  being  found  in  salt  lakes,  while 
a  few  CladocerL,  are  marine.  They  seem  to  be  the  most 
primitive  of  all  the  Crustacea  and  present  the  greatest  variation 
in  the  number  of  metameres  composing  the  body,  some  spe- 
cies possessing  over  forty  pairs  of  appendages,  while  in  others 
again  the  number  is  reduced  to  nine.  All  the  thoracic  ap- 
pendages, however,  as  a  rule  bear  branchial  lobes,  and  in  some 
cases  (Apus)  present  the  many-lobed  and  imperfectly-jointed 
condition  which  has  been  considered  the  most  primitive  form  of 
the  Crustacean  limb  (see  p.  373).  The  antennules  are  usually 
small  and  abundantly  provided  with  olfactory  hairs,  while 
the  antennoe  (except  in  Apus,  in  which  they  entirely  disappear) 
are  long  and  serve  as  locomotor  organs.  The  mandibles  are 
reduced  to  simple  masticatory  plates  without  palps,  and  the 
maxillae  undergo  likewise  considerable  reduction.  A  heart  is 
always  present,  but  no  blood-vessels  exist,  the  blood  passing 
from  the  heart  into  lacunar  spaces. 

1.  Suborder  Branchiopoda. 

The  Branchiopoda  have  all  a  plainly-segmented  body  con- 
sisting of  many  segments,  and,  with  the  exception  of  Branchi- 
pus  and  Artemia,  are  provided  with  a  fold  of  the  body- wall 


TYPE  CliUSTAGEA. 


387 


which  may  form  a  dorsal  carapace,  as  in  Apus,  or  a  bivalveJ 
shell,  as  in  Limnadia,  Limnetis,  and  Esther ia  (Fig.  174),  an 
adductor  muscle  being  developed  for  the  closure  of  the  shell 
within  which  the  entire  body  may  be  withdrawn.  The  an  ten- 
nules  are  as  a  rule  small  and  are  provided  with  olfactory 
hairs ;  the  antenna?,  on  the  other  hand,  are  well  developed  ex- 
cept in  ApiiSy  in  which  they  are  in  some  species  quite  small 
and  in  others  entirely  wanting.  In  the  shelled  forms  they  are 
biraraous,  consisting  of  a  several-jointed  protopodite  termi- 
nated by  two  many-jointed  fiagella,  and  serve  as  oarlike  loco- 
motor organs,  but  in  Branchipm  they  are  short  strong  struc- 


FiG.  Vti.—EatTieria  eompleximanua  (after  Packard). 
aV  =  auteuuule.  «<*  =  auteuna,  m  =  sbell-muscle. 


tures  without  any  locomotor  function,  serving  in  the  males 
as  clasping  organs  of  use  in  copulation.  The  mandibles  are 
reduced  to  toothed  plates,  lacking  a  palp,  and  the  first  max- 
illae show  an  almost  similar  reduction,  while  the  second  are 
entirely  wanting  in  some  genera,  such  as  Limnetis.  The  suc- 
ceeding appendages  are  not  limited  to  the  thoracic  region  of 
the  body,  taking  the  genital  opening  as  the  limit  between  the 
two  regions.  Thus  in  Apus  cancri/ormis  there  are  eleven 
thoracic  appendages,  while  behind  the  genital  ring  there  are 
no  less  than  over  fifty  locomotor  limbs,  and  in  such  forms  as 
Limihetis  and  Estheria  (Fig.  174)  it  is  difficult  to  distinguish 
between  the  thorax  and  the  anterior  abdominal  segments. 

The  heart  of  the  Branchiopoda  is  a  more  or  less  elongated 
organ  with  several  ostia  and  is  usually  limited  to  the  anterior 
portion  of  the  thoracic  cavity,  though  in  Branchipus  it  extends 


388 


IN  VERTEBRA  TE  MORPHOLOQ  Y. 


into  the  anterior  abdominal  region.  Lateral  eyes  are  present 
in  addition  to  the  unpaired  median  eye.  In  Branchip.is  they 
are  situated  upon  the  sides  of  the  head  upon  well-deiined 
stalks,  but  in  Apus  they  are  closely  approximated  on  the 
dorsal  surface  of  the  cephalo-thoracic  carapace,  while  in  the 
shelled  forms  they  are  united  together  to  form  a  single  eye 
whose  double  nature  is  revealed  only  by  a  study  of  the  details 
in  its  arrangement. 

A  peculiar  feature  in  the  life-history  of  the  members  of  this  group  is  the 
comparative  infrequency  of  males,  their  proportion  to  females  being  so 
small  that  for  some  time  they  were  not  known  to  exist.  The  females  are 
able  to  reproduce  parthenogenetically — males  appearing  only  under  certain 
conditions  which  are  not  as  yet  satisfactorily  understood.  The  eggs  de- 
velop generally  in  brood-pouches  situated  upon  certain  of  the  thoracic  ap- 
pendages {Apus,  Limnadia)  or  else  are  affixed  to  filamentar  processes  of 
these  appendages  {Estheria). 

2.  Suborder  Cladocera. 

The  Cladocera  are  distinguished  from  the  Brancbiopoda  by 
the  segmentation  of  the  body  being  much  less  clearly  defined 
and  by  the  small  and  more  definite  number  of  appendages, 
there  being  only  from  four  to  six  pairs  of  thoracic  limbs.  A 
bivalved  shell  arising  from  the  maxillary  segments  and  pro- 
vided with  an  adductor  muscle  is  always  present ;  it  does  not 
enclose  the  head,  but  the  rest  of  the  body  may  be  completely 
withdrawn  within  it  except  in  some  genera,  such  as  jKvadne 
and  Polyphemus,  in  which  it  is  transformed  into  a  brood- 
chamber,  leaving  the  body  almost  unprotected. 

The  antennules  are  always  small  unjointed  structures  pro- 
vided with  a  bunch  of  olfactory  hairs  usually  terminal  in  po- 
sition, and  the  antennae  are  strong  biramous  locomotor  organs. 
The  mandibles  are  simple  toothed  plates  without  palps,  and 
the  second  maxillae  are  usually  entirely  wanting  in  the  adults. 
The  thoracic  limbs  are  six  in  number  in  the  genus  Sida  and 
are  all  lamellate  and  abundantly  supplied  with  marginal  setae, 
but  in  Daphnia  (Fig.  175),  Moina,  and  allied  forms  the  number 
is  reduced  to  five,  and  the  more  anterior  ones  are  more  or  less 
modified  towards  simple  cylindrical  jointed  appendages,  a 
condition  found  in  all  the  four  thoracic  appendages  of  Evadiie 


TYPE  CliUSTACEA. 


389 


Pig.  175. — Daphnia  pulex  (from  Hkrtwig). 
h  =s  brood-poucb.  o  =  ovary. 

e  =  immature  ova.  a  =  shell  gland. 

g  =  cerebral  gauglion.  1  =  antennule. 

go  =  optic  ganglion.  2  =  antenna. 

h  =  heart.  3  =  mandible. 

k  =  germinal  region  of  ovary.  5-9  =  thoracic  limbs. 


390 


IN  VEltTEBlU  TE  MOIWIIOLOQ  Y. 


and  PolyphemuHy  the  braiichiui  lobes  beiug  at  the  same  time 
rudimentary  or  entirely  wanting.  The  abdomen,  which  is 
composed  of  four  segments,  possesses  on  its  dorsal  surface 
elevations  for  the  closure  behind  of  the  brood-chamber,  and 
on  its  terminal  segment  setiB  are  usually  developed ;  it  does 
not,  however,  bear  any  appendages. 

The  heart  is  an  oval  structure  situated  in  the  thoracic  re- 
gion and  possesses  but  a  single  pair  of  ostia.  The  lateral 
eyes  are  in  all  cases  fused  to  form  a  double  eye  situated  in 
the  median  line  of  the  head  and  capable  of  movement  within 
a  socket  by  means  of  muscles  which  are  attached  to  it. 

The  majority  of  the  Cladocera  are  fresh- water  forms, 
though  some,  such  as  £/vadne,  are  marine.  The  ova  undergo 
development  in  a  brood-chamber  formed  by  the  space  in- 
cluded between  the  shell-valves  and  the  dorsal  surface  of  the 
abdomen,  and  in  Evadne  and  Polyphemus,  as  already  stated,  the 
entire  shell,  which  is  somewhat  reduced  in  size,  is  adapted  to 
serve  as  walls  for  the  chamber. 

As  in  the  Brauchiopoda,  collections  of  Cladocera,  es- 
pecially if  made  during  the  spring  or  summer,  will  show  an 
enormous  preponderance  of  females,  and  several  generations 
may  be  reared  without  a  single  male  making  its  appearance. 
The  eggs,  which  have  a  thin  egg-membrane  and  little  yolk, 
develop  jjartheuogenetically  and  produce  females,  and  this 
method  of  reproduction  will  continue  so  long  as  the  condi- 
tions, such  as  temperature  and  food,  remain  satisfactory ; 
hence  the  eggs  of  this  kind  are  generally  known  as  "  summer 
eggs."  Towards  autumn,  however,  or  whenever  the  condi- 
tions tend  to  become  unfavorable,  males,  distinguishable  by 
their  smaller  size,  the  absence  of  a  brood-pouch,  and  their 
more  highly-developed  sense-organs,  as  well  as  by  the  de- 
velopment of  hooked  set^e  on  the  anterior  appendages  which 
serve  as  clasping  organs,  make  their  appearance,  and  at  the 
same  time  the  females  begin  to  deposit  ova  much  larger  in 
size  than  the  summer  eggs  and  containing  a  considerable 
amount  of  yolk.  These  "  winter  eggs  "  develop  apparently 
only  after  fertilization.  In  Polyphemus  they  possess  a  thick 
shell,  but  in  other  forms  special  arrangements  occur  to  render 
them  resistent  to  cold,  drying,  etc.     In  some  forms  the  ma- 


TYPK  CIW8TACEA. 


391 


ternal  shell  is  sloughed  and  serves  rr  a  protecting  case,  hut 
moru  usually,  as  iu  I)nj>hma,  Moimi,  and  others,  a  saddlo- 
shaped  thickeniug,  the  epln'jtjnum,  appears  on  the  dorsal  wall 
of  thej  blood-pouch  at  the  time  of  the  passr.ge  of  the  winter 
egg  into  it,  and  this  thickening  is  thrown  off  with  the  egg  and 
forms  a  protective  covering  for  it. 

2.  Order  Ostracoda. 

The  Ostracoda  resemble  the  Cladocera  iu  the  segmentation 
of  the  body  being  but  slightly  marked  and  in  possessing  a 
bivalved  shell  pro'  ided  with  an  adductor  muscle.  The  shell, 
however,  encloses  the  head  as  well  as  the  thoracic  and  ab- 
dominal regions,  and  furthermore  but  two  thoracic  limbs 
exist. 

The  antenuulea  and  antennop  are  both  uniramous  append- 
ages and  serve  for  creeping,  though  the  former  are  also  pro- 
vided with  olfactory  hairs.  The  mandible  consists  of  a  tooth- 
bearing  plate  and  a  strong  jointed  palp  which  iu  some  forms 
also  functions  as  a  creeping  limb,  and  behind  it  are  two  well- 
developed  maxillaj.  The  first  of  these  is  distinguished  by  the 
development  of  the  jaw  portion  and  the  reduction  of  the  palp, 
and  in  Cypris  and  Cythere  bears  a  large  i)late  M'ith  numerous 
marginal  setae  which  is  usually  termed  a  branchial  lobe.  The 
second  maxilla,  on  the  other  hand,  shows  considerable  modi- 
fication in  different  genera.  In  Cypridina  (Fig.  176,  J/.c')  it  is 
jawlike  and  bears  a  large  branchial  lobe  (wanting  on  the  first 
pair),  and  in  (Jypris  is  adapted  for  the  same  function,  but  bears 
in  addition  to  the  rudimentary  branchial  lobe  a  short  two- 
jointed  palp,  which  iu  Halocijpris  becomes  enlarged  to  form  a 
three-  or  four-jointed  limu",  while  finally  in  Cythere  the  append- 
age is  practically  a  walking  limb,  its  jaw  function  not  being 
developed.  The  first  thoracic  appendage  is  an  elongated 
many-jointed  limb  except  in  Cypridina  (Fig.  176,  t'),  where  it 
possesses  a  jaw  function,  and  the  second  is  also  limblike.  In 
Jlahcypris  this  latter  appendage  is,  however,  rudimentary,  and 
iu  Cypris  and  Cypridina  (Fig.  176,  T'')  it  is  dorsally  directed 
and  serves  for  cleansing  the  inner  surface  of  the  shell  from 
foreign  bodies,  iu  the  latter  genus  arising  some  distance  up 


•   '    il 

I; 


if 


392 


INVEliTEBRA  TE  MOIWUOLOG  T. 


upou  the  sides  of  the  body  aud  forniiiig  a  h)Jipj  cylindrical 
imjoiuted  appendage. 

liespiiatiou  is  usually  effected  by  the  j^eueral  surface  of 
the  body  aud  the  iuuer  walls  of  the  shell  duplicature,  though 
iu  certaiu  OypiidinidtC  a  double  row  of  respiratory  processes 
are  situated  upou  the  dorsal  surface  of  the  body  uear  the 
secoud  thoracic  appeudage.  The  so-called  branchial  lobes  on 
the  maxillio  probably  subserve  the  respiratory  function  only 
by  renewing  the  water  iu  contact  with  the  body  surface.     A 

H 


Fig   176. — Cypridina  mediteri'anea,  Female  (after  Claus). 


At^  =  auteiiiiule. 
At^  =  iuiteuua. 
h  =  heart. 
Map  =  mtuulibular  palp. 
Mx^,  Mjc^  —  lii'st  aud  secoud  maxilla. 


o  —  simple  eye. 
Oc  =  conipoimd  eye. 
Pr  =  fioiitul  organ. 
8m  =  shell-mviscle. 
fi,  T''^  =  lirst  and  second  thoracic 
appendages. 


single  median  eye  alone  is  present  in  Cypru  aud  Cythere,  but 
in  addition  a  pair  of  lateral  compound  eyes  occurs  in  Cypri- 
<li)i((.  The  frontal  sense-organ  is  a  single  strong  process,  in 
certain  forms  lying  slightly  above  aud  between  the  antennules. 
A  heart  is  present  in  Cypridina  and  Halocypris  as  a  saclike 
organ  with  two  lateral  ostia  and  is  not  })rolonged  into  arteries. 
In  ()fpris  aud  Cythere  it  is  entirely  wanting. 

The  Ostracoda  occur  both  in  fresh  water  aud  in  the  ocean. 
The  genus  Cypris  and  its  allies  are  for  the  most  part  aquatic, 
while  the  other  genera  mentioned  are  exclusively  nuirine. 


TYPE  CRUSTACEA. 


393 


3.  Order  Copepoda. 

The  members  of  the  order  Copepochi  present  great  variji- 
tious  ill  form,  tlue  to  the  fuct  that  there  are  a  number  of  para- 
sitic forms  beh)ii<4iuj^  to  it  some  of  whicii  sliow  so  much  tle- 
geueratiou  that  their  rehitiouships  to  the  non-parasitic  forms 
only  become  api)arent  b}'  a  study  of  their  deveh)pment. 
Typically,  however,  the  body  is  j^euerally  elongated  (Fig.  177) 
and  consists  of  ten  segments  in  addition  to  those  of  the  head, 
the  tive  anterior  ones  usually  bearing  ai)pendages  and  con- 
stituting the  thorax,  while  the  live  posterior  Lick  ap})endages 
and  form  the  abdomen,  the  terminal  segment  of  which  bears 
a  pair  of  caudal  furcje  provided  with  setje.  In  female  indi- 
viduals the  two  anterior  abdominal  segments  fuse  together  to 
form  a  genital  double  segment,  and  in  all  cases  the  head  seg- 
ments fuse  together,  while  the  anterior  thoracic  segment 
usually  fuses  with  this  consolidated  mass.  No  shell-duplica- 
ture  occurs.  Iii  the  parasitic  forms  there  is  a  tendency  for 
the  various  segments  to  become  indistinct  and  all  trace  of 
them  may  vanish,  the  abdomen  in  some  cases  becoming  also 
extremely  reduced  in  size.  Add  to  this  that  lobes  and  pro- 
cesses are  frequently  developed  upon  the  body  and  it  will  be 
understood  how  far  these  degenerate  forms  depart  from  the 
typical  arrangement. 

The  antennules  (Fig.  177,  aV)  in  all  free-swimming  Cope- 
poda form  long  many-jointed  swimming-organs  used  in  an 
oarlike  manner.  They  consist  of  a  certain  number  of  stout 
basal  joints,  terminated  by  a  single  hmg  multiarticulate 
tlagtdlum,  no  trace  of  a  biramous  condition  being  apj)arent.  In 
addition  to  their  locomotor  function  they  also,  as  in  other 
t'oiiiis,  serve  as  sense-organs,  olfactory  hairs  being  scatte/ed 
along  the  tlagellum,  and  in  male  individuals  they  are  s])ecially 
moditied  tt)  ft)rni  clas})ing  organs  for  usi^  in  co})ulation.  The 
ant«3nn!e  ((//')  are  much  smaller  and  are  fitHpiently  biramous, 
and  the  mandible  (//<«)  has  usually  a  ])alp,  while  the  first 
maxilhe  (mx"'),  bearing  strong  masticatory  bristles  on  their 
basid  joints,  alst)  show  more  or  less  indication  of  a  biramous 
condition.  The  second  maxilhe  (wu*"),  sometinu^s  termed  the 
maxillipeds,    show    a    peculiar    arrangement    in    that    each 


ii 


394 


INVERTEBRATE  MORPHOLOGY. 


appendage   separates  into  two  portions  inserted   separately 

into  the  body-wall.  The  an- 
terior one  is  a  comparatively 
small  plate  provided  with 
numerous  masticatory  setse  on 
its  inner  edge,  while  the 
posterior  is  an  elongated 
limblike  structure.  It  is  this 
combination  of  a  maxillary 
and  limblike  portion  that  has 
gained  for  this  appendage  the 
term  maxilliped,  though  it 
must  be  recognized  that  it  is 
a  true  cephalic  appendage  and 
not  comparable  to  the  maxil- 
lipeds  of  the  higher  forms. 
The  five  thoracic  appendages 
(<' — t")  are  typically  biramoua 
and  serve  for  swimming. 

This  description  refers  to 
the  free-swimming  forms  ;  in 
parasitic  species  much  modi- 
fication of  the  appendages 
ensues.  The  antennules  lose 
their  long  oarlike  character 
and  may  even  be  degenerated 
to  strong  hooks  which  serve 
to  fasten  the  animal  to  its 
host,  a  degeneration  which 
the  autennaj  may  also  under- 
go.    The  mouth-parts  become 

Fig.  m.—Calanns  hypcrboreua  (after  adapted    to   a   piercing  func- 

QlKSBRECHT). 

an  =  nnus. 
aV  =  autuiiDule. 
at*  =  antenna. 
mn  =  mandible. 
WW!',  mx^  =  first  and  second  maxillse. 
<'-<*  =  thoracic  appendages. 


thus    produced.     The    first 


tion,  and  the  mandibles  are 
represented  by  sharp  stylet- 
like structures,  sometimes  en- 
closed in  a  tube  formed  by  the 
union  of  the  upper  and  lower 
lips,  a  sucking-organ  being 
maxilloB   undergo    considerable 


ji>j^ai^4t8,ii3Btte 


TYPE  CHU8TACEA. 


395 


reduction,  while  the  second  pair  is  frequently  adapted  to  form 
organs  for  adhering  to  the  host,  and  finally  the  thoracic 
appendages  may  undergo  various  stages  of  degeneration,  in 
some  forms  entirely  disappearing. 

Branchial  organs  are  entirely  wanting  throughout  the 
order,  respiration  taking  place  over  the  entire  body  surface. 
A  heart  is  present  in  a  few  forms  (Calanidae)  consisting  of  a 
saclike  organ  with  but  a  single  pair  of  ostia,  but  in  the 
majority  of  cases  it  is  wanting.  A  single  median  eye  is  gen- 
erally present,  and  in  a  few  forms,  Pontella,  Corycceus,  and 
Argulus,  lateral  eyes  are  also  present,  though  absent  as  a 
rule  throughout  the  group.  Each  lateral  eye  in  Corycoius 
consists  of  a  single  ommatidium,  but  in  Argulus  is  compound 
and  similar  to  the  lateral  eyes  of  the  Brauchiopoda. 

The  Copepoda  are  throughout  bisexual  even  in  the  cases 
of  the  parasitic  forms.  The  vasa  deferentia  are  provided  with 
an  enlargement  in  which  the  spermatozoa  are  included  within 
a  capsule,  forming  a  spermatophore  which  during  copulation 
is  deposited  in  the  neighborhood  of  the  female  genital  open- 
ing. The  spermatozoa  being  discharged  from  the  spermato- 
phore-capsule,  by  a  special  discharging  apparatus  with  which 
it  is  provided,  make  their  way  into  a  receptaculum  seminis 
which  communicates  with  each  oviduct,  the  ova  being  fertilized 
during  their  passage  to  the  exterior.  These  are  usually 
carried  in  one  or  two  masses  attached  to  the  first  abdominal 
segment  of  the  female,  though  in  some  forms,  such  as  Notodel- 
phys,  they  undergo  their  development  in  a  brood-chamber 
formed  by  the  duplication  of  the  integument  of  the  dorsal 
surfaces  of  the  fourth  and  fifth  thoracic  segments.  A  peculiar 
dimorphism  of  the  sexes  occurs  in  some  of  the  most  highly 
modified  parasites,  such  as  ChondracantJms,  Achtheres,  and 
others,  the  male  being  very  much  smaller  than  the  female 
and  showing  much  less  degradation,  frequently  presenting 
well-developed  eyes  and  more  or  less  perfectly-developed 
appendages,  so  that  it  is  able  to  lead  for  a  time  a  free  exist- 
ence. It  is  to  be  regarded  as  a  larval  stage  sexually  mature, 
since  it  resembles  closely  the  female  when  in  the  stage  immedi- 
ately before  fixation  to  its  host,  the  greater  part  of  the  degen- 
eration taking  place  after  that  has  been  accomplished. 

Two  suborders  are  recognizable. 


I 


pk.. 


« .■  ■ 


396 


INVERTEBRATE  MORPHOLOGY. 


1.  Suborder  Eiicopepoda. 

This  suborder  includes  the  majority  of  the  Copepoda,  and 
its  members  are  characterized  by  havinj^  only  the  first  thoracic 
segment  fused  with  the  head  and  by  possessing  usually  a 
well-developed  abdomen.  Many  are  free-swimming,  some  in- 
habiting fresh  water,  as  Cyclops  and  Canthocamptus,  while 
others  are  more  especially  marine,  such  as  llarpacticuSy  Calanus 


,mx« 


Fig.   l'7S.—A,  Philichthi/s  xiphm  seen  puom  the  Dorsal  Surface  (after 
Clai-s);  B,  Achtheres  percarum  (from  Bronn). 

(Fig.  177),  and  Cetochihts,  the  latter  sometimes  occurring  in 
enormous  schools,  and  forming  an  important  food-su})ply  for 
fish  and  the  baleen  whales.  Some,  on  the  other  hand,  lead  a 
commensalistic  life,  occurring  in  the  branchial  chamber  of 
Tuuicates,  e.g.  Notodelphys,  while  a  large  number  of  forms  are 
parasitic.  The  degree  of  parasitism  varies  greatly  in  different 
forms  ;  thus  many  are  capable  of  free  existence,  becoming 
parasitic  only  occasionally,  such  as  Cort/cceiis  and  the  brilliantly- 
colored  Sapphirina,  while  others,  such  as  Ergasilus,  parasitic 
on  the  gills  of  fishes,  and  Caligus  and  Pandariis,  though  epsen- 


TYPE  CRUSTACEA. 


397 


1 


tiiilly  parasitic,  still  retain  more  or  less  perfectly  the  segnieu- 
tatiou  aud  geueral  appearance  of  free-swirainiug  forms,  the 
modifications  which  they  have  undergone  affecting  principally 
the  antennsB,  which  are  modified  for  purposes  of  adhesion  to 
the  host,  the  mandibles,  which  are  piercing  organs,  and  in  some 
cases  the  maxilhe,  which  may,  like  the  anteuuiP,  become  hook- 
like. Frequently,  however,  the  body  assumes  aberrant  forms, 
as  in  PhilicMhys  (Fig.  178,  A),  and  the  segmentation  may  en- 
tirely disappear,  as  in  Penella,  Lerncea,  Chondracanth  us,  Achtherea 
(Fig.  178,  /?),  and  Anchorella,  these  last  two  forms  presenting 
a  peculiar  modification  of  the  second  maxillas  in  the  females, 
the  two  appendages  fusing  at  their  tips  to  form  a  chitinous 
adhesive  disk  which  serves  as  au  organ  of  adhesion.  In  the 
majority  of  these  forms,  as 
already  noted,  the  thoracic 
appendages  may  become  more 
or  less  rudimentary;  indeed 
even  in  the  less  modified 
forms,  such  as  ErgasUm,  the 
appendages  of  the  fifth  thora- 
cjic  segment  may  be  wanting. 


IllX 


2.  Suborder  Brancldnra. 

In  the  Branchiura  the 
cephalic  and  thoracic  seg- 
ments are  fused  together  to 
form  a  shield-shaped  cephalo- 
thorax,  while  the  abdomen  is 
small  and  divided  into  two 
platelike  halves  which  have 
a  rich   blood-supply,    appar-  ^lo.    m.-Artjulm    foliaceus 

.  .       ,  Claus). 

ently  serving  respiratory  pur- 
poses, and  in  the  males 
contain  the  testes. 

The  basal  joint  of  the  an- 
tennules  (Fig.  179,  at')  is  devel- 
oped into  a  strong  hooked  process,  aud  the  mandibles  and 
first  maxillsB,  which   are   stylet-like,  are   enclosed  iu  a  tube 


(after 


at'  =  antennules. 

I  =  digestive  glaud 
mx  =  second  muxilla. 
oc=  eye. 
t  =  testis. 


f 


*!      !  ! 

il 


398 


INTERTEBRATE  MORPIIOLOOT. 


formed  by  tlie  fusion  of  the  upper  and  lower  lips.  The 
second  maxillse  (mx)  develop  at  their  bases  large  suckers, 
while  the  first  thoracic  appendages,  here  termed  maxilli- 
peds,  are  limblike  and  have  also  hooked  processes  upon  the 
basal  joints.  These  are  succeeded  by  four  pairs  of  biramous 
swimming  appendages. 

A  well-developed  heart  is  present,  giving  rise  to  arteries 
extending  throughout  the  length  of  the  body.  A  pair  of 
lateral  compound  eyes  (oc)  are  also  present,  and  a  further 
difference  from  the  majority  of  the  Eucopepoda  lies  in  the 
fact  that  the  eggs  are  not  carried  by  the  female,  but  are  de- 
posited on  fo'  eign  bodies.  > 

All  the  forms  are  parasitic,  in  some  cagfes,  as  Arguhis^ 
upon  fresh-water  fishes,  but  they  also  possess  the  power  of 
swimming  actively. 

4.  Order  Cirrhipedia. 

The  Cirrhipedia  or  Barnacles  are  without  exception  ma- 
rine forms,  and  in  the  adult  condition  either  adhere  to  foreign 

P 


W 


Fig.  180.— Cypkis  Larva  of  Lepas  (after  ChAva). 
Ab  =  iibdomeu.  Oc  =  eye. 

At^  =  autennules.  Ov  =  ovary. 

Cg  =  duct  of  cement-gland.  p  =  penis. 

0  =  opening  of  oviduct.  2'^  =  third  thoracic  foot 

bodies,  leading  a  perfectly  sessile  life,  or  olse  bore  in  the  shells 
of  certain  Mollusca,  or  finally  are  parasitic.  It  will  be  con- 
venient to  describe  first  of  all  the  organization  of  the  sessile 
and  boring  forms,  later  considering  briefly  the  parasitic  forms 
which  show  many  peculiarities  due  to  degeneration. 

During  the  course  of  development  the  Cirrhipedia  all  pass 
through  a  larval  stage  similar  in  general  appearance  to  an 


TYPE  CRUSTACEA. 


399 


Ostracotle  and  hence  termed  the  Cypris-stage  (Fig.  180). 
The  body  is  enclosed  in  extensive  fokls  of  the  bod^'-wall 
termed  the  mantle,  and  the  antennules  (a<')  are  characterized 
by  being  directed  forwards  and  terminating  in  an  adhesive 
disk  upon  which  open  the  dacts  of  cement-glands.  Adher- 
ing to  a  foreign  body  by  these  lisks,  the  adhesion  being 
made  permanent  by  the  secretion  of  the  glands,  a  rotation  of 
the  body  upon  the  antenum  through  90°  takes  place,  so  that 
the  animal  comes  to  lie  upon  its  back,  the  ventral  surface 
looking  away  from  the  point  of  fixation.  The  antennules 
persist  as  rudimentary  structures,  and  the  adult  animal  really 
seems  to  be  fixed  by  the  dorsal  surface  of  the  head,  which 
may  elongate  to  form  a  stalk  bearing  the  body  proper  at  its 
extremity  {Lepas,  Fig.  181). 

The  body  shows  no  indication  of  segmentation,  but  a  head 
region  may  be  distinguished  from  the  thorax  and  this  from  a 
short  abdomen  by  means  of  the  appendages.  The  character 
of  the  antennules  has  already  been  mentioned ;  the  antennae 
are  wanting  in  adults,  and  the  mandibles  and  first  maxillso 
are  simple  toothed  plates  destitute  of  palps,  while  the  second 
maxillsB  are  small  and  fused  together  to  form  a  kind  of  lower 
lip.  The  thoracic  appendages  (Fig.  181,  B)  are  biramous,  the 
basal  portion  supporting  two  long  multiarticulate  and  usually 
setose  filaments.  In  typical  cases  six  p.iirs  of  these  append- 
ages occur,  but  they  may  be  reduced  to  four  (Alcippe)  or 
three  pairs  {Cryptophialus).  In  the  living  animal  flexions  of 
these  appendages  towards  the  ventral  surface  of  the  body 
take  place  almost  rhythmically,  currents  of  water  being  thus 
impelled  towards  the  mouth  together  with  any  food-particles 
they  may  contain.  The  abdomen  does  not  bear  appendages, 
but  from  it  arises  a  long  slender  cirrus  (Fig.  181,  B,  cir)  which 
contains  the  terminal  portions  of  the  vasa  deferentia. 

The  mantle-folds  which  occur  in  the  Cypris-larva  persist 
in  the  adult,  and  calcification  of  their  walls  takes  place,  giving 
rise  to  a  calcareous  shell,  composed  of  several  pieces,  which 
encloses  the  animal.  In  the  genus  Lepas,  the  goose-barnacle, 
this  shell  consists  of  five  pieces.  On  the  dorsal  side  there  is 
a  single  unpaired  piece  which  receives  the  name  of  the  carina 
(Fig.  181,  A,  car) ;  at  the  sides  and  resting  below  on  the  upper- 


■if: 


400 


IN  VEli TEDRA TE  MOliPHOLOO  T. 


§ 


most  part  of  the  stalk  are  the  two  scuta  (sc),  while  above 
these  are  the  terija  (te),  also  paired,  the  opeuiug  iuto  the  in- 
terior l}'iuf<  between  the  terga  and  the  scuta  of  opposite  sides. 
Ill  ScaJpcUuin  between  the  two  scuta  a  sixth,  unpaired,  piece, 
the  rosfrmn,  is  inserted,  and  in  the  same  genus  between  the 
scuta,  terga,  and  carina  and  the  summit  of  the  stalk  small 
accessory  pieces  occur;  and  if  one  imagines  a  disappearance 


Fig.  181  —  Lepas fascicularis.    A,  exterior;  B,  structure. 


ag  =  auleunary  gland. 
Car  =  carina. 
Cir  =  cirrus 
M  =  sliell-muscle. 
Od  —  oviduct. 


Ov  =  ovary. 
pe  =  peduncle. 
Sc  —  scutum. 

i  =  testis. 
Te  =  terguin. 
Vd  =  vas  deferens. 


of  the  stalk  of  such  a  form,  an  enlargement  of  these  accessory 
pieces,  usually  six  in  number,  and  their  articulation  to  form 
a  wall-like  circle  around  the  body  of  the  animal,  the  scuta 
and  terga  closing  it  in  and  formin^-'  as  it  were  a  roof,  an  idea 
of  the  arrangement  of  the  shell  of  Balanus^  the  acorn-barnacle, 
will  be  obtained. 

No  special  respiratory  organs  exist,  the  entire  surface  of 
the  body  probably  performing  this  function,  nor  does  a  heart 
seem  to  occur   in  any  member  of  the  group.     The  nervous 


TYPE  CRUSTACEA. 


401 


system  consists  iu  Lepus  of  a  syiicerebrum  ami  five  or  six 
ventral  ganglia, — of  which  the  last  is  probabl}'  composed  of  at 
least  two  fused  ganglia,  and  a  certain  amount  of  fusion  has 
also  probably  occurred  in  the  first.  In  Jialanius  the  fusion 
has  reached  its  greatest  extent,  the  entire  ventral  chain  of 
ganglia  having  fused  to  a  single  mass.  The  median  unpaired 
eye  is  usually  represented,  and  in  some  forms  rudimentary 
lateral  eyes  are  presen*^^,  showing,  however,  a  marked  degen- 
eration from  the  large  compound  eyes  which  occur  in  the 
Cyprislike  larva. 

As  a  rule  the  Cirrhipedia  are  hermaphrodite  in  accord- 
ance with  their  sessile  or  parasitic  life.  The  testes  (Fig. 
181,  /y,  t)  lie  one  on  each  side  of  the  digestive  tract,  and  the 
vasa  deferentia  {vd)  after  dilating  into  seminal  vesicles  pass 
to  the  long  cirrus  {cir)  borne  by  the  abdomen,  at  the  tip  of 
which  thej'  open  by  a  short  common  duct.  The  ovaries  lie 
in  Lepas  (Fig.  181,  B,  oo)  in  the  stalk,  and  iu  stalkless  forms, 
such  as  BaUinus,  in  the  Ijasal  fold  Avhich  corresponds  to  the 
stalk,  and  the  oviducts  (od)  passing  upwards  and  then  back- 
wards open  on  the  basal  joints  of  the  anterior  thoracic  ap- 
peudages.  Although  hermaphroditism  is  the  rule  throughout 
the  order,  yet  in  some  cases  small  males  have  been  found 
which  have  received  the  name  of  "  complemental "  males. 
These  occur  in  the  genus  Ibla  and  iu  some  spe  ies  of  Scalpel- 
lum  and  live  like  parasites  in  folds  of  the  mantle  of  the  her- 
maphrodite forms.  In  form  they  do  not  advance  greatly 
beyond  the  Cypris  stage,  and  possess  iu  addition  to  the  anten- 
uules  only  four  pair  of  small  thoracic  limbs,  the  mandibles  and 
maxillse  as  well  as  the  mouth  being  entirely  wanting,  while 
the  digestive  tract  is  rudimentary.  In  other  species  of  Scal- 
pellum,  and  in  the  genera  Alcippe.  and  Cryptophialus,  these 
pigmy  males  are  also  present,  but  the  forms  in  which  they 
live  are  no  longer  hermaphrodites  but  females,  so  that  bi- 
sexuality  with  sexual  dimorphism  occurs  in  these  forms. 

It  might  be  supposed  from  the  general  occurrence  of  bisexuality  among 
the  Crustacea  that  these  last  cases  represented  the  first  stage  iu  the  dis- 
appearance of  the  males,  leading  Anally  to  hermaphroditism.  Since,  how- 
over,  Alcippe  and  Cryptophialus  are  the  most  degenerate  of  the  Cirrhi- 
peds  so  far  discussed,  it  would  seem  that  this  is  not  the  case,  but  rather 


m 

lit 


n 


403 


INVERTEBRATE  MORPHOLOGY. 


I   W 


I 


that,  on  the  assumption  of  a  sossile  life  hermaphroditism  became  character- 
istic of  the  order,  tlio  biseximlism  of  tiieso  boring  forms  being  secondarily 
acquired.  The  fact  that  the  pigmy  males  present  larval  characters  sug- 
gests the  idea  tluit  their  occurrence  may  be  an  extreme  ease  of  proterandry. 
If  in  the  hermaphrodite;  forms  it  is  a  rule  tliat  the  spermatozoa  mature 
earlier  than  the  ova,  thus  preventing  self-fertilization,  it  is  conceivable  that 
this  early  maturation  of  the  testes  might  be  carried  back  almost  to  the 
Cypris  stage  and  pigmy  males  be  thus  developed. 

Not  unfrequently  baruacles  choose  the  bodies  of  other 
auimals  upon  which  to  fasten,  as  for  instance  upon  the  cara- 
pace of  Zirnuliis,  or  on  the  skin  of  whales,  and  the  genus 
Anelasma  fastens  itself  upon  the  surface  of  the  body  of  a 
Shark,  its  stalk  penetrating  into  the  tissues  and  developing 
rootlike  processes  and  so  enabling  it  to  lead  a  parasitic  life. 
As  a  result  of  this  the  calcareous  plates  cease  to  develop, 
the  mantle  having  merely  a  leathery  consistency  and  the 
mandibles  and  maxillte  remain  rudimentary.  This  degenera- 
tion is  carried  still  further  in  Proteolepas  (Fig.  182),  which 

lives  as  a  parasite  in  the 
mantle-cavity  of  other  Cir- 
rhipeds  and  has  a  maggotlike 
appearance,  the  body  being 
distinctly  divided  into  eleven 
segments  and  lacking  all 
traces  of  a  mantle.  The 
mouth-parts  are  modified  so 
as  to  be  suctorial,  and  the 
thoracic  feet  are  entirely 
wanting,  while  the  digestive 
tract  becomes  rudimentary. 
Finally,  a  group  of  forms, 
known  as  the  EMzocephcda,  fasten  themselves  to  the  abdomen 
of  crabs  and  become  transformed  into  cylindrical  or  saclike 
structures  entirely  destitute  of  digestive  tract  and  appendages, 
rootlike  processes  arising  from  the  anterior  end  of  the  body 
and  traversing  the  body  of  the  host,  by  whose  juices  the 
parasite  is  nourished.  The  genus  Sacculina  consists  of  an  an- 
terior short  cylindrical  portion  from  the  extremity  of  which 
the  rootlike  processes  arise  and  which  perforates  the  integu- 
ment of  the  host.     From  the  base  of  this  a  circular  fold  arises 


vs 


Fig.  183. — Proteolepas  (from  Bronn). 

m  =  muscle. 

ov  =  ovary. 

vs  =  vesicula  seminalis. 


TYPE  CRUST  ACE  A. 


403 


which  eucloses  between  its  walls  aucl  the  wall  of  the  body  a 
cavity  which  serves  as  a  brood-pouch  and  communicates  with 
the  exterior  by  a  terminal  opening  capable  of  being  closed  by 
a  sphincter.  The  body  proper  contains  only  the  nervous 
system,  reduced  to  a  single  ganglion,  and  the  ovaries  and  the 
paired  testes,  as  well  as  a  pair  of  cement-glands  connected 
with  the  female  genital  openings. 

The  development  of  Saceidina  presents  some  extraordinary  featnros. 
It  resembles  in  its  early  stages  the  development  of  the  other  Cirrhlpeds 
and  reaches  a  typical  Cypris  stage  during  which  it  fastens  itself  by  the 
antennules  to  the  body  of  a  crab.  The  tissues  of  the  larva  then  retr.act 
themselves  from  the  cuticle,  and  a  remarkable  degeneration  of  tlie  body 
together  with  an  amputation  of  the  entire  thoracic  and  abdominal  regions 
then  ensues,  leaving  an  oval  mass  of  tissue,  richly  pigmented,  attached  to 
the  body  of  the  crab  by  the  empty  cuticle  of  the  antennules.  At  the 
anterior  end  of  this  mass  a  hollow  dartlilce  process  arises  which  is 
pushed  forward  through  the  hollow  cuticle  of  the  antennules  and  pierces 
tJje  body-wall  of  the  host,  the  panisite  apparently  flowing  then  through  the 
dart  and  so  becoming  an  endoparasite.  Within  the  body  of  the  crab  the 
development  of  the  Saccnlfna  takes  place  from  the  apparently  undiffer- 
entiated mass  of  tissue  by  which  it  is  represented,  and  growing  rapidly 
produces  an  absorption  of  tlie  ventral  integument  of  the  host,  which  allows 
the  saclike  body  to  protrude  to  the  exterior.  It  is  to  be  noted  that  para- 
sitic Cirrhipeds  (Laura)  have  been  found  in  the  stem  of  a  Gorgonian  and 
also  in  the  body-cavity  of  Echinoderms  (Dendrogaster).  These  forms 
show  many  peculiarities  of  structure  and  have  been  grouped  together  in 
the  suborder  Ascothoracida. 


■)  il 


m 


m 


1' 
i 


II.  Class  Malacostraca. 

The  Malacostraca  are  distinguished  from  the  Entomo- 
straca  by  the  definiteness  throughout  the  entire  class  of  the 
number  of  metameres  entering  into  the  composition  of  the 
body.  The  head  consists  of  five  segments  which  are  invari- 
ably fused,  and  the  thorax  is  composed  of  eight,  of  which  the 
anterior  one,  or  indeed  all,  may  unite  with  the  head  to  form 
a  perfect  or  imperfect  cephalothorax.  The  abdomen  is  the 
only  region  in  which  variation  of  number  takes  place,  and 
this  variation  is  confined  to  a  single  group  of  forms  (Lepto- 
straca).  In  these  the  abdomen  is  composed  of  eight  segments, 
while  in  all  other  forms  it  possesses  only  seven,  counting  in 
both  these  cases  the  terminal  segment  which  bears  the  anus 


404 


IN VEHTEtiliA TE  MOliPlIOLOO  Y. 


aud  is  kuowu  as  the  telson.  All  these  segiueuts  with  the 
exceptiou  of  the  telsoij,  uud  iu  the  Leptostracu  of  the  se^- 
meut  immediately  iu  frout  of  it,  bear  appendages.  Folds  of 
the  integumeut  forming  a  cephalothoracic  carapace  are  fre- 
quently present,  but  it  is  rare  that  a  bivalved  shell  occurs. 

The  stomach  is  always  provided  with  chitinous  teeth  aud 
forms  an  elKtieut  masticatory  organ,  aud  lateral  eyes  are 
present  except  iu  some  Cumacea  aud  iu  some  forms  belong- 
ing to  other  groups  which  inhabit  caves  or  the  depths  of  the 
ocean,  under  which  conditions  the  eyes  become  rudimentary. 
The  openings  of  the  female  reproductive  organs  are  always 
situated  on  the  basal  joiuts  of  the  appendages  of  the  sixth 
thoracic  segmeut,  and  the  male  openings  on  the  appendages 
of  the  eighth  segment.  The  antennary  gland  is  usually  well 
developed,  while  the  shell-gland  is  either  rudimentary  or 
wanting  in  the  adult. 

Although  numerous  rather  small  forms  belong  to  this 
class,  yet  on  the  whole  they  much  surpass  iu  size  the  Euto- 
mostraca,  some  forms  even  reaching  a  length  of  over  50  cm. 
A  few  forms,  such  as  Euphausia  and  Pen(pm,  leave  the  egg  as  a 
Naui)lius,  but  iu  the  majority  this  stage  is  passed  before 
hatching,  the  embryo  first  leading  a  free  existence  at  a  later 
stage  iu  the  larval  form  known  as  the  Ziiea,  though  in  some 
cases  hatching  may  be  retarded  until  later  stages,  in  fact 
sometimes  until  the  adult  form  is  acquired. 


bean 

app| 

struj 

the 

biral 

app^ 

plat^ 

four 

swinl 

moul 


I.  Subclass  Leptostbaca. 

The  Leptostraca  are  exceedingly  interesting  forms,  present- 
ing similarities  to  the  Entomostraca  on  the  oue  hand  and  to 
the  Malacostraca  on  the  other,  thus  connecting  the  two 
classes.  They  are  exclu  Ai%'t  ly  marine  in  habitat  and  possess 
a  thin  bivalved  shell- dupl'oature  which  is  provided  with  an 
adductor  muscle  and  is  prolonged  in  front  into  an  unpaired 
plate  which  covers  the  dorsal  surface  of  the  head. 

The  antennules  (Fig.  183,  aV)  consist  of  a  three-jointed 
basal  portion  bearing  in  addition  to  the  multiarticulate  flagel- 
lura  a  scalelike  exopodite,  a  structure  wanting  iu  the  antennse 
{af),  which  otherwise  have  a  similar  form.     The  mandibles 


TYPE  VltUSTACKA. 


406 


bear  a  palp,  as  do  also  tlie  first  niaxillio,  it  beiuj;  in  these  latter 
appeudaj^es  prolonged  into  a  long  slcnider  liniblike  {nix) 
structure  which  is  directed  dorsally  and  serves  for  cleansing 
the  inner  surface  of  the  shell.  The  second  niaxilliu  are 
biramous  foliate  structures,  as  are  also  the  eight  thoracic 
appendages  (/),  each  of  which  bears  upon  its  basal  joints  a 
platelike  epipodite  which  is  respiratory  in  function.  The 
four  anterior  abdominal  appendages  {nb*)  are  strong  biramous 
swimming-legs,  while  the  two  posterior  are  small  and  unira- 
mous.      Behind  the  last  appendage-bearing  segment  are  two 


Kin  nux. 


Fio.  183. — Nebalia  Oeoffroyi,  Malk  (after  Claus). 
rtft*  =  aUilominal  appendage.  h  =  heart. 

adr  =  an  ten  nary  gland  mx  —  process  of  first  maxilla. 

aO  =  tintennule.  am  =  shell-muscle. 

«<*  =  antenna.  t  =  thoracic  appendage. 

te  =  testis. 

others  without  appendages,  the  terminal  one  being  the  telson, 
the  Leptostraca  possessing  one  more  metamere  than  the  rest 
of  the  Malacostraca. 

The  heart  is  au  elongated  organ  extending  from  the 
maxillary  region  as  far  back  as  the  fourth  abdominal  seg- 
ment ;  it  possesses  several  ostia,  and  is  prolonged  anteriorly 
and  posteriorly  into  aortfe.  The  antennary  gland  is  present 
and  a  rudimentary  shell-gland  also  persists.  The  lateral  eyes 
are  borne  upon  short  stalks. 

The  group  contains  but  few  species,  the  majority  belong- 
ing to  the  genus  Nehcdia  (Fig.  183). 


406 


INVEliTEBRATE  M0RPH0L0O7. 


I    • 


U 


II.  Subclass  Thobacostbaca. 

The  Thoracostraca  are  characterized  by  the  occurrence 
throughout  the  group  of  a  well-developed  duplicature  of  the 
body-wall,  arising  from  the  posterior  hesid-segmeuts  and 
covering  in  a  greater  or  less  number  of  the  thoracic  segments, 
constituting  what  is  termed  a  carapace.  On  the  dorsal  sur- 
face it  fuses  with  the  body-wall,  but,  at  the  sides  encloses  a 
respiratory  chamber  in  which  the  branchiae,  when  present,  lie. 
According  as  the  carapace  extends  over  all  or  only  over  the 
anterior  thoracic  segments  a  more  or  less  perfect  cephalo- 
thorax  is  formed,  a  fusion  of  the  covered  thoracic  segments 
with  each  other  and  with  the  head-segments  occurring,  the 
abdominal  segments  remaining  in  all  cases  distinct. 

Branchiae,  consisting  of  bunches  of  hollow  thin-walled 
processes  whose  cavities  communicate  with  the  lacunar  spaces 
of  the  body,  are  borne  by  certain  of  the  appendages  except  in 
the  MysidesB.  Thfe  lateral  eyes  except  in  the  Cumacea  are 
stalked  and  the  antennary  gland  is  usually  well  developed. 

1.  Order  Schizopoda. 

The  carapace  in  the  Schizopoda  covers  in  the  entire 
thorax,  but  a  certain  number  of  the  posterior  thoracic  seg- 
ments remain  ununited  with  it.  The  antenrmles  are  bira- 
mous,  as  are  also  the  antennfie  (Fig.  184),  the  exopodite  in  the 
latter  case  being  represented  by  a  scalelike  structure.  The 
thoracic  appendages  are  all  similar  and  are  biramous,  the 
endopodites  being  limblike  structures  tipped  by  claws, 
while  the  exopodites  are  multiarticulate  tiagella.  In  the 
genus  Euphausia  the  two  last  pairs  are  quite  rudimentary, 
their  branchiae  remaining,  however,  well  developed.  The  two 
anterior  pairs  in  the  genus  Mysis  have  their  basal  joints  en- 
larged to  form  jaws  and  consequently  are  distinguished  as 
maxillipeds,  but  in  Euphausia  this  distinction  does  not  occur. 
The  abdominal  appendages  in  the  females  are  generally  small 
with  the  exception  of  the  sixth  pair,  and  in  the  genus  Mysis 
are  quite  rudimentary.  In  the  males  of  all  genera  they  are, 
however,  well-developed  biramous  swimming-feet,  and  the 
sixth  pair  in  both  sexes  forms  with  the  telson  a  tail-fin. 


I 


TYPE  CRUSTACEA. 


407 


BrauchisB  are  present  in  Mysis  only  in  the  form  of  small 
epipodial  elevations  of  the  thoracic  appendages,  and  in  Siriella 
as  coiled  tubular  structures  on  the  protopodites  of  the  abdom- 
inal appendages  of  the  males.  In  Euphausia,  however,  they 
form  large  ramided  bunches  attached  to  the  protopodites  of 
the  thoracic  limbs  and  are  present  even  on  the  rudiments  of 
the  seventh  and  eighth  pairs ;  they  are  not,  however,  enclosed 


Fig.  184.  —Mysts  relicta  (after  Sars). 
bp  =  brood-poucb.  ot  =  otocyst. 

within  a  chamber  formed  by  the  lateral  portions  of  the  cara- 
pace, but  project  freely  to  the  exterior. 

Otocysts  occur  in  the  inner  lamellae  of  the  sixth  abdominal 
appendages  (Fig.  184,  ot),  and  in  Euphausia  a  number  of  eye- 
like phosphorescent  organs  occur  on  the  liisal  joints  of  the 
second  and  seventh  thoracic  appendages  as  well  as  upon  the 
ventral  surface  of  the  four  anterior  abdominal  segments. 
They  are  spherical  in  shape  and  each  consists  of  a  cup  of 
cells  containing  red  pigment  covered  in  by  a  lens. 

The  Schizopoda  are  essentially  marine,  though  some 
species  of  the  genus  Mysis  (Fig.  184)  occur  in  fresh  and 
brackish  water. 


4ii  !i 


iilF  1 


2.  Order  Gumacea. 

In  this  order  the  carapace  covers  only  the  anterior  three 
or  four  thoracic  segments,  five  or  four  of  them  remaining  dis 
tinct.  The  antennules  are  short  and  in  the  male  biramous, 
while  the  antennie,  though  in  the  female  almost  rudimentary, 
may  be  as  long  as  the  entire  bod'^'.  The  two  anterior  thoracic 
appendages  form  maxillipeds,  their  basal  joints  serving  for 
mastication  while  the  succouvUng  six  pairs  are  limblike,  all 
but  the  last  or  three  list  poHseritiiug  small  exopodites.     The 


408 


IN VEliTEDliATE  MORPUOLOU  Y. 


!l  1 


sixth  abdominal  segment 
bears  a  pair  of  biramous  ap- 
pendages with  a  long  single- 
jointed  protopodite,  the  re- 
maining segments  being  in 
the  female  destitute  of  ap- 
pendages, but  in  the  male  the 
anterior  2  {Diastylis),  3,  or  5 
{Campylaspis)  segments  may 
bear  biramous  swimming-feet. 

The  lateral  eyes  are  never 
stalked  and  may  be  closely 
approximated  or  even  fused 
on  the  dorsal  surface  of  the 
cephalothorax.  They  are 
generally  composed  of  but 
few  ommatidia  and  in  some 
species  are  entirely  wa^tjug. 

The  Cumacea  ar«  c-\oia- 
sively  marine  and  arc  more 
especially  characteristic  of 
the  colder  seas. 

3.  Order  Stomatopoda. 

As  in  the  Cumacea  the 
carapace  covers  only  some  of 
the  anterior  thoracic  seg- 
ments, the  last  three  or  four 
remaining  distinct,  but  the 
abdomen,  instead  of  being 
slender,  is  even  stouter  than 
the  thorax  and  ends  in  a  tei- 
minal    tail-fin.     The   anterior 

«      .«-     ^.      ....    «  portion  of  the  head,  bearing 

Pig.  185.— 2)mj»/yfo«  stygia,  Malb  (after  f  i    ,i       .  • 

SARsfromLANQ).  the  eyes   and   the  two   pairs 

rt,  =  antemmle.  eM=eiido|MKliie.       of  anteunPB,  is  separated  from 

«s  =  antenna.  <;j!  =  exop()dite.  ,  ,  ,  ,,  - 

a6= abdominal  ap      ;>  =  abdominal  ap-  a^d  movable  upon  the  rest  of 

pendages.  pendages.      the  cephalo-thorax,  and   onlv 

c<A= carapace.         /K-Ki// =  thoracic  ^,  ^  i     •         ,, 

segments,      the    more    anterior    thoracic 


.■^^iJKlV*"l^ltt 


TYPE  CRUSTACEA. 


409 


segments  are  fused  with   the  carapace,  though  it  covers  iu 
several  others. 

The  aiiteiiuules  consist  of  au  elougated  three-jointed  basal 
portion  bearing  three  many-jointed  Hagella,  while  the  anten- 
nas are  generally  shorter,  the  exopodite  being  represented  by 
a  large  scale.  The  maxillse  are  comparatively  small,  and  the 
appendages  of  the  five  anterior  thoracic  appendages  are 
crowded  forwards  and  are  termed  maxillipeds,  being  limb- 
like structures  destitute  of  exopodites,  but  possessing  well- 
developed  epipodites,  and  with  the  terminal  joint  capable  of 
fiexion  upon  the  next  succeeding  one.  The  second  maxilli- 
])ed  is  especially  long  and  large,  and  with  its  strong  terminal 
and  penultimate  joints  forms  a  very  efficient  Aveapon  for  secur- 
ing prey.     The  three  posterior  appendages  of  the  thorax  are 


Fig.  ISO. — Squilla  mantis  (from  LKrms). 
a'  =  antenuulcs.  pm  =  luaxillipcds. 

a*  =  tiuteniiie. 


oc 


p  =  tUoracic  limbs, 
coinpouud  eyes.  pa  =  abdomiujil  limbs. 

slender  biramous  structures,  the  somewhat  stronger  abdomi- 
nal appendages  being  also  biramous  and  somewhat  lamellar 
swimming-feet.  The  last  pair  are  especially  enlarged  and  di- 
rected backwards,  forming  with  the  telsou  the  strong  tail-fiu. 

Bunches  of  branchial  filaments  occur  upon  the  outer  lamel- 
1.10  of  the  abdominal  appendages  with  the  exception  of  the  last 
pair.  The  heart  is  much  elongated,  extending  from  the  ante- 
rior thoracic  region  as  far  back  as  the  fifth  abdominal  seg- 
ment and  possessing  numerous  pairs  of  ostia.  It  is  prolonged 
anteriorly  and  posteriorly  into  aortas  and  gives  off  laterally 
in  each  segment  a  pair  of  arteries. 

The  Stomatopods  are  all  marine  and  pass  through  a  com- 
plicated series  of  metamorphoses  during  development.  Some 
of  the  principal  genera  are  Squilla  (Fig.  186),  Lysiosquilla,  and 
Gonoddctylus.  , 


III  ''I 


U 


I 


ill 

11  ii 


r 


Iff' (8 
''if 


I 


■( 


410  INVERTEBRATE  MORPHOLOGY. 

4.  Order  Decapoda. 

In  the  Decapods  the  carapace  is  well  developed,  covering 
in  the  thorax  completely  (Fig.  162),  the  segments  of  that  region 
of  the  body  fusing  with  it  dorsally,  so  that  a  perfect  cephalo- 
thorax  is  present.  The  antennules  generally  possess  two 
terminal  multiarticulate  flagella,  and  the  antenuse  frequently 
lack  the  scalelike  exopodite  which  occurs  in  other  groups 
(e.g.,  Schizopoda).  In  the  second  maxillae  the  exopodite  is 
transformed  into  a  platelike  structure  which,  swinging  to  and 
fro,  serves  to  renew  the  water  in  the  branchial  chamber  lying 
between  the  lateral  portions  of  the  carapace  and  the  body- 
walls.  On  account  of  this  action  this  appendage  is  usually 
spoken  of  as  the  scaphognathite.  The  three  anterior  thoracic 
appendages  are  raaxillipeds,  the  third  one  frequently  becom- 
ing aim  '5 1  limblike,  a  characteristic  which  distinguishes  the 
live  post  J  »airs  of  appendages  which  are  adapted  for  walk- 

ing and  ai  hence  termed  the  pereiopods.  They  lack  all 
traces  of  exopodites,  though  usually  bearing  epipodites  and 
branchiae,  and  a  certain  number  of  the  anterior  ones  are  fre- 
quently chelate,  thus  ser  ang  for  the  prehension  of  food.  The 
number  of  the  pereiopods  has  suggested  the  name  given  to 
the  order.  The  abdominal  appendages  are  sometimes  want- 
ing or  very  rudimentary,  but  when  present  are  biramous  swim- 
ming-feet and  are  hence  termed  pleopods — a  term  equally 
applicable  in  some  other  groups. 

The  branchiae  lie  entirely  within  the  branchial  chamber 
and  are  developed  in  connection  with  the  thoracic  append- 
ages. They  may  be  seated  upon  the  basal  joints  of  the  ap- 
pendages (podobiauchia),  or  upon  the  joint  between  the  ap- 
pendage and  the  body-wall  (ai'throbranchia),  or  finally  upon 
the  body-wall  itself  (pleurobrauchia).  All  three  kinds  may 
occur  on  the  same  segment,  so  that  the  entire  number  of  gills 
may  be  much  greater  than  that  of  the  appendages,  amount- 
ing in  the  Lobster  to  no  less  than  twenty  in  each  branchial 
chamber. 

The  heart  is  a  short  saclike  organ  lying  in  the  thorax  and 
possessing  as  a  rule  three  pairs  of  ostia,  one  pair  being  situ- 
ated on  the  dorsal  surface,  one  upon  the  sides,  and  the  third  on 


TYPE  CRUSTACEA. 


411 


the  ventral  surface.  Arteries  pass  off  from  both  ends  of  the 
heart.  Otocysts  are  always  developed  in  the  basal  joints  of 
the  auteunules. 


1.  Suborder  Macrura. 

In  the  Macrura  the  abdomen  is  well  developed  and  usu- 
ally as  long  as  the  cephalothorax,  and  is  provided  with  its 
full  complement  of  appendages,  the  sixth  pair  forming  with 
the  telson  a  tail-fin.  Exceptions  to  tliese  arrangements  oc- 
cur ;  in  the  Hermit-crabs,  EupaguruSy  which  inhabit  the  empty 
shells  of  Gasteropod  Mol- 
lusks,  the  abdomen  is  gener- 
ally soft  and  unsymmetricul, 
since  it  is  coiled  around  the 
columella  of  the  shell,  but 
terminates  in  a  movable  tail- 
fin  which  serves,  together  with 
the  remaining  pleopods  and 
the  last  (and  sometimes  also 
the  penultimate)  pereiopod, 
which  is  bent  dorsally,  to  re- 
tain the  animal  in  the  shell. 
The  chelsB  of  the  anterior 
pereiopods  are  generally  un- 
equal in  size,  serving  to  oc- 
clude the  mouth  of  the  shell, 
and  occasionally  the  abdomi- 
nal appendages  of  only  one 
side  are  developed.  In  the 
genus  Hippa  too  the  abdomen, 
though  with  a  well-developed 
and  calcified  cuticle,  iss  short, 
the  terminal  half  being  bent 
up     under     the     thorax,    the  Fig.  187.—^,  a  young  Lucifer  (adapted 


from   Brooks);    B,    Eupagurus    bem 
hardns  (after  LcuNid). 


condition  characteristic  of 
the  Crabs  being  thus  ap- 
proached. In  some  forms,  such  as  Sergestes  and  Limfer^ 
the  fourth  and  fifth  pereiopods  may  be  rudimentary  or  even  ab- 
sent, but  more  usually  all  these  appendages  are  well  devel- 


;3 


•iji 

m 


I 


W. 


il' 


t 


il'lf 


lii 


i;  i  i 


IP 


III 


)l    : 


41:^ 


IN VEHTEBHA TE  MOliPUOLOG  Y. 


oped,  the  anterior  ones  becoming  chelate.  In  the  Crayfish, 
Cambarus,  and  the  Lobster,  Homarus,  the  first  pereiopod  is  an 
exceedingly  strong  chela,  and  the  same  arrangement  is  found 
in  Alpheiis,  while  in  the  Shrimp,  Palti'monetes,  the  second  pe- 
reiopod is  somewhat  longer  than  the  first. 

The  branchiytj  are  usually  numerous  and  are  for  the  most 
part  bunches  of  cylindrical  processes,  but  in  Pakemonetes  and 
the  shrimps  and  prawns  in  general,  which  form  the  familj^  Ca- 
ndida}, and  in  the  Hermit-crabs  they  are  lamellate.  In  Lucifer 
branchiae  are  entirely  wanting.  The  Macrura  are  essentially 
marine,  a  few  forms,  such  as  Cambarus  and  some  species  of 
Paloimon,  occurring  in  fresh  water.  The  genus  Birgus,  one  of 
the  Hermit-crabs,  commonly  known  as  the  robber-crab,  is 
almost  entirely  terrestrial,  living  in  holes  in  the  ground  and 
climbing  cocoa-nut  palms  for  the  sake  of  the  nuts,  on  which 
it  lives.  In  harmony  with  its  terrestrial  life  the  inner  surface 
of  the  branchial  chamber  is  thrown  into  folds  richly  supplied 
with  blood-lacunae,  a  lunglike  structure,  recalling  the  lungs  of 
the  Puimonate  Gasteropods,  being  thus  developed. 

2.  Suborder  Brachyura. 

In  the  Brachyura  the  body  is  exceedingly  compact,  the 
abdomen  being  very  much  reduced  in  size  and  usually  desti- 
tute of  a  tail-fin,  an^  in  addition 
is  bent  up  so  as  to  lie  in  a  groove 
upon  the  ventral  surface  of  the 
cephalothorax.  In  some  cases 
the  cephalothorax  is  almost  glo- 
bular, though  prolonged  anterior- 
ly into  a  strong  rostral  spine,  as 
in  Libinia,  the  spider-crab ;  while 
in  other  cases  it  is  more  flattened 
and  triangular  in  shape  and  lacks 

a  distinct  rostrum,  as  in  the 
Fig.   188. — Panopmia    depressus      -,-■,■,  ,     ^  n-       ,       .i       i    i 

(after  EMEBTON  from  VERBAL).  ^^ible  crab,  GaUmectes,  the  lady- 

crab,  Platyonychus,  and  the  com- 
mon crab,  Cancer,  and  in  others  again  is  more  or  less 
quadrangular  and  thicker,  as  in  Pinnotheres,  the  03'ster-crab, 
Ocypoda,  the  sand-crab,  and  Gdasimus,  the  tiddler-crab.    The 


'....jj^:ls&iiSiM 


TYPE  CRUSTACEA. 


413 


uuteuuules  are  small  and  they  and  the  eyes  can  be  partially 
concealed  iu  a  f^roove  on  the  anterior  edge  of  the  cnrapace. 
The  abdominal  appendages,  with  the  exception  of  the  anterior 
one  or  two  pairs  which  are  adapted  for  coi)ulation,  are  ab- 
sent in  the  males,  while  the  females  generally  possess  four 
pairs,  to  which  the  ova  are  attached. 

The  gills  are  generally  few  iu  number,  except  in  Porcel- 
lana  and  some  allied  forms,  and  are  usually  lamellate  iu  form. 
AVhile  essentially  marine  in  habit,  the  Brachyura  are  fre- 
quently more  or  less  terrestrial,  the  sand-crabs,  (Jcyjxxia,  and 
the  liddler-flrabs,  Gelasimus,  living  in  holes  in  the  sand  just 
above  high-tide  mark,  while  the  land-crabs,  Gecarcinvfi,  of  the 
tropics  may  live  some  distance  from  the  sea,  migrating  to  it 
in  armies  during  the  breeding-season.  A  few  forms,  such  as 
the  genus  Telphusa,  are  aquatic. 

III.  Subclass  Asthrostraca. 

The  Arthrostraca,  with  the  exception  of  the  small  group 
of  the  Anisopoda,  are  destitute  of  a  carapace,  and  the  tho- 
racic appendages,  with  the  exception  of  the  first  pair,  are 
jointed  walking-limbs  lacking  an  exopodite.  The  anterior, 
or  in  some  cases  the  anterior  two  thoracic  segments  fuse 
with  the  head,  the  appendages  of  these  segments  differing 
from  those  of  the  free  segments,  being  modified  to  assist  iu 
the  process  of  mastication,  whence  they  are  termed  maxilli- 
peds.  The  abdomen  is  composed  of  six  segments  provided 
with  appendages,  and  of  a  terminal  telson  ;  occasionally  the  va- 
rious segments  fuse  together,  and  in  some  forms  the  abdomen 
is  reduced  to  a  small  unsegmented  structure.  Platelike  ap- 
pendages attached  to  the  basal  joints  of  some  of  the  thoracic 
limbs  form  by  their  meeting  and  overlapping  a  brood-pouch 
in  which  the  ova  undergo  their  development. 

The  lateral  compound  eyes  are  not,  except  in  Tanais,  sup- 
ported on  stalks,  a  characteristic  which  has  suggested  the 
term  Edriophthalmata  sometimes  applied  to  the  group. 

1.  Order  Anisopoda. 

The  Anisopoda  are  exclusively  marine  forms  iu  which  the 
two  anterior  thoracic  segments  are  fused  with  the  head  and 


m 


■■'■  i) 


I'  I  '  'i 


!?»:'» 


'm  iji 


414 


INVEnTEBHATE  MORPHOLOGY. 


covered  in  at  the  sides  by  duplicatures  of  the  body-wall, 
wliich  enclose  a  small  respiratory  cavity. 

The  anteuuules  and  antennae  are  uuiramous  except  in 
Apsevdes  in  wliich  the  anteunules  carry  two  terminal  liagella. 
The  palps  of  the  anterior  maxilla?  project  into  the  respiratory 
chamber  and  serve  for  cleansing  it,  and  the  first  thoracic 
limbs,  the  maxillipeds,  bear  each  an  epipodial  branchial  ap- 
pendage lying  in  the  respiratory  chamber.  This  limb  and 
the  succeeding  one  are  chelate,  the  inner  angle  of  the  penul- 
timate joint  being  prolonged  into  a  process  against  which  the 
terminal  joint  may  be  apposed.  The  abdominal  appendages 
are  biraraous  swimming-feet  in  Tanaia  and  Apsevdes,  the  last 
pair  being  in  AntJmra  especially  enlarged  to  form  with  the 
telson  a  terminal  finlike  structure. 

2.  Order  Isopoda. 

The  majority  of  the  Isopoda  are  marine,  the  genus  Aselhis 
(Fig.  189),  however,  occurring  in  fresh  water,  while  Oniscus, 
PorcelUo,  and  Armadillidium  are  terrestrial,  being  commonly 
known  as  Wood-lice  or  Sow-bugs.  The  body  in  all  forms  is 
more  or  less  flattened  dorso-veutrally  and  only  the  anterior 
thoracic  segment  is  fused  with  the  head,  the  remaining  seven 
remaining  perfectly  distinct.  There  is  no  trace  in  the  adult 
of  a  carapace,  and  the  abdominal  segments  are  usually  small 
and  may  be  fused  more  or  less  completely.  ^ 

The  maxilhiB  are  destitute  of  palps  and  the  maxillipeds 
(mxp)  usually  fuse  together  to  form  a  sort  of  lower  lip.  The 
remaining  thoracic  appendages  are  limblike  and  d^  not  bear 
any  respiratory  appendages,  though  lamella3  are  attached  to 
the  basal  joints  of  several  of  them  in  female  individuals, 
serving  to  form  a  brood-pouch.  The  five  posterior  abdom?nal 
appendages  are  biramous  and  lamellar  (ab),  serving  both  for 
swimming  and  for  respiration,  the  anterior  pair  (op)  usually 
becoming  hard  and  forming  an  operculum  which  covers  in 
the  posterior  more  delicate  appendages  and  in  the  terrestrial 
forms  may  have  branching  spaces  containing  air  (tracheae) 
ramifying  through  them. 

The  heart  (Af\  in  conformity  to  the  position  of  the  respira- 
tory organs,  is  situated  principally  in  the  abdomen,  extending 


!ii 


TYPE  CliUSTACEA. 


415 


forwards  only  a  short  distance  into  the  thorax  segment.  It 
possesses  one  or  two  pairs  of  ostia  and  is  closed  behind, 
giving  off  in  front  and  at  the  sides  numerous  aortse.  A  shell- 
gland  has  been  observed  in  some  Isopoda,  but  the  antennary 
gland  is  wanting. 

Although  the  majority  of  the  marine  forms,  such  as  Idotea 
and  Sphreroma,  lead  a  free  existence,  nevertheless  there  are 
certaiu  parasitic  forms.  Thus  the  genera  Oymothoa  and  yEj/rt 
are  parasitic  on  the  skin  or  in  the  mouth  of  fishes,  but  also 
retain  the  power  of  swimming  and  consequently  are  not  much 


Fig.  ISQ.—Asellus  communis.  Diagram  oi   Structure, 

ab  =  abdominal  appendages.  I  =  liver-csecn. 

ao  —  aorta.  mnp  =  uiaudibuliir  palp. 

at^  =  antennule.  mrp  =  maxillipcd. 

at?  =  auteuna.  r  =  rectum. 

ce  =  cerebral  ganglion.  «  =  stomach. 

c?i  =  chelate  limb.  t  =  thoracic  appendage. 

ht  =  heart.  vn  =  ventral  nerve-cord. 

modified.  The  genus  Bopyrus,  which  lives  in  the  branchial 
cavity  of  shrimps,  becomes  in  the  female  somewhat  distorted 
in  shape  and  asymmetrical,  and  the  mouth-parts  become 
transformed  into  a  suctorial  proboscis  and  the  eyes  disap- 
pear. The  male,  however,  which  is  much  smaller  than  the 
female,  retains  the  eyes  and  does  not  depart  from  the  usual 
symmetrical  body  form.  The  degeneration  of  the  female 
proceeds  much  farther  in  the  genus  Entoniscus,  which  lives 
either  partly  or  wholly  included  within  the  body-cavity  of 
other  Crustacea  and  assumes  a  saclike  unsymraetrical  form, 
recalling  to  a  certain  extent  that  of  some  of  the  parasitic 
Copepoda.  At  the  time  of  pairing  both  sexes  are  alike  fully 
segmented  and  with  an  almost  full  complement  of  appendages. 


■J 


n 


t»i« 


m 


i 


tv 


"  ill 


I  r,  ji 


416 


JN  VEHTEBRA TE  MOltPUOLOO  Y. 


After  copulation,  however,  the  female  assumes  tlie  degener- 
ated  form,  while  the  male  dies. 

3.  Order  Amphipoda. 

Like  the  Isopoda  these  are  essentially  marine  forms, 
thouj^h  the  genus  Gammarus  is  aquatic  and  Orchestia 
(Fig.  190)  partly  terrestrial,  living  among  the  wrack  on  sea- 
beaches  just  beyond  the  reach  of  the  waves.     The  body  in 

ht 


Fig.  190.  — Diaguam  of  Stkucture  op  Orchestia  eavimana  (after  Nebkski). 

aO  =  antetiuule.  m  =  mouth. 

aP  =  auteniia.  mt  =  Malpigbian  tiihule. 

br  =  brauchia.  oc  =  eye.                , 

ce  =  cerebnil  ganglion.  •        r  =  rectum.           v 

ch  =  chelate  limb.  rd  =  reproductive  duct. 

ht  =  heart.  ro  =  reproductive  organ. 

I  =  liver-caeca.  en  =  ventral  nerve-cord. 

the  Amphipoda  is  laterally  flattened  and  presents  therefore  a 
very  different  appearance  from  that  of  the  Isopoda,  though, 
as  in  that  group,  lacking  all  traces  of  a  carapace.  The  first 
thoracic  segment  is  fused  with  the  head,  and  in  Caprella  and 
Cyamus  the  second  segment  likewise.  The  appendages  of  the 
head  and  the  maxillipeds  resemble  those  of  the  Isopoda,  and 
the  remaining  thoracic  appendages  are  limblike,  a  certain 
number  of  the  anterior  ones  frequently  possessing  a  terminal 
joint  capable  of  flexion  upon  the  succeeding  one,  or  even 
being  chelate  The  five  posterior  limbs  or  the  third  and 
fourth  only  bear  epipodial  lobes  which  serve  as  branchiae,  and 


-^iU. 


TYPE  CItUSTACEA. 


417 


a  uumber  of  the  limbs  also  in  females  bear  lamellas  which 
may  enclose  a  brood-pouch.  The  three  anterior  abdominal 
limbs  are  biramous  and  serve  for  swimming,  while  the  three 
posterior  ones,  also  biramous,  are  frequently  directed  back- 
wards and  serve  as  springing  organs,  the  springing  powers  of 
Orchesfta  having  gained  for  it  the  name  of  the  Beach-flea,  In 
Caprella,  which  crawls  about  over  colonies  of  Hydroids  and 
Polyzoa,  and  Cyamns,  which  is  parasitic  upon  the  skin  of 
whales,  the  abdomen  becomes  almost  rudimentary  and  is  des- 
titute of  appendages. 

The  heart  (ht)  lies  in  the  thoracic  region  in  the  anterior 
five  or  six  segments  and  possesses  from  one  {Corophiiun)  to 
three  ostia.  It  is  prolonged  into  an  aorta  at  either  end.  In 
connection  with  the  mid-gut  portion  of  the  digestive  tract,  in 
addition  to  the  four  so-called  liver-cseca  (/)  is  a  pair  of  gland- 
ular ca3ca  which  seem  to  be  excretory  in  function  and  have 
been  termed  Malpighian  tubules  {mt).  An  antennary  gland 
occurs,  but  the  shell-gland  is  apparently  unrepresented  in 
adults. 

Development  of  the  Crustacea. — The  majority  of  the  Crus- 
tacea pass  through  a  more  or  less  complicated  series  of 
metamorphoses,  the  larval  forms  being  highly  suggestive 
when  studied  from  the  phylogenetic  standpoint.  A  few  forms, 
especially  those  inhabiting  fresh  water,  abbreviate  their  de- 
velopment considerably,  so  that  the  young  animal  when  it 
leaA'es  the  egg  practically  may  differ  from  the  parent  onl}'  in 
ii\ze{Gamharus),  and  among  the  higher  forms  the  development 
is  generally  abbreviated  to  the  extent  that  a  greater  or  less 
number  of  the  larval  stages,  characteristic  of  lower  forms,  are 
passed  through  while  the  young  animal  is  still  within  the  egg- 
membrane,  only  the  final  stages  being  free-swimming. 

Throughout  the  Entomostraca  th^  drst  larval  form  which 
hatches  from  the  egg  is  termed  the  Nauplius  (^ig.  191)  and 
differs  markedly  from  the  adult,  chiefly,  however,  ^n  the  small 
number  of  appendages  it  possesses.  The  body  in  typical 
forms  shows  no  trace  of  segmentation  and  possesses  a  single 
median  eye  generally  x-shaped.  But  three  pairs  of  limbs 
are  present,  which  become  transformed  later  into  the  anten- 
nules,  antennee,  and  the  msindibles  of  the  adult.    The  Naupliar 


'Mi.  '■ 


I 


, 


418 


IN  VEHTEBUA  TE  MOHPUOLOO  Y. 


autenuules  are  uuirumous  auil,  like  the  other  limbs,  but  iudis- 
tiuctly  jointed,  the  autenuules  und  uiundibles  being,  however, 
biruuioiiH  und  possessing  strong  setie  at  their  bases  which 
function  as  jaws,  though  both  pairs  of  appendages  are  essen- 
tially locomotor.  Judging  from  the  appeudages,  therefore,  the 
Nauplius  may  be  regarded  as  consisting  of  five  segments,  one 
corresponding  to  the  prostomial  lobe  of  Auuelids  and  contain- 
ing the  primitive  cerebral  gangliou  (archicerebrum),  one  cor- 


Fig.  191.— Nauplius  of  Cetochilm  aeptentrionalia  (atter  QnoBBatt). 

responding  to  each  pair  of  appeudages  and  one  to  the  region 
of  the  body  behind  the  mandibles. 

A  Nauplius  of  this  simple  form  may  be  regarde  ,s  typical 
and  is  that  which  is  found  in  the  majority  of  the  C  oda  and 

in  the  Cirrhipedia  as  well  as  in  some  Branchiopoda  {Astheria, 
JLrmnadia).  In  the  Ostracoda  the  arrangement  of  the  limbs 
and  segments  is  the  same,  but  the  bivalved  shell  characteristic 
of  the  adult  is  already  developed,  giving  the  Nauplius  an  ap- 
pearance very  different  from  that  of  the  Copepoda.  Not  un- 
frequently,  however,  as  for  instance  in  Apits  among  the 
Branchiopoda,  and  Leptodora  among  the  Cladocera  (the  re- 
maining Cladocera,  so  far  as  is  known,  leave  the  egg  with  the 
adult  form),  the  Nauplius,  though  possessing  only  the  three 
pairs  of  appendages,  yet  shows  indications  in  the  post-maudib- 
ular  region  of  a  varying  number  of  additional  segments,  and 
to  this  form  it  is  convenient  to  apply  the  name  Metanauplius. 

As  a  rule  in  the  Eutomostraca  further  development  con- 
sists of  a  series  of  moults  (ecdyses),  an  increase  in  the  number 
of  segments  and  appendages  and  modifications  of  the  latter 
taking  place  at  each  ecdysis,  until  the  adult  form  is  attained. 
No  special  larval  forms  beyond  the  Nauplius  are  common  to 


TYPE  VHV8TACEA. 


419 


all  the  members  of  the  class,  and  it  is  only  in  the  Civrliipedia 
that  a  Hecoutl  definite  larval  form  can  be  diHtinj^'uished,  the 
Oypris-larva,  to  which  attention  has  already  been  called  (p.  M99). 
In  the  Malacostraca  the  occurrence  of  a  free-swinuninfj; 
Naiipliiis  is  the  exception  rather  than  the  rule,  and  indeed 
larval  forms  are  practically  wanting  in  some  groups,  such  as 
the  Leptostraca  and  Arthrostraca,  and  iu  certain  species  or 
families  of  other  groups  (e.g.  My8id(Vy  Camharvs).  In  the 
genus  Penceits  among  the  Decapods,  and  iu  Ei(plutusia  among 
the  Schizopods,  a  typical  free-swimming  Nauplius  occurs,  and 
in  Lvjcifer  the  embryo  leaves  the  egg  in  the  form  of  the  Meta- 
nauplius.  In  the  majority  of  forms  these  stages  are  passed 
over  while  the  embryo  is  still  within  the  egg-shell,  and  it 
hatches  only  when  it  has  acquired  a  greater  degree  of  develop- 
ment. In  such  forms  as  Pfenrews,  Ewphmma^  and  Lucifer  the 
Metanauplius  stages  pass  into  what  is  termed  the  Protozoea 
(Fig.  192,  A)  a  stage  also  passed  over  within  the  egg  by  the 


mx« 


Fig.  193.—^,  PaoTOZofiA  op  Zt/ct/fer  (after  Brooks);  B,  ZofiiA  OF  Palamonetes 

(after  Faxon). 
At^  =  autenuule.  m  =  muDdible. 

At^  =  antenna.  7nx\  mx'^  =  miixillse. 

c  =  cerebral  gaDglion.  mp\  mp^  =  maxilllpeds. 

E  =  compound  eye.  oc  =  simple  eye. 

h  =  heuit.  r  =  rostrum. 

8  =  stomach. 

majority  of  Malacostracaus,  though  occurring  as  the  first 
larval  stage  of  some  Stomatopods.  It  is  characterized  by  the 
development  of  two  maxillae  and  the  two  or  three   anterior 


:;!' 


r 

ii 

it 

III 

m 

i 

' 

>■  ! 


ii  f 


'      1 


ill 


>• 


1    iliH 

■    I' 


ill 


420 


IN  VERTEBRA TE  MORPHOLOG  Y. 


thoracic  appeudages  iu  addition  to  those  already  present  in 
the  Nauplius,  and  furthermore  by  the  distinct  separation  of 
the  body  into  an  anterior  cephalo-thoracic  portion  covered  by 
a  carapace  and  a  posterior  abdomen  which  is  usually  but 
imperfectly  segmented.  This  stage  is  succeeded  sometimes 
after  two  or  more  ecdyses  by  the  Zo'ca  (Fig.  192,  B),  a  stage  iu 
which  the  majority  of  Deeapoda  leave  the  egg.  It  is  distin- 
guished from  the  Protozoea  principally  by  the  perfect  segmen- 
tation of  the  abdominal  region,  though  it  still  possesses  no 
appendages,  unless  it  be  rudiments  of  the  sixth  pair,  and  it 
is  furthermore  characterized  by  the  compound  eyes  being 
stalked,  a  feature  but  slightly  indicated  in  the  Protozoea,  in 
which  stage  they  make  their  appearance.  The  Zoea  stage  in 
the  Brachyura  is  generally  characterized  b}'  the  development 
of  spines,  sometimes  of  enormous  length  {Porcellana\  upon  the 
dorsum  and  sides  of  the  carapace. 

In  such  a  form  as  Euphausia  the  next  stage  is  the  adult, 
but  in  the  Decapods  other  larval  stages  intervene  before  the 
adult  condition  is  reached.  The  first  of  these  is  characterized 
in  the  majority  of  the  Macrura  by  the  appearance  of  the  re- 
maining thoracic  appendages  which  were  unrepresented  in 
the  Zoea,  in  the  form  of  biramous  structures  closely  resem- 
bling the  thoracic  appendages  of  the  Schizopoda,  AVdience  the 
stage  is  generally  termed   the  Alysis   stage  (Fig.  193).     The 


Pig.  193.  -  My8I8  8TAOE  op  Lobstbu,  Uomarus  americanus  (after  S.  I.  Smith). 


abdominal  appendages  also  develop  during  this  stage. 
Among  the  Hermit-crabs  (Paguridie)  and  the  Brachyura  the 
development  is  to  a  certain  extent  abbreviated,  the  pereiopods 
never  being  represented  by  biramous  appendages,  but  being 
from  the  first  uniramous,  and  in  these  forms  therefore  a 
true  Mysis  stage  never  occurs.     To  the  corresponding  stage, 


,■.  ii*aSaij*aiiii»i*MA-i 


TYPE  CRUSTACEA. 


421 


or  rather  to  one  in  which  the  pereiopods  are  indicated  but 
not  fully  developed,  the  term  Metazola  is  applied.  Further- 
more in  certain  Macrura,  such  as  Scyllarns  and  Pulinurm,  the 
Mysis  stage  is  represented  by  peculiarly-shaped  transparent 
larvae  which  have  been  termed  FhyUosoma,  or  glass-crabs. 
The  carapace  is  divided  into  two  portions,  of  Avhich  the  an- 
terior or  larger  covers  in  the  head  region  and  the  posterior 
the  thorax,  the  body  being  throughout  flat  and  the  ab- 
domen very  small.  The  pereiopods,  of  which  in  the  earliest 
stages  there  are  but  three,  are  biramous,  and  the  first  maxil- 


Pia  194.— Megalopa-stagb  op  Cancer  iiroratua  (after  Emkrton  from  Verrill). 

lipeds  are  either  entirely  wanting  or  very  rudimentary.  Dur- 
ing successive  ecdyses  the  missing  appendages  are  gradually 
developed,  though  the  actual  transformation  of  the  PlruUosoina 
into  the  youngest  Scyllanifi  or  Palinurus  stage  (which  is  de- 
cidedly smaller  than  the  oldest  Phyllosomu)  has  not  yet  been 
observed. 

The  change  from  the  Mysis  stage  to  the  adult  is  usually 
gradual,  and  no  specially  definite  larval  forms  are  to  l)o  fountl 
as  a  rule  among  the  Macrura.     In  the  Brachyura,  however, 


Hh 


u 


m 

IIS' 


s  ■  I 


422 


INVERTEBRATE  MORPHOLOGY. 


J 


■1 


the  Metazoea  becomes  transformed  into  a  well-marked  form, 
the  Megalopa  (Fig.  194),  so  called  from  the  usually  large  size 
of  the  cephalothorax.  It  resembles  closely  a  Macrurau, , 
differing  only  in  the  abdomen  being  relatively  small,  and 
becomes  converted  into  the  adult  form  by  the  doubling  of 
the  abdomen  beneath  the  thorax.  A  Megalopa  stage  occurs 
also  in  the  Hermit-crabs,  but  is  not  so  well  marked  off 
from  the  young  fully-formed  animals  as  in  the  Brachyura. 

Affinities  of  the  Crustacea. — The  rehitionships  of  the  higher  groups  of 
the  Malacostraca  to  one  another  are  clearly  shown  Ijy  their  larval  forms, 
the  Megalopa  showing  the  origin  of  the  Brachyura  from  Macruran  forms, 
and  the  Mysis  stage  that  of  the  latter  from  Schizopod  ancestors.  When 
attempts  are  made  to  go  still  further  difficult'^s  stand  in  the  way.  As 
regards  the  Stomatopoda  it  is  to  be  noted  th.  ^ey  pass  through  a  stage, 
the  Erichthus,  in  which  the  thoracic  appendages  which  are  present  are 
biraraous,  and  it  seems  probable  that  both  they  and  the  Cumacea  are  re- 
ferable back  to  Schizopod  ancestors.  The  Arthrostraca,  on  the  other  hand, 
are  probably  traceable  to  Curaacealike  ancestors,  while  the  Leptostraca 
represent  more  nearly  the  Entomostracan  ancestors  than  any  other  group, 
though  widely  differentiated  from  them  in  certain  particulars.  It  is  even 
still  more  diflScult  to  trace  out  relationships  of  the  various  Entomostracan 
orders,  but  it  seems  fairly  clear  that  Phyllopodau  forms  such  as  Apus  are 
to  be  considered  as  representing  more  nearly  than  any  others  the  primitive 
Crustacea. 

As  regards  the  affinities  with  other  groups  very  interesting  questions 
arise,  two  possibilities  seeming  to  be  open.  According  to  one  tlie  Crustacea 
have  been  derived  directly  from  segmented  Annelids,  through  fprms  repre- 
sented in  a  modified  condition  to-day  by  Ajms.  The  lobed  appendage  of 
Apits  is  a  moditied  parapodium,  and  the  segmentation  of  the  body  has  been 
mherited.  What  then  as  to  thoNauplius?  According  to  this  view  it  has 
practically  no  aticostral  significance,  or  at  best  can  be  considered  only  as 
representing  a  Trochophoro  larva  highly  modified  and  with  many  adult 
characters  tlirown  back  upon  it.  Tiiis  latter  idea  does  not  seem,  however, 
to  agree  with  tl»o  f.'icts.  since  the  Trochophore  is  an  unsegmented  structure 
and  can  be  comparable  only  to  the  prostomial  and  first  appendage-bearing 
segments  of  the  Nauplius.  In  other  wonls,  the  Nauplius  is  comparable, 
if  comi)arable  at  all,  to  a  Trochophore  plus  certain  additional  segments. 
It  has  recently  been  suggested  that  possibly  the  Nauplius  may  represent 
not  the  Trochophore  but  the  larval  Annelid  with  three  parai)odia,  which,  as 
indicated  (p.  215),  is  a  well-marked  stage  in  the  development  of  many 
Polychajta.  The  luimber  of  segments  is  apparently  similar  in  the  two 
forms,  and  the  idea  is  plausible.  If,  howev(>r,  ia  all  Crustacea  a  ganglion, 
representing  a  segment,  intervenes  between  the  archicerebral  ganglia  and 
the  autennulary  (see  p.  378),  then  the  Nauplius  has  potentially  one  seg- 


iVi^a^^^Mi^-i 


TYPE  CRUSTACEA. 


423 


ment  more  than  the  Annelid  lai'va  and  the  comparison  will  not  hold.  If 
the  direct  Annelid  origin  is  to  be  accepted,  it  seems  most  satisfactory  at 
present  to  regard  the  Nauplius  as  a  secondarily  acquired  larval  stage 
without  any  ancestral  significance. 

Another  suggestion  has,  however,  been  made  which  gives  the  Nauplius 
a  significance  and  traces  the  Crustacea  back  to  unsegmented  ancestors.  It 
is  to  the  effect  that  the  Nauplius  can  be  referred  to  Rotiferlike  ancestors, 
the  remarkable  Hexarthra  with  its  six  processes  being  supposed  to  indi- 
cate the  line  of  descent.  It  is  exceedingly  doubtful,  however,  whether 
this  similarity  can  be  regarded  .is  anything  more  than  a  superficial  one 


!i   ! 


M! 


TYPE   CRUSTACEA. 

I.  Class  Entomostbaca. — Number  of  segments  varies  ;  abdomen  without 
appendages  ;  larva  a  Nauplius. 

1.  Order  Phyllopoda. — Number  of  segments  variable ;  appendages 

with  branchiae. 

1.  Suborder  Branchiopoda. — Body  plainly  segmented  and  seg- 
ments of  thorax  more  numerous  than  six.  Apus,  Bratiehipus, 
Estheria,  Limnadia,  Limnetis. 

2.  Suborder  Cladocera. — Body  indistinctly  segmented  ;  with  bi- 
valved  shell ;  four  to  six  thoracic  appendages.  Daphnia^ 
Moina,  8ida^  Ewidne,  Polyphemus. 

2.  Order  Ostracoda. — With  bivalved  shell ;  body  indistinctly  seg- 

mented ;  two  thoracic  appendages.    Cypris,  Cythere,  Cypri- 
diiia,  Halocypris. 

3.  Order  Copepoda. — Without  shell ;  five  pairs  of  thoracic  limbs ; 

many  forms  parasitic  and  degenerate. 

1.  Suborder  Eucopepoda. — First  thoracic  segment  only  fused  with 
head;  abdomen  cylindrical  and  segmented  except  in  highly 
degenerated  forms.  Cyclops,  Canthocamptus,  Harpactictts, 
Calanus,  Cetochilns  (free-swimming)  ;  Notodelphys  (commeii- 
salistic) ;  Corycceus,  tS(ij>p/iirina,  Eryasilus,  Caligus,  Panda- 
rus  (partly  parasitic)  ;  PhilicJithys,  Penella,  Lerima,  Cfion- 
dracnnthus,  Achtheres,  Anchorella  (parasitic). 

2.  Suborder  Branchinra. — All  thoracic  segments  fused  with  head  ; 
abdomen  small  and  lamellar,  partly  parasitic.     Argultis. 

4.  Order  Cirrhipedia. — Sessile  or  parasitic ;    segmentation  indis- 

tinct ;  six  pairs  of  thoracic  appendages ;  pass  through  Cypris 
stage.  Lepas,  Scalpellnm,  Ibia,  Balaims  (sessile) ;  Alcippe, 
Cryptophialus  (boring)  ;  Proteolepas,  Saactdina,  Laura, 
Dendrogaster  (parasitic). 
U.  Class  Malacostraca. — Number  of  segments  constant ;  thoracio  seg- 
ments eight,  abdominal  seven  or  eight. 
1.  Subclass  Leptostraca.  With  bivalved  shell;  abdomen  with  eight 
segments.    JVebalia. 


I  il 


M 


It;'.- 


ij- 


t, 


424 


INVERTEBRATE  MORPUOLOOT. 


3.  Subclass  Thoracostraca. — With  carapace  covering  the  whole  or  a 
part  of  the  thorax ;  abdominal  segments  seven. 

1.  Order  Scliizopoda. — Thorax  completely  covered  ;  thoracic  append- 

ages biramous.    Mysis,  Euphausia,  Siriella. 

2.  Order  Cumacea. — Last  four  or  five  thoracic  segments  not  covered 

by  the  carapace  ;  eyes  sessile   or   rudimentary.    Diastylis, 

Campylaspis. 
8.  Order  Stomatopoda. — Last  three  or  four  thoracic  segments  not 

covered    by    the    carapace;  'eyes   stalked;    five  maxillipeds. 

Squilla,  Lysiosquilla,  Gonodactyltis. 
4.  Order  Decapoda.  —  Thorax  completely  covered ;  five  posterior 

appendages   uniramous  and  three  maxillipeds  ;   otocysts  in 

antennules. 
1.  Suborder  Macrura. — Abdomen  usually  well  developed.    8er- 

gestes,   Lucifer,  Penceus,  l^lcemmietes,   Alphetts,  Cambarus, 

Homarus,  Eupagurus,  Birgus,  Hippa. 
a.  Suborder  ^rac^wm. — Abdomen  small  and  concealed  beneath 

cephalothorax  more  or  less  perfectly.     Porcellana,  Libinia, 

CatUneeteSf    Platyonychus,    Cancer^    Pinnotheres,    Ocypoda^ 

Oelasimus,  Gecarcinus. 
8.  Subclass  Arthrostraca. — No  shell  or  carapace  as  a  rule;  with  seven 

(or  six)  walking-limbs  ;  eyes  sessile. 
1.  Order  ^7moj9oc?a,— Carapace  slightly  developed ;  first  two  tho- 
racic segments  fused  with  head  ;  branchiae  on  anterior  maxillae. 

A  seudes,  Tanais,  Antlmra. 

3.  Order /jsoporfa.— No  carapace  ;  first  thoracic  segment  fused  with 

head  ;  body  flattened  dorso-ventrally ;  branchiae  on  abdomi- 
nal appendages.  Aselliis,  Onisctis,  Porcellio,  ArmadilUd- 
iitm,  Idotea,  Sphcproma  (free) ;  Cymothoa,  JElga,  Bopyrus, 
Entoniscus  (parasitic). 
8.  Order  Amphipoda. — No  carapace,  first  thoracic  segment  fused 
with  head ;  body  flattened  laterally  ;  branchiae  on  thoracic 
appendages.  Gammarus,  Orchestia,  Corophiumf  Cyainus, 
Caprella. 

LITERATURE. 


OBNEHAL. 

H.  Milne-Edwards.    Histoire  Naturelle  des  Grtutaeea.    Paris,  1884-1840. 

F.  Mttller.    Fur  Darwin.    Leipzig,  1864. 

H.  Oerstaeoker.    Arthropoda.    Bronii's  Elassen  und  Orduungen  des  Thier- 

reichs,  Bd.  v.  Abth.  1.     1866-  (not  yet  completed). 
C.  Claus.     Untermehungen  zur  Erforaehung  der  genealogisehen  Gfrundlage  des 

Grustaceen-Syatems.    Wien,  1876. 
0.  Orobben.    Die  Aniennendriise  der  Crustaceen.    Arbeiten  a.  d.  Zoolog.  Inst. 

Wien,  III.  1880. 


TYPE  CRUSTACEA. 


425 


J.  £.  V.  Boaa.    Studien  iiber  die  Verwandtachaftsbeziehungen  der  Malakoitrdken. 

Morpholog.  Jahrbuch,  viir,  1883. 
J.  Carriere.     Die  Sehm'gane  der   Thiere,  vergleiehend-anatomisch  dargestelU. 

Munich  und  Leipzig,  1885. 
C.   Clans.     Neue  Beitriige  zur    Morphologie  der   Cruataeeen.     ArbeiteD  a.   d. 

Zoolog.  Inst.  Wiiu,  vi,  188C. 
S.  Watase.     On  the  Morphology  of  the  Compound  Eyes  of  Arthropoda.    Studies 

from  the  Biolog.  Laboratory,  Johns  Uopliius  Univ.,  iv,  1890. 
O.  H.  Parker.     The   Compound  Eyes  in   Crustaceans.    Bulletin  Museum  of 

Comp.  Zoology,  xxi,  1891. 
C.  Orobben.    Zur  Kenntniss  des  Stambaumes  und  des  Systems  dor  Orustaceen. 

Sitzungsber.  Akad.  wissensch.  Wien,  ci,  1893. 


■::l 


PHYLLOPODA. 

A.  S.  Packard.     A   Monograph  of  North   American   Phyllopod    Orustacea. 

Twelfth  Annual  Report  U.  S.  Qeolog.  Survey.    Washington,  1883. 
0.  L.  Herrick.     A  Final  Report  on  the  Crustacea  of  Minnesota.     Twelfth  An- 
nual Report  of  the  Geolog.  and  Natural  History  Survey  of  Minnesota. 

Minneapolis,  1884. 
C.  Clans.    Zur  Kenntniss  des  Baues  und  der  Entwicklung  von  Branchijms  stag- 

nalis  und  Apus  cancriformis.    Abhandl.  k.  Akad.  wissensch.  G()ttiugen, 

xvin,  1878. 
A.  Weismann.     Veber  Bau  und  Lebensersch^inungen  non  Leptodora  hyalina. 

Zeitschr.  fttr  wissensch.  Zoologie,  xxiv,  1874. 
Beitriige  zur  Naturgesehichte  der  Daphnoiden.    Zeitschr.  fUr  wissensch. 

•Zoologie,  xxvii-xxxHi,  1876-1879. 
E.  Bay  Lankester.     Observations  and  Reflections  on  the  Appendages  and  on  the 

Nervous  System   of  Apus  cancriformis.     Quarterly  Journ.  Microscop. 

Science,  xxi,  1881. 
C.    Clans.      Untersuchungen   uber   die   Organisation   und   Entwicklung   von 

Branchipus  und  Artemia.     Arbeiten  a.  d.  Zoolog.  Inst.  Wien,  vi,  1886. 
P.  Pelseneer.     Observations  on  the  Nervous  System  of  Apus.    Quarterly  Journ. 

of  Microscop.  Science,  xxv,  1885. 
P.  Samassa.     Untersuchungen  fiber  das  centrale  Nervensystem  der  Crustaceen. 

Archiv  flir  mikr.  Auat.,  xxxviii,  1891. 
O.  M.  Bernard.     The  Apodidce,  a  Moi'phological  Study.     London,  1892. 
C.  Orobben.    Die  Entwicklungsgeschichte  der  Moina  reciirostris,  etc.    Arbeiten 

a.  (1.  Zool.  Inst.  Wien,  ii,  1879. 
P.  Samaasa.    Die  Keimbldtterbildung  bei  den  Cladoceren.    I.  Moina  rectiroatris 
Brady.    Archiv.  filr  mikrosk.  Anat.,  XLi,  1893. 


1:1   \ 


" 


OSTIIACODA. 

0.  E.  Brady.    A  Monograph  of  the  Becenc  British  Ostracoda.    Traoaactions 

Linnsean  Soc.  London ,  xm. 
0.  Clans.     Ueber  die  Organisation  der  Cypridinen.     Zeitschr.  fttr  wissensch. 

Zoologie,  xv.  1865. 
Beitrdge  zur  &nntni88  der  aumocmer  Oatracoden.     Arbeiten  a.  d. 

zoolog.  Inst.  WieD.  zd,  1892. 


426 


JNVEHTEBRATE  MORPHOLOGY. 


COFEPODA. 

C.  L.  Herriok.  A  Final  Report  on  the  Q'uttacea  of  Minnesota.  Twelfth  An- 
nual Report  of  the  Geol.  uud  Nat.  Hist.  Survey  of  Minnesota.  Min- 
neapolis, 1884. 

W.  Oiesbreoht.  Pelagiaclte  Gopepoden.  Fauna  und  Flora  des  Golfes  von 
Neapel.    Monogr.,  xix,  1892. 

C.  ClauB.  Ueber  die  Entwiekiung,  Organisation  und  syatematiacfie  Stellung  der 
Arguliden.    Zeitscbr.  filr  wisseusch.  Zoologie,  xxv,  1875. 

C.  Haider.  Die  Oattung  Lernanthropus.  Arbeiten  a.  d.  zoolog.  Inst.  Wien, 
II,  1879. 

M.  Hartog.  Ihe  Morphology  of  Cyclops  and  tlie  Relations  of  tlie  Copepoda. 
Traus.  Linntean  Soc.  London,  2d  Series,  v,  1888. 

F.  Leydig.     Ueber  Argulus  foliaceus.    Arcbiv.  filr  mikr.  Anat. ,  xxxiii,  1889. 

C.  Orobben.  Die  Entvricklungsgeschichte  von  Cetochilus  septentrionalis,  Ooodsir, 
Arbeiten  a.  d.  zoolog  lust.  Wien,  iii,  1881. 

CIltUHIFBDIA. 

0.  Darwin.    A  Monograph  of  the  Subclass  Cirrhipedia.    London,  1851-1854. 
H.  de  Lneaie-Dnthiers.    Histoire  de  la  Laura  Oerardice.    Archives  de  Zool. 

exp6r.  et.  gen.,  vui,  1880. 
P.  P.  C.  Hoek.    Report  oi.  the  Cirrhipedia.    Scientific  Results  of  Voyage  of 

H.M.S.  Challenger.     Zool.,  viii,  1883;  x,  1884. 
Tves  Delage.    Evolution  de  la  Sacculine.    Archives  de  Zool.  exper.  et.  gen., 

2me  ser.,  ii,  1884. 

LEPTOSTRACA. 

C.  Clans.  Ueber  den  Organismus  der  Nebaliiden,  und  dm  systenatischen 
Stellung  der  Leptostraken.    Arbeiten  a.  d.  Zool.  Inst.  W^n,  viii,  1888. 

SCHIZOPODA.  V 

0.  0.  Bars.  Report  on  the  Schizopoda.  Scientific  Results  of  the  Voyage  of 
H.M.S.  Challenger,    xiii,  1885. 

CUMACEA. 

A.  Dohm.     Ueber  Bau  und  Entuncklung  der  Cumaeeen.    Jenaische  Zeitschr. 

fUr  Naturwiss.,  v,  1870. 
0.  0.  Sara.    Report  on  the  Cumacea.    Scientific  Results  of  the  Voyage  of 

H.M.S.  Cbiilleiiger.     xix,  1887. 

STOMATOPODA. 

C.  Claoi.    Die  Kreislaufsorgane  und  Blutbeicegung  der  Stomatopoden.    Arbeiten 

a.  d.  Zoolog.  Inst.  Wien,  v,  1883. 
W.  K.  Brooks.    Report  on  the  Stomatopoda.    Scientific  Results  of  the  Voyage 

of  H.M.S  Challenger,    xvi,  1886. 

DBCAPOUA. 

8. 1.  Smith.  Various  Papers  in  Trans.  Connecticut  Academy  and  in  Reports 
of  the  U  S.  Commissioner  of  Fish  and  Fisheries. 


TYPE  CRUSTACEA. 


427 


y.  Henaen.      Studien    liber  das  OeJwrorgan   der  Decapoden.     Zeitscbr.   fUr 

wissenscb.  Zoul.,  xiii,  1863. 
C.   Grobben.    Beitriige  zur  Kenntnisa  der   mdnnlichen  Ouchleehtaorgane  der 

Decapoden.    Arbeiteu  a.  d.  Zoolog.  Inst.  Wieu,  i,  1878. 
T.  H.  Huxloy.     Tlie  Crayfish.     London  and  New  York,  1881. 
W.  K.  Brooks.     Lucifer:  a  Study  in  Morphology.     Pbilosoph.  Trans.  Royal 

Soc.  London,  clxxiii,  1882. 
H.  Beiehenbach.     Studien  zur  Enttoicklungsgeschichte  dea  Fluaskrebaea.     Ab- 

baudl.  Senckeuburg.  Gesellsch.  Frankfurt,  xiv.,  1886. 
W.   F.   B.   Weldon.     Calom  and   Nephridia  of  Palcemon  serratua.    Journal 

Marine  Biolog.  Assoc,  i,  1889. 
6.   H.   Parker.     The  Utstology  and  Development  of  tlie  Eye  in  tlie  Lobster. 

Bulletin  Museum  Comp.  Zoolog.,  xx,  1890. 
P.  Marchal.     Rechercliea  anatomiques  et  physiologiquea  sur  I'appareil  excreteur 

des  Cruslaces  decapodea.     Archives  de  Zool.  exper.  et  gen.,  2me  ser.,  x, 

1893. 
W.  K.  Brooks  and  F.  H.  Herriok.     The  Embryology  and  Metamorphosis  of  the 

Macroura.    Proc.  U.  S.  National  Acad.,  v,  1892. 
£.  J.  Allen.    Nephridia  and  Body-cavity  of  some  Decapod  Crustacea.    Quarterly 

Journ.  Microscop.  Science,  xxxiv,  1893. 

ARTBUOSTHACA. 

0.  Harger.  Report  on  the  Marine  Isopoda  of  New  England  and  Adjacent 
Waters.  Report  of  the  U.  S.  Commissioner  of  Fish  and  Fisheries  for 
1878. 

A.  Delle  Valle.     Oammarini  del  Oolfo  diNapoli.    Fauna  und  Flora  des  Golfes 

vou  Neapel.     Monogr.,  xx,  1893. 
P.Mayer.     Caprelliden.    Fauna  und  Flora  des  Golfes  von  Neapel.    Monogr., 

VI  and  XVI,  1883,  1890. 
0.  Nebeski.    Beitriige  zur  Kenntnisa  der  Amphipoden  der  Adria.    Arbeiten  a. 

d.  Zoolog.  Inst.  Wien,  ni,  1890. 

B.  Kossmann.     Studien  iiber  Bopyriden.    Zeitschr.  filr  wissensch.  Zoologie, 

XXXV,  1881 ;  Mitlh.  a.  d.  Zoolog.  Station  zu  Neapel,  iii,  1883. 
Yves  Delage.     Contribution  d  I'etude  de  I'appareil  circulatoire  dea  Crustacea 
idriophthalmes  marins.    Archives  de  zool.  exper.  et  gen.,  ix,  1881. 


APPENDIX  TO  THE  TYPE  CRUSTACEA. 

Order  Xiphosura. 

The  Xiphosura  is  a  group  which  possesses  many  Crus- 
tacean peculiarities,  and  also  many  foreign  to  that  group  and 
more  especially  characteristic  of  the  Araclmida ;  consequently 
it  is  advisable  to  consider  it  as  an  order  by  itself,  intermediate 
between  the  two  types. 

A  single  genus,  lAmulus  (Fig.  195),  with  few  species  consti- 
tutes the  order,  the  members  of  which  are  popularly  known 


I 
I 


\'\\ 


W 


'■ 


'  i  ^ 


f.t  . 


!  1 


fis 


III 


|!l|  |! 


428 


INVERTEBRATE  MORPHOLOGY. 


as  King-crabs  or  Horseshoe-crabs.  They  are  large  forms 
measuring  a  foot  or  so  in  diameter,  and  the  body  is  composed 
of  three  portions.     The  anterior  is  a  broad  semicircular  ceph- 


PiG.  I%b.—Limulu8  polypJiemus,  Fbmalb,  from  the  Ventral  Surface. 

ab  =  abdomen  cp  =  cepbalotborax, 

an  =  anus.  ol  =  olfactory  organ. 

ch  =  cbelicera.  op  =  operculum. 

ehi  =  cbilariuin.  sp  =  spiue. 

alothorax  (cp),  prolonged  backwards  into  sharp  points  at  its 
posterior  angles  and  bearing  upon  its  dorsal  surface  a  pair  of 
compound  eyes  towards  the  sides  and  near  the  median  line 
two  simple  eyes.     The  middle  region  is  the  abdomen  {ab), 


TYPE  CRUSTACEA. 


429 


showing  but  faint  indication  of  segmentation,  and  bearing  on 
its  terminal  segment  the  anus,  behind  which  is  a  long  mova- 
ble spine  {sp\  the  post-abdomen,  forming  the  third  region 
and  to  be  regarded  probably  as  a  movable  prolongation  of 
the  dorsum  of  the  last  abdominal  segment. 

The  cephalothorax  bears  seven  pairs  of  appendages.  The 
first  pair,  the  chelicerw  (Fig.  195,  eh),  which  lie  in  front  of  the 
mouth,  are  small  and,  like  the  following  four  pairs,  are  chelate. 
These  together,  with  the  sixth  are  much  longer  and  surround 
the  mouth,  their  basal  joints  beiug  provided  with  strong  bris- 
tles and  serving  as  jaws.  The  sixth  pair  of  appendages  difter 
from  their  predecessors  in  not  being  chelate  and  in  possessing 
upon  their  basal  joints  a  peculiar  process  which  has  been 
termed  the  flabellum  and  by  some  is  regarded  as  representing 
an  exopodite  The  seventh  pair  of  appendages  is  very  differ- 
ent from  the  others,  forming  a  broad  fiat  plate,  the  two 
appendages  of  the  opposite  sides  meeting  in  the  middle  line. 
This  plate  covers  in  the  abdominal  appendages  to  a  certain 
extent  and  hence  is  termed  the  operculum  {op).  The  abdom- 
inal appendages,  of  which  there  are  five  pairs,  resemble  the 
operculum  in  form,  and  like  it  allow  an  external  larger  exopo- 
dite and  an  inner  smaller  endopodite  to  be  distinguished. 
They  carry  upon  their  posterior  surfaces  series  of  large  leaf- 
like, thin-walled  folds  which  function  as  brauchijB. 

The  heart  (Fig.  196,  ht)  is  an  elongated  tubular  organ  lying 
in  the  posterior  part  of  the  cephalothorax  and  the  anterior 
part  of  the  abdomen,  and  possesses  eight  ostia.  Arteries 
arise  from  it  which  carry  the  blood  to  various  parts  of  the 
body,  eventually,  however,  opening  into  the  general  lacunar 
system.  The  blood  has  a  distinct  bluish  color  which  deepens 
on  exposure  to  the  air  and  is  due  to  a  copper-containing 
respiratory  pigment,  hsemocyanin. 

The  body  is  enclosed  in  a  hard  chitinous  cuticle,  and  in 
addition  a  peculiar  fibro-cartilaginous  plate,  the  endosternite, 
is  found  in  the  cephalothorax  between  the  intestine  and  the 
nervous  system.  It  is  formed  by  the  fusion  of  a  number  of 
tendons  and  may  be  regarded  as  an  endoskeleton. 

The  mouth  is  an  elongated  opening  lying  between  the 
bases   of  the   anterior   cephalothoracic    appendages   and   is 


I! 

I 


I) 


M 


11     '1  i  '■ 


430 


INVERTEBRATM  MORPHOLOGY. 


bounded  behind  by  a  pair  of  processes  which  represent  a 
lower  lip  and  are  known  as  the  chilaria  (Fig.  195,  chi).  The 
OBSophagus  passes  upwards  and  forwards  and  dilates  into  a 
large  proventriculus  (Fig.  196,  pr)  iu  the  front  part  of  the 
ceplialothoracic  shield,  and  this,  bending  upon  itself  and 
constricting  again,  opens  into  the  stomach  (s),  from  which  the 
intestine  {i)  passes  straight  back  to  open  on  the  ventral  sur- 
face of  the  body  at  the  base  of  the  terminal  spine.  The  inner 
wall  of  tlie  hind-gut,  cesophagus,  and  proventriculus  is  lined 
by  chitin,  which  in  the  last-named  structure  is  thrown  into 


vn 


Pig.  196. — Longitudinal  Section  through  a  young  Limulus  polyphemua, 

DiAGUAMMATIC  (after  Packard). 

ce  =  cerebral  giiuglion.  I  =  liver.  ( 

ht  =  heart.  jn'  =  proven  triculus^; 

i  =  iutestiue.  s  =  stomacb. 

vn  =  ventral  nerve-cord. 

folds  and  recalls  the  masticatory  apparatus  in  the  stomach 
of  the  Decapodous  Crustacea.  Into  the  stomach  there  open 
the  ducts  of  two  pairs  of  voluminous  digestive  glands  {1} 
which  occupy  the  greater  portion  of  the  cephalothorax  and 
are  much  branched  greenish  structures. 

The  nervous  system  consists  of  a  syncerebrum  (ce)  com- 
posed apparently  of  three  pairs  of  ganglia.  It  lies  in  front  of 
the  oesophagus,  sending  branches  to  the  compound  and  simple 
eyes.  Behind  the  oesophagus  and  united  with  the  syncere- 
brum by  circumoesophageal  connectives  comes  a  series  of 
seven  pairs  of  ganglia  closely  approximated,  the  first  pair 
innervating  the  chelicerre  and  the  remaining  six  the  other 
thoracic  limbs  in  succession.  A  chain  of  six  pairs  of  ganglia 
lying  in  the  abdomen  is  connected  with  the  cephalothoracic 
series  and  innervate  the  abdominal  appendages. 


iif 


TYPE  CRUSTACEA. 


431 


As  already  noticed,  a  pair  of  simple  eyes  are  borne  upon  the 
dorsal  surface  of  the  carapace,  one  on  each  side  of  the  mediuu 
line,  while  a  pair  of  larger  conipuuud  eyes  are  situated  lat- 
erally. The  structure  of  these  compound  eyes  is  ])eculiar 
(Fig.  197).  Over  their  surface  the  cuticle  is  considerably 
thickened  and  shows  upon  the  outer  surface  no  indication  of 
corneal  facets,  but  its  inner  surface  is  prolonged  into  a  num- 
ber of  papillae  (I)  each  one  of  which  projects  into  a  depression 
of  the  ectoderm.    At  the  bottom  of  each  depression  is  a  bulb- 


PiG.  197.— Compound  Eye  op  Limulus  polyphemus,  Two  Ommatidia  (after 

Watase). 

c  =  centnvl  cell.  ms  =  niesoderm. 

I  =  lens.  opn  =  optic  uerve. 

rt  =  retiuula. 

like  structure  composed  of  a  number  of  cells  arranged  in  a 
circle  and  constituting  a  retinula  (rt),  the  lower  ends  of  the 
cells  being  continued  inwards  to  form  part  of  the  optic  nerves 
{opn).  Upon  the  face  which  is  turned  towards  its  fellow  each 
retinular  cell  secretes  a  layar  of  chitin,  and  these  various  chit- 
inous  rods  being  in  contact  there  is  formed  a  structure  com 
parable  to  the  rhabdom  of  the  Crustacean  eye.  In  the  centre 
of  the  retinular  cells  and  below  the  rhabdom  is  a  single  clear 
cell  (c)  whose  lower  end  is  also  prolonged  into  a  nerve-fibre. 
Each  depression  with  its  retiuula  and  the  chitinous  pn[)i]la 
which  fits  into  it  and  represents  its  cornea  is  an  ommatidium, 
and  the   development  shows  that  the   ommatidia  arise   as 


' 


m 


■^ 


I 


432 


INTERTEBRA  TE  MORPUOLOO  Y. 


number  of  separate  iuvaginations  of  the  ectoderm,  the  sides 
of  the  retiuuhir  cells  which  secrete  the  rhabdom  beiug  iu 
reality  those  sides  which  before  iuvagiuatiou  were  at  the 
surface  of  tlio  body,  and  the  rhabdom  aiay  therefore  be 
rej^arded  as  composed  of  portions  of  the  general  cuticle  which 
have  been  separated  by  the  invagination. 

On  the  under  surface  of  the  carapace  in  the  median  line 
in  front  of  the  chelicerio  is  a  small  tubercle  (Fig.  195,  ol) 
which  contains  an  organ  supposed  to  be  olfactory  in  function, 
and  probably  some  of  the  setre  upon  the  basal  joints  of  the 
limbs  may  also  possess  a  similar  function. 

Nephridia  are  represented  by  a  single  pair  of  large 
reddish  bodies  lying  at  the  sides  of  the  cephalothorax.  They 
have  no  communication  with  the  exterior  in  the  adult,  but  in 
the  early  stages  of  development  open  upon  the  basal  joint  of 
the  fifth  appendage,  and  are  at  first  tubular  organs  ami 
nephridialike,  later  becoming  much  contorted  and  complex. 
What  their  function  in  the  adult  may  be  is  uncertain,  aud  to 
avoid  possible  misconceptions  it  seems  preferable  to  speak 
of  them  as  coxal  glands,  a  term  indicating  their  original  point 
of  opening  on  the  basal  joints  (coxbb)  of  one  of  the  pairs  of 
limbs. 

The  Xiphosura  are  bisexual,  the  genital  ducts  opening  on 
both  males  and  females  on  thf  posterior  surface  of  the  oper- 
culum near  its  base.  The  ovaries  are  much  branched  paired 
structures,  the  various  branches  frequently  anastomosing 
even  across  the  median  line.  Tho  testes  are  numerous 
spherical  bodies  scattered  through  the  body  aud  situated  on 
branching  and  anastomosing  vasa  deferentia. 

Development  and  Affinities  of  the  Xiphosura. — When  the 
young  /Amulus  leaves  the  egg  it  presents  i  remarkable  resem- 
blance to  a  Trilobite  and  suggests  a  possible  •ftiuity  with 
these  forms  which  are  known  to  occur  >nl  in  the  Palaozoic 
rocks.     In  these  same  rocks  there  o  Uso  the  remains  of 

forms  known  as  the  Eurypterida>  w  u  seera  to  luive  been 
even  more  nearly  related  to  Limidus  than  wf^re  the  Trilobites. 
In  them  the  cephalothorax  bore  ai)parently  only  six  pairs  of 
appendages  which  resembled  more  or  less  closely  those  of 
Limidus,  except  that  the  sixth  pair  was  broad  and  oarlike, 


TYPE  CRUSTACEA. 


433 


probably  aerviug  for  swimming.  The  abdomen  was  com- 
posed of  twelve  segments,  the  anterior  six  of  which  were 
much  more  massive  than  the  others  and  bore  five  pairs  of 
platelike  appendages  on  whoso  posterior  surface  were  the 
branchijB.  The  terminal  segment  bore  a  spine  or  tinlike 
structure.  Such  a  form  as  this,  represented  by  the  genus 
iLhirypterus  (Fig.  198),  presents  strong  similarities  to  Limulus 
and  also  to  the  Scorpions,  bearing  out 
the  numerous  similarities  of  structure 
occurring  between  Limidm  and  those 
forms.  This  side  of  the  affinity  may 
be  postponed,  however,  until  the 
next  chapter,  and  the  comparison  of 
Limidus  with  the  Crustacea  discussed 
here.  Its  chitiuous  cuticle,  its  jointed 
and  biramous  appendages,  and  its 
branchial  respiration  show  similari- 
ties to  the  Crustacea,  as  do  also  the 
form  of  the  heart  and  the  compound 
eyes.  Whether  or  not  the  coxal 
gland  is  comparable  to  the  shell- 
gland  is  at  present  uncertain,  but  the 
other  similarities  are  sufficient  to 
justify  the  recognition  of  a  Crusta- 
cean origin  for  Limvlus.  It  iov\x\H  Y\Q.\9Q--Eurypteru8remipe8 
indeed  a  connecting  link  between  the  (from  Nicholson). 

Crustacea  and  t.'ie  Arachnida,  presenting  probably  on  the 
whole  more  affinities  with  this  latter  group  than  with  the 
former. 

Since,  however,  a  Crustacean  ancestry  is  probable,  a  com- 
parision  of  the  appendages  of  Limulus  with  those  of  a  repre- 
sentative of  the  ancestral  group  ouguu  *o  be  possible.  It  has 
already  been  noticed  that  the  brain  of  Limulus  is  a  syncere- 
brum  composed  of  three  segments ;  it  represents,  therefore, 
two  segments  of  which  the  appendages  and  other  parts  have 
disappeared.  Furthermore,  recalling  that,  in  the  higher 
Crustacea  at  leafjtj  a  ganglion  occurring  between  the  cerebral 
antennary  ganglia  in  the  embryo  indicates   a  lost   pair    of 


■:.i' 


*■; 


Ilr 


434  INVERTEBRATE  MORPHOLOGY. 

appendages  in  these  forms,  the  following  table  may  represent 
the  homologies  of  the  appendages  of  the  two  groups. 

Crustacean.  Limulus. 

1  segment no  appendage  no  appendage 

8  **  antenuules  "  " 

4  **  auteuujc  chelicera) 

5  " mandibles  Ist  pair  of  legs 

6  **  1st  raaxillffl  2d     "     "     " 

7  "  2d      "  3d     "      "     " 

8  "  1st  thoracic  appendages    4th  "      "     " 

9  "  2d        "  «*  5th  "     "     " 

10  "  3d        "  "  operculum 

LITERATURE. 

A.  Oerstaecker.    Cruataceen.  Bronn's  Eltiaaeu  uud  Ordtiungcn  des  Thierreicbs, 

B(l.  V.  t.  Abth.,  1866-79. 
A.  S.  Packard.     The  Anatomy,  Histology,  and  Embryology  of  Limulus  poly- 

phemus.     Memoirs  Boston  Soc.  Nut.  History,  1880. 
£.  B.  Lankester.     Limulus   an  Arachnid.    Quarterly  Jouru.   Microscopical 

Scieucc,  XXI,  1881. 
8.  Watase.     On  the  Morphology  of  the  Compound  Eyes  of  Arthropods.    Studies 

from  the  Biol.  Lab.  Johns  Hopkins  Univ.,  iv,  1890. 
W.  Fatten.     On  the  Origin  of  Verlebraten  from  Arachnids.    Quarterly  Jouru. 

Microscop.  Science,  x.xxi,  1890. 
J.  8.  Kingsley.     Tlie  Embryology  of  Limulus.    Journ.  of  Morphology,  vii, 

1893  ;  VIII,  1893. 
W.  Fatten.     On  the  Morphology  and  Physiology  of  the  Brain  and  Sense-organs 

of  Limulus.    Quarterly  Jouru.  Microscop.  Science,  zxxv,  1893. 


^;i 


:  i 


TYPE  AliACUNlDA, 


435 


CHAPTER  XIV. 


TYPE  ARACIINIDA. 


1 


The  Aracbuida  are  esHeiitiully  terrestrial  forms,  for  though 
a  few  species  lead  au  aquatic  or  mariue  life,  they  are  evi- 
dently desceudauts  of  forms  which  led  a  terrestrial  existence 
and  have  only  secondarily  acquired  the  power  of  living  under 
water.  In  all  members  of  the  group  the  body  is  covered  by 
a  more  or  less  thick  chitiuous  cuticle  and  the  appendages  are 
as  a  rule  jointed. 

A  characteristic  feature  of  the  group  is  the  fusion  of  the 
head  and  thorax  to  form  an  unseguiented  cephalothorax  bear- 
ing usually  six  pairs  of  limbs.  The  first  pair  of  these  are 
the  cheUcenv  (Fig.  2(>1,  cA),  composed  of  one  to  tlirce  joints 
and  terminated  either  b}'  a  claw  or  a  chela ;  they  lie  in  front 
of  the  mouth,  which  is  bounded  at  the  sides  by  the  basrJ 
joints  of  the  second  pair  of  a])pendages,  the  pedipalps  (p), 
which  may  be  long  and  limblike,  or  chelate,  or  in  some  cases 
clawlike,  their  basal  joints  serving  in  all  cases  as  mandibles. 
Behind  these  follow  four  pairs  of  legs  composed  of  six  or  seven 
joints,  the  basal  joint  Ixuiig  termed  the  co.rn,  the  next,  usually 
short,  the  troclhrnfn',  tl»«^  third  the  femur,  the  next  two  to- 
gether form  the  tibin,  iXx&n  follows  in  some  forms  a  tncfatdrsus, 
while  the  terminal  one,  provided  with  two  claws,  toniied 
HtKjtu's,  and  in  some  mites  also  with  a  suctori'il  disk,  consti- 
tutes the  fiirfins.  Variations  from  this  structure  of  course 
occur,  the  chelicerje,  for  exam])le,  in  some  mites  being  re- 
duced to  short  stylets, antl  in  otheis  the  two  posterior  pairs  of 
legs  maybe  quite  rudimentary  (/*lii/iopfm).  Tho  most  iuqjor- 
tant  variation  is,  however,  that  found  in  the  members  of  the 
order  Solifugje,  in  which  a  heavl  is  distinctly  marked  off  from 
a  thorax  composed  of  three  segments. 

The  abdomen  in  some  forms  is  segmented,  in  others  all 
trace  of  the  segmentation  is  lost,  and,  finally,  in  the  Mites  it 


'i! 


i  IF 


436 


INVEUTEBRATE  MORPUOLOOY. 


>r 


!   IE 


may  be  united  with  the  ceplialotLorax.  lu  the  Scorpions  it 
is  divisible  into  an  anterior  portion,  the  prseabdoraen,  much 
broader  and  stouter  than  the  posterior  postabdomen,  an  ar- 
rangement also  indicated  in  certain  other  forms.  In  the 
adults  the  abdomen  is  usually  destitute  of  appendages,  though 
they  may  be  present  in  tlie  embryos ;  the  Scorpions,  however, 
possess  two  highly-modified  pairs,  and  it  seems  probable  that 
the  four  or  six  papilho  ui)on  which  the  ducts  of  the  spinning- 
glands  open  in  the  Spiders  represent  also  modified  append- 
ages. 

A  special  respiratory  system  is  entirely  wanting  in  a  few 
forms.  In  the  majority  there  occur  on  the  sides  of  the  body 
from  one  to  four  pairs  of  pores  termed  stigmata  (Fig.  201,  st^*). 
In  the  Scorpions  and  some  other  forms  each  stigma  opens 

into  a  cavity  lined  with  chitiu 
continuous  with  that  which  covers 
the  general  surface  of  the  body, 
and  into  this  cavity  there  project 
a  number  of  lamellae  arranged 
like  the  leaves  of  a  book  (Fig. 
199),  whence  the  term  lung-books 
frequently  applied  to  them.  Each 
lamella  is  hollow,  trabecuh«  ex- 
tending across  the  cavity  trom 
one  wall  to  the  other,  and  tho 
cavities    communicate    with    the 

fpVft:.^*'  ____,^««-***=-rt«''       crelomic  lacumc,  so  that  blood  can 

V|^«rm^.-r:;v.-Trrr¥?i^^  readily   flow    into    them    and    so 

Fm.  109.-TUAN8VEU8K  SKCTr«N«^^"?«  ^<^«  «'^«^«  ihvoM^Xx  the  thin 
TimoiioH  Tuifi  LuNo-BooK  OF  A  walls  of  the  lamelhc.  In  other 
Sim iJKK  (after  McLkod).  cases  there  occurs  in  connection 

eh  =  chitinogenous  tissue.  ^'^^\^  ^\^q  lung-book  apparatus,  or 

,  ,'  1      11  else   entirely  replacmg   it,  a  tra- 

Ip  =  pulmonary  lamella.  -^        *  .     . 

st  =  stigma.  cheal  system  consisting  of  a  num- 

t  =  lust  compart nient    of  lung-  ber  of  tubes  ramifying   through 

book,  tiacbeulike  in  cbar-  the  body.     In  some  cases  a  strong 

^^^^'  tube   or   trachea   arises   at  each 

stigma  and  traverses  the  body,  giving  otf  branches  to  all  parts 

i>s  it  goes;  in  others  there  is  in  connection  with  each  stigma 


TYPE  ARACHNID  A. 


437 


a  bunch  of  uubrancbed  tracbese,  and  all  gradations  between 
these  two  conditions  occur.  The  trachef©  are  lined  with 
chitin,  which  is  sometimes  thickened  to  form  rings  or  spiral 
bands  which  serve  to  keep  the  lumen  of  the  tubes  open  and 
thus  permit  a  free  passage  of  air  into  them. 

The  coelom  is  tilled  for  the  most  part  with  the  various 
organs  and  is  reduced  to  a  series  of  lacunar  spaces  containing 
blood,  sometimes  rich  in  hiemocyanin  and  assuming  a  blue 
color  when  oxygenated.  A  heart  is  wanting  in  some  Mites, 
but  is  present  in  the  majority  of  forms,  varying  from  a  saclike 
organ  with  a  single  pair  of  ostia  guarded  by  valves  to  an 
elongated  cylinder  with  as  many  as  eight  pairs  of  ostia  (Scorpi- 
ons). It  is  for  the  most  part  situated  in  the  abdominal  region, 
and  in  the  Spiders  is  enclosed  within  a  space  with  definite 
walls  which  is  termed  the  pericardium,  though  it  cannot  be 
considered  homologous  with  the  pericardium  of  the  Mollusca, 
since  it  contains  blood ;  muscle-bands  extend  from  it  to  the 
walls  of  the  body  and  by  their  contraction  cause  its  expan- 
sion, fibres  in  its  wall  diminishing  its  cavity  and  forciug  the 
blood  through  the  ostia  into  the  heart.  Arteries  in  many 
forms  arse  from  the  heart,  but  after  usually  a  short  course 
open  into  the  lacunar  ca^lom. 

The  digestive  tract  pursues  a  more  or  less  straight  course 
through  the  body,  but  shows  a  tendency  to  develoj)  cdjcal  out- 
growths which  sometimes  reach  a  considerable  size.  The 
anterior  and  posterior  portions  of  the  tract  are  ectodermal, 
while  the  middle  region  or  mid-gut  is  endodermal  and  is  the 
portion  with  which  the  cojca  are  connected.  In  the  Scorpi- 
ons the  ducts  of  a  digestive  gland  open  into  the  mid-gut,  and 
in  many  forms  there  is  connected  with  the  jjosterior  portion 
of  this  same  region  a  pair  of  tubular  Mali)ighian  vessels 
which  are  presunuibly  excretory  in  function  and  recall  the 
similar  structures  of  the  Amphipoda.  The  end-gut  is  fre- 
quently dilated  into  a  large  bladderlike  structure,  the  rectal 
bladder. 

The  nervous  system  consists  of  a  suprauesophageal  syn- 
oerebrum  composed  of  three  pairs  of  ganglia  fused  together, 
and  in  some  forms  even  four  pairs  may  be  included,  since  the 
chelicerte  may  be  innervated  from  the  mass,  their  ganglia  in 


t  ' 


I     ' 


!ii; 


II 


438 


INVEHTEBRA  TE  MORPHOLOG  Y. 


embryonic  life  being,  however,  distinct  and  postoral  in  posi- 
tion, only  later  moving  forward.  The  succeeding  ganglia  are 
generally  more  or  less  fused,  and  indeed  in  some  forms  all 
the  ganglia  of  the  limb-bearing  segments  of  the  cephalothorax 
may  be  united  with  those  of  the  abdominal  region  to  form  a 
single  ganglionic  mass.  In  some  forms  a  single  ganglion 
occurs  behind  this  mass  at  the  junction  of  the  cephalothorax 
and  the  abdomen,  and  in  the  Scorpions  the^e  is  posteriorly  a 
ventral  nerve-cord  with  seven  pairs  of  ganglia,  the  anterior 
pair  corresponding  with  the  fifth  abdominal  segment.  A 
sympathetic  nervous  system  occurs  in  the  Scorpions,  Spiders, 
and  Harvest-spiders,  consisting  of  a  nerve  arising  from  the 
syncerebrum  and  passing  to  the  digestive  tract. 

Hairs  situated  upon  the  body  and  appendages  serve  as 
sense-organs  of  touch  and  apparently  also  of  audition,  since 
Spiders  are  sensitive  to  air-vibrations  and  possess  no  definite 
auditory  organs.  Eyes  are  very  generally  present  and  vary 
considerably  in  number,  there  being  in  the  Spiders  three  or 
four  pairs ;  in  the  Scorpions,  in  which  there  are  from  two  to 
six  pairs,  one  pair  become  closely  approximated  on  each  side 
of  the  niid-liue  and  recall  the  median  eyes  of  Limulus,  while 
the  remaining  pairs  are  situated  more  to  the  sides  of  the 
cephalothorax.  In  structure  the  median  eyes  differ  from  the 
lateral  ones ;  the  chitinous  cuticle  is  thickened  over  them  to 
form  a  simple  unfaceted  lens  below  which  lies  a  layer  of 
transparent  cells  continuous  with  the  general  ectoderm  (hypo- 
dermis)  of  the  body  and  which  may  be  termed  the  corneal 
hypodermis,  though  more  usually  they  are  known  as  the 
vitreous  cells.  Below  them  comes  the  retina,  consisting  of  a 
single  layer  of  elongated  cells  with  their  nuclei  situated 
toward  their  inner  ends,  with  which  the  fibres  of  the  optic 
nerve  come  into  connection.  The  various  retinal  cells  are 
arranged  in  groups  of  five  (retinuhe),  which  secrete  a  thin 
chitinous  rod  upon  their  contiguous  faces,  producing  thus  a 
rhabdom  composed  of  five  i)arts.  Behind  the  retina  is  a  thin 
layer  of  cells,  the  postretiuular  layer,  and  numerous  pigment- 
cells  occur  between  the  various  retinulro.  The  lateral  eyes 
are  courV'ucted  upon  a  very  different  plan,  lacking  a  corneal 
hypodermis  between  the  retina  and  the  cornea.     They  are 


TYPE  AliACHNIDA. 


439 


ut- 
es 
eal 
ire 


cup-shaped  structures,  the  cavity  of  the  cup  being  filled  by 
the  cuticular  cornea  and  its  wall  transformed  into  the  retina, 
which  is  continuous  at  the  margins  of  the  cup  with  the  gen- 
eral hypodermis.  The  retinal  cells  are  of  two  kinds,  viz., 
large  sensory  cells  and  smaller  interstitial  cells.  Each  sen- 
sory cell  is  surrounded  by  pigment  and  bears  upon  its  lateral 
walls  a  chitiuous  secretion  which,  with  the  corresponding 
secretion  of  contiguous  cells,  forms  a  rhabdom.  The  nuclei 
of  the  cells  are  situated  nearer  their  inner  than  their  outer 
ends,  and  behind  them  in  EiLScorpius  highly  refractive  spheres 
occur  imbetlded  in  the  cells,  constituting  what  have  been 
termed  the  phaospheres.  Upon  its  inner  wall  the  retina  is 
lined  by  a  basement-membrane  continuous  with  that  lying 
below  the  general  hypodermis  and  perforated  by  the  fibres  of 
the  optic  nerves  which  come  into  contact  with  the  inner  ends 
of  the  sensory  cells. 

In  the  Spiders,  in  which  there  are  six  or  eight  eyes  arranged 
upon  the  dorsum  and  sides  of  the  cephalothorax,  the  ante- 
rior dorsal  pair  difiers  in  structure  from  the  remaining  ones. 
In  both  forms  of  eyes  (Fig.  200)  the  cuticular  cornea  (c)  rests 


Fig.  200.— Eyks  of  Simuek.    A,  Antkkioh,  and  /?,  Postkuiou  Eye  (after 

Hkrtkau  from  Kokm^hklt  mul  IlKiiiKit). 
b  =  rods.  r  =  relinii. 

I  =  lens.  t  =  tapetuui  liicidum. 

V  =  vitreous  layer. 

upon  a  corneal  hypodermis,  (the  vitreous  cells,  v),  but  the 
arrangement  of  the  retina  differs  greatly.  In  the  anterior 
dorsal  pair  (A)  it  is  composed  of  a  layer  of  elongated  cells  (r) 
whose  nuclei  are  situated  towards  their  inner  ends,  while  near 


440 


IN  VERTEBRA  TE  MORPHOLOQ  Y. 


the  outer  ends  are  situated  a  number  of  rodlike  bodies  (rhab- 
doins,  />),  whence  these  eyes  have  been  termed  prebacillar ; 
the  uerve-tibres  are  coutiuuoua  witli  the  inner  ends  of  the  cells. 
In  the  posterior  dorsal  and  lateral  eyes  (/?)  an  inversion  of 
the  retina  (r)  has  taken  place,  so  that  the  rods  (6)  are  situated 
at  the  apparently  inner  ends  of  the  cells  and  the  nuclei  at 
their  outer  ends,  whence  the  term  postbacillar  applied  to 
these  eyes.  The  optic  nerve-fibres  enter  at  the  sides  of  the 
eye  and  are  distributed  to  the  nuclear  ends  of  the  retinal 
cells,  recalling  the  arrangement  occurring  in  Pecten  among  the 
Mollusoa.  The  innermost  layer  of  the  eye  upon  which  the 
ends  of  the  rods  rest  is  cellular,  numerous  minute  crysti^ls 
being  deposited  in  the  cells,  whence  it  has  the  function  of  a 
reflector  and  is  termed  the  tapetwii  {t).  It  is  quite  wanting  in 
the  prebacillar  eyes. 

The  significance  of  the  structure  of  the  Arachnid  eye  may  be  under- 
stood by  supposing  it  to  have  been  derived  from  a  compound  eye  simihir  to 
that  of  TAmuhis  (see  p.  431),  the  individualities  of  the  various  ornniatidia 
being  more  or  less  subordinated.  The  cuticular  cornea  in  fJnmlus  is 
smooth  upon  its  outer  surface,  the  inner  surface  being  produced  into 
jiapillffi,  one  of  which  corresponds  to  each  ommatidium.  In  the  Arachnids 
even  these  i)apilla  are  wanting,  the  cornea  showing  no  evidence  of  the 
presence  of  omnuitidia.  The  hiteral  eyes  of  the  Scorpions  approach  more 
nearly  in  their  general  structure  the  eyes  of  TJinulns,  (hough  the  conden- 
sation of  the  ommatidia  has  been  cirried  further  than  in  the  median  eyes 
of  that  form,  or  in  the  posterior  dorsal  and  lateral  eyes  of  the  Spiders.  But 
ill  these  eyes  the  condensation  is  associated  with  an  invagination  of  the  en- 
tiro  eye,  a  process  which,  it  may  be  remarked,  is  indicated  in  tiie  median 
eyes  of  Limiting.  Tliis  invagination  has  been  regarded  as  a  pushing  in, 
under  and  parallel  to  the  hypodermis,  of  a  pouch  of  that  layer,  a  process 
which  gives  in  cross-section  the  appearance  of  an  S-shaped  fold.  Tiie 
outernuxst  layer  of  the  fold  forms  the  vitreous  ci^lls  or  corneal  liypodermis, 
tlie  middle  layer  the  retina,  tiie  inversion  of  which  is  plainly  seen  in  the 
posterior  dorsal  and  lateral  eyes  of  the  Spiders,  while  the  innermost  layer 
forms  the  post  retinal  layer  in  the  Scorpions  and  the  tapetum  of  the 
Spiders.  The  ominatidial  retinulat  are  mon*  or  less  retained  in  these  eyes, 
as  is  shown  by  tlu*  structure  of  the  rhabdom,  which  in  the  Scorpions  is 
composed  of  tivo  parts,  in  the  Spiders  of  two,  and  in  the  Harvest -spiders 
of  three.  The  anterior  dorsal  eyes  of  the  Spiders  do  i.ot  seem  to  have 
undergone  an  invagination,  hence  the  absence  of  a  tapetum  and  th(^  pnuba- 
cillar  structure  of  the  retina;  a  corneal  hypodermis  is,  however,  present, 
and  woidd  seem  to  indicate  an  invagination,  but  its  mode  of  origin  .scinis 
at  present  but  imperfectly  understood.    If  a  generalization  is  to  be  made,  it 


TYPE  ARACUNIDA. 


441 


will  be  to  the  efftjct  that  the  eyes  of  the  Aruohiiids  have  been  derived  from 
compound  eyes  similar  to  those  of  Limtdus.  and  tliat  in  the  median  eyes  of 
the  Scorpions,  and  tlie  posterior  <lorsal  and  hiteral  (syos  of  the  Spiders  the 
entire  optic  area  has  been  invaginated,  making  them  comparable  to  the 
median  eyes  of  Limidus,  while  the  lateral  t-yes  of  the  Scorpions  ami  tiie 
anterior  dorsal  eyes  of  the  Spiders  have  not  undergone  invagination  and 
hence  are  comparable  to  the  lateral  eyes  of  Litntdu.s.  Whether  the  com- 
parability indicates  also  the  homology  from  a  phylogenetic  standpoint  of 
eye  to  eye  must  remain  at  present  uncertain  (see  p.  457). 

In  additiou  to  the  Malpliigliiaii  tubules  iilready  meutioned 
as  excretory  oijjfaus  occurriii'^  in  couueetion  with  tlie  digestive 
tract  of  the  Spiders,  there  exist  in  many  forms  additional 
glands  wliich  probably  are  also  excretory  in  funeti»)u  or  sig- 
uiticauce.  These  are  the  coxal  glands,  so  called  on  account  of 
their  openings  when  present  being  on  the  basal  joints  (coxje) 
of  one  of  the  pairs  of  legs.  In  the  Scorpions  and  Spiders 
the  ducts  of  the  glands  open  on  the  third  pair  of  legs  (i.e.,  the 
fifth  pair  of  appendages)  in  the  embryo,  but  are  usually 
wanting  in  the  adults.  In  the  Solifugte  and  Harvest-spiders 
coxal  glands  also  occur  in  connection  with  the  fourth  pair 
of  legs,  and  similar  glands  have  also  been  observed  in  several 
genera  of  Mites,  opening,  however,  at  varying  points. 

Glands  are  also  of  frequent  occurrence  in  connection  with 
the  pedipalps,  having  apparently  varying  functions  in  differ- 
ent genera.  They  do  not,  however,  seem  to  belong  to  the  same 
category  as  the  coxal  glands  and  are  in  no  case  excretory. 

The  Arachnida  are  bisexual  throughout.  The  ovaries  not 
infrequently  fuse  to  form  a  single  mass  or  a  circular  band, 
and  in  connection  with  the  oviducts,  which  are  in  direct  com- 
munication with  them,  there  is  usually  d«neloped  a  receptac- 
ulum  semiuis,  and  in  the  Harvest-K))id('rs,  an  elongated  ovi- 
]>ositor.  The  testes  are  also  freijuently  fused,  and  the  vasa 
deferentia  are  provided  with  vesicuhi'  seiuinales  and  usually 
terminate  in  a  co})ulatory  organ.  The  majority  of  forms  are 
oviparous,  exceptions  to  the  rule  being  found,  however,  as  in 
the  genus  Phrynns  nm\  in  the  Scorpions,  which  are  viviparous. 


1.  Order  Scorpionida. 

In  the  Scorpions  (Fig.  201)  the  bod}'  is  composed  of  an 
unsegmented    cephalothorax    and   an    elongated   segmented 


!  ;L 


442 


INVERTEBRATE  MORPHOLOGY. 


abdomen.  The  seveu  anterior  segments  (the  praeabdomen) 
of  the  abdomen  are  broader  and  thicker  than  the  remaining 
five  segments  (the  postabdomen),  the  last  one  of  which  ter- 
minates in  a  curved  stout  sjjine  which  bears  at  its  extremity 
the  openings  of  two  ducts  leading  from  a  pair  of  glands  lying 
in  the  twelfth  abdominal  segment  and  secreting  a  poisonous 
fluid. 

The  chelicerju  {ch)  are  small  chelate  appendages  situated 
in  front  of  the  mouth,  while  the  pedipalps  {pe)  are  long  and 

provided  with  strong  chehe,  their 
basal  joints  and  those  of  the  two 
succeeding  appendages  surround- 
ing the  mouth  and  serving  as  jaws. 
The  four  pairs  of  appendages  be- 
hind the  pedipalps  are  all  similar 
in  form,  being  six-jointed  walking- 
limbs.  Upon  the  abdomen  modi- 
fied appendages  are  also  found, 
the  second  abdominal  segment 
bearing  a  pair,  each  member  of 
which  consists  of  a  single  joint 
whose  posterior  edge  is  beset  with 
a  number  of  processes  which  give 
it  the  appearance  of  a  comb, 
whence  the  name  pectiues  [pt) 
applied  to  these  appendages.  In 
front  of  the  pectines  lies  the  geni- 
tal opening,  protected  by  a  small 
genital  operculum  {op)  which  may 
possibly  represent  another  pair  of 
appendages  belonging  to  the  first 
abdominal  segment. 

Upon  the  ventral  surfaces  of 
third,  fourth,  fifth,  and  sixth  abdominal  segments  elongated 
pores  are  to  be  found  which  are  stigmata  (s/'"')  leading  into 
the  respiratory  cavities  containing  the  lung-books,  of  which 
there  are  in  all  four  pairs  in  this  group.  No  traeheaj  occur. 
The  intestine  is  quite  straight  in  the  Scorpions  and  lacks 
ciecal  outgrowths  excepting  the  two  Malpighian  tubules  sit- 


FlG.  201. — SCOKPION  (after  Owkn) 
ch  =  (•lielicorte. 
op  =  genital  operculum. 
pe  =  pedipalp. 
pt  =  pecteu. 
«<'-•»  =  sligmuta. 


TYPE  AliACHNlDA. 


443 


uated  at  the  posterior  e>  -1  of  the  mid-gut.  The  digestive 
gland  is  a  large  five-lobed  structure  which  empties  through 
several  ducts  iuto  the  uiid-gut. 

The  nervous  system  consists  of  a  syncerebrum  lying 
above  the  oesophagus  and  giving  rise  to  nerves  for  the  eyes 
and  for  the  chelicera?.  It  is  connected  with  a  suboesophageal 
mass  from  which  the  pedipalps  and  the  three  anterior  legs  are 
innervated,  the  fourth  pair  of  legs  receiving  its  nerves  from  a 
pair  of  distinct  ganglia  separated  only  b}'  a  short  distance 
from  the  suboesophageal  mass.  Behind  this  in  the  abdomen 
is  a  chain  consisting  of  seven  pairs  of  ganglia  united  by  long 
<  unectives.  The  eyes  vary  in  number  from  two  to  six  pairs, 
one  pair  being  situated  on  or  near  the  median  line,  while  the 
others  are  lateral. 

Coxal  glands  occur  in  connection  with  the  third  pair  of 
legs,  and  the  heart  is  an  elongated  structure  lying  in  the  an- 
terior portion  of  the  abdomen  and  possessing  eight  pairs  of 
ostia. 

The  Scorpions  are  viviparous.  The  ovaries  are  situated 
in  the  anterior  abdomin.al  region  and  are  elongated,  that  of 
one  side  of  the  body  being  united  with  the  other  by  several 
transverse  connections.  The  oviducts,  which  are  short,  serve 
as  uteri,  and  open  to  the  exterior  by  a  single  median  opening 
situated  on  the  ventral  surface  of  the  tirst  abdominal  segment. 
The  testes  consist  of  four  tubes,  those  of  the  same  side  being 
connected  by  transverse  anastomoses,  and  unite  together  to 
open  into  a  protrusible  penis,  accessory  glands,  vesiculse  sem- 
inales,  occurring  in  connection  with  each  vas  deferens.  The 
single  genital  orifice  occupies  the  same  position  as  in  the 
female. 

The  Scorpions  are  confined  to  the  warmer  regions  of  the 
globe,  but  few  genera  being  known.  Of  thes<  lie  genera  Eu- 
scorpiiis  and  Buthus  are  perhaps  the  conif        at. 


2.  Order  Pseudoscorpionida. 

This  order  includes  a  number  of  small  forms  which  are 
found  under  the  bark  of  trees  or  among  dead  leaves  or  moss, 
one  genus,  Ohelifer  (Fig.  202),  occurring  occasionally  between 


F 


m\ 


: 


444 


7iV  VKHTKIiUA  TK  MOIWIIOLOU  Y. 


the  pages  of  books,  aud  heuce  being  kuowu  popularly  as  the 
Book-scorpioii.  The  cephalothorax  is  uusegiueuted,  auil  is 
followed  by  a  bnmd  flatteuetl  abdomeu  composed  of  eleveu 
segments.  A  pnuabdomen  and  a  postabdomeu,  such  as  cau 
be  distinguisheil  in  the  Scorpiouida,  does  not  occur,  nor  is 
there  a  terniinal  poison-spine  nor  a  poison-gland. 

The  chelicenu  and  pedipalps  resemble  those  of  the  Scor- 
pions, being  chelate,  and  the  four  succeeding  appendages  are 

walking-legs,  while  the  abdomen  pos- 
sesses no  appendages  in  the  adult.  Both 
the  second  and  third  abdominal  seg- 
ments bear  upon  their  ventral  surfaces  a 
pair  of  stigmata  which  are  the  openings 
of  tubular  trachese  which  extend  through 
the  bodj'  sending  oft'  branches,  except 
in  Chernes,  in  wiiich  bunches  of  un- 
brauched  trachea>  arise  from  each  stig- 
ma. A  heart  is  present,  but  consists  of 
Fio.  202.  -Chelifer  card-  a  simple  tube  with  either  a  single  pair 
Mo<(/e;«  (from  ciiviKB).        yf    ostia   near    its   posterior   extremity 

{Ohisium)  or  with  four  ostia  (Chernes). 
The  eudodermal  portion  of  the  digestive  tract  gives  rise 
to  a  pair  of  lateral  crecal  diverticula  branched  at  the  apex  and 
to  one  unpaired  ventral  one.  Two  eyes  are  present  in  Chelifer 
and  four  in  (Jfnsiuin,  while  they  are  entirely  wanting  in 
Chernes.  The  reproductive  organs  open  ui)on  the  ventral  sur- 
face of  the  second  abdominal  segment,  and  the  opening  is 
surrounded  with  glands  which  secreve  a  fluid  which  quickly 
hardens  to  silky  filaments  and  serves  to  fasten  the  eggs  to 
the  abdomen  of  the  parent.  These  glands  are  hypoderinal  in 
origin  and  correspond  to  the  spinning-glands  of  the  Spiders. 


3.  Order  Solifugee. 

The  members  of  this  order  are  characterized  by  the  head- 
region  being  separated  from  a  thorax  consisting  of  three  seg- 
ments and  bearing  the  three  posterior  pairs  of  legs.  The 
abdomen  is  also  segmented,  its  ten  segments  showing  no  dif- 
ferentiation into  praeabdomen  and  postabdomen,  nor  does  it 


TYPE  AHACIINIDA. 


445 


le 
f- 
it 


possess  anj'  stiug  or  poison-gluud.  The  clielicerm  are  rlielato, 
but  the  pedipiilps  are  loug  auil  leglike  and  posscsH  glands 
which  in  Gtdeixte.s  have  been  siH)p()sed  to  bo  [xjjsonous.  TIih 
anterior  pair  of  legs  lacks  tht^  terniiual  ungues  found  on  the 
others,  and  functions  as  a  second 
pedipalp  rather  than  a  walking-leg. 
No  appendages  occur  on  the  abdo- 
men. 

Three  pairs  of  stigmata  occur 
ou  the  ventral  surface  of  the  body, 
the  most  anterior  i)air  being  situ- 
ated on  the  first  thoracic  segment, 
while  the  other  two  are  on  the 
second  and  third  abdcuninal  seg- 
ments. The  anterior  posititm  of  the 
first  pair  is  probably  to  be  regarded 
as  secondary,  and  produced  by  a 
forward  migration  of  the  pair  which 
should  occur  upon  the  first  abdomi- 
nal segment.  The  stigmata  lead 
into  tubular  tracheso  which  branch 
extensively.  A  comparatively  simple  heart  is  situated  in  the 
abdomen. 

The  mid-gut  possesses  numerous  branched  diverticula  as 
well  as  Malpighian  tubules.  The  nervous  system  consists  of 
a  syucerebrum  connected  with  a  snliMSophageal  mass  which 
represents  all  the  thoracic  and  abdominal  ganglia  fused  to- 
gether. Two  eyes  are  present,  situated  on  a  common  eleva- 
tion at  the  front  edge  of  the  head. 

The  reproductive  organs  resemble  those  of  the  Scorpions 
except  that  transverse  anastomoses  do  not  occur,  and  the  gen- 
ital o})ening  is  situated  upon  the  ventral  surface  of  the  first 
abdominal  segment. 

The  Solifugje  is  a  small  order  living  more  especially  in 
warm  sandy  regions.  The}-  are  usually,  ou  rather  insufficient 
grounds,  supposed  to  be  capable  of  inflicting  a  poisoned 
wound.  Only  two  genera,  Solpitga  and  Galeodes,  belong  to 
the  order. 


Fio.  20ii.—Onleo<ie8  spiniptlpus 
(from  CrviKK). 


446 


iiV  VEHTEDKA TR  MOKPUOLOU  Y. 


4.  Order  Pedipalpi. 

The  order  Peilipalpi  iuclude«  two  j^euera,  Phrynits  aid 
ThelyphmnH,  both  of  which  are  iuhabitauts  of  tho  warunsr 
re^ious  of  the  earth.  The  cephalothorax  Ih  uiiHegiueiited  ; 
the  iibdoineu  iu  Phrynws  Ih  elongated  aud  oval,  aud  conipoued 
of  eleven  segments  showing  little  ditferentiatiou  .of  form, 
while   iu    Thdyphonm   there  are   twelve   segments,  the   lust 

three  of  which  are  much  smaller  than 
the  others  and  bear  a  lung,  many- 
jointed  terminal  tilument.  The  cheli- 
cera«  are  not  chelate,  but  the  terminal 
joint  may  be  Hexed  upon  the  basal 
one  and  contains  the  duct  of  a  poison- 
gland  which  opens  at  its  extremity. 
The  pedipalps  in  Phrynus  are  long 
and  leglike,  though  richly  provided 
with  spines,  and  termiuate  with  un- 
gues, but  iu  Thelyphoniis  they  are  rel- 
atively short  aud  stout  with  a  flexible 
terminal  joint  as  in  the  chelicerie ; 
in  both  genera  the  basal  joints  of  the 
Yxoym.-ThelypJionnscau.  ^^^,^    pepipalps  are    fused.       The    first 

leg  IS  long  and  slender  and  termi- 
nates in  a  filament-like  structure,  the  other  three  pairs  being 
typical  walking-legs. 

Four  stigmata  occur,  one  pair  situated  in  the  second  and 
another  in  the  third  abdominal  segment,  and  they  open  into 
cavities  containing  lung-books.  The  digestive  tract  is  com- 
paratively simple,  but  the  nervous  system  shows  a  concentra- 
tion of  the  postcesophagea!  ganglia  similar  to  that  described 
for  tjie  SolifugtB,  except  that  a  single  pair  of  ganglia  occurs 
in  the  abomiual  region  united  by  loug  connectives  with  the 
cephalothoracic  mass.  Eight  eyes  are  present,  two  of  which 
are  larger  than  the  others  aud  situated  at  the  anterior  edge 
of  the  dorsal  surface  of  the  cephalothorax,  while  the  other 
three  pairs  are  situated  laterally. 

The  reproductive  organs  are  paired  and  open  by  a  median 


TYPE  AHAVUMDA, 


447 


orifice  situated  ou  the  ventral  surface  of  the  first  abiloraiual 
segment.     Phrynua  is  viviparous. 


Tt.  Ui-ilur  Phalangida. 

The  Phalangida  (Fig.  205),  popularly  known  as  the  Harvest 
8})iders,  possess  an  uuseguiented  cephalothorax  {d)  and  have 
from  six  to  nine  segments  composing  the  abdomen  {ah).  The 
cheiicertK  are  chelate,  while  the  pedipal])s  {pe)  are  long  and 
leglike,  with  terminal  ungues.  The  eight  walking-legs  are 
usually  exceedingly  long,  though  in  the  genera  CyphophtlnihnHs 
and  (libbocellum  they  are  shorter.  A  single  pair  of  stigmata 
are  usually  all  that  occur;  they  are  situated  upon  the  first 


ab  =  abdomen. 


Fio.  205. — Leiohunum. 
ct  =  ccpimlutliorux. 


pe  =  pedipalps. 


abdominal  segment  and  open  into  branching  trachesi'.  In 
(ribhoceUum,  however,  two  pairs  occur,  situated  upon  the 
second  and  third  abdominal  segments,  the  anterior  pair  open- 
ing into  branched  tracheje,  while  a  bunch  of  sim])le  uubranched 
tracheje  arises  from  each  of  the  posterior  ones.  The  heart 
is  somewhat  elongated  and  possesses  three  pairs  of  ostia  ;  ar- 
teries are  entirely  wanting,  the  blood  passing  from  the  heart 
directly  into  the  lacunar  spaces. 

The  digestive  tract  dilates  into  a  sac-like  stom.ich  from 
which  numerous  much-branched  ccecal  diverticula  pass  oflF. 
Malpighian  vessels,  two  in  number,  are  found  in  ( 'yphnphfhal- 
mvs  and  Gibhocdlum,  and  have  been  described  as  occurring 


fflf 


■■    M 


148 


INVKHTEBRATE  MOUPIIOLOOT. 


?.  \ 


lil, 


\\     > 


in  other  forms  jilso,  though  it  is  probable  that  two  glandular 
tubes  which  ()j)eu  to  the  exterior  ou  the  sides  of  the  cephalo- 
thorax  hav»^  in  some  forms  been  mistaken  for  these  organs. 
Odoriferous  glands  are  also  found  in  the  abdomen  of  some 
forms,  and  so-called  salivary  glands  occur  in  connection  with 
the  pedipn  ps. 

The  nervous  system  shows  a  marked  concentration  of  the 
postoral  ganglia,  a  single  pair  only  remaining  separate  from 
the  fused  mass  formed  of  the  remainder.  The  majority'  of 
forms  possess  but  a  single  pair  of  eyes  on  the  dorsum  of  the 
cephalothorax,  but  in  Gthboccllmn  two  lateral  pairs  are 
found. 

Coxal  glands  have  been  described  in  connection  with  the 
eoxal  joints  of  the  third  pair  of  legs  and  have  been  observed 
to  communicate  with  the  exterior,  differing  therefore  from 
those  of  other  Arachnoids  in  being  functional  in  the  adult. 
The  reproductive  organs  are  unpaired,  a  condition  which 
results  from  the  fusion  of  originally  paired  structures,  and 
the  genital  pore  lies  in  both  sexes  at  tiie  junction  of  the 
cephalothorax  and  abdomen  or  on  the  tirst  abdominal  sog- 
nieut.  The  vasa  deferentia  and  oviducts  are  paired,  each  of 
the  former  communicating  with  a  j)rotrusible  penis,  while 
similarly  each  oviduct  unites  with  a  long  protrusible  ()vi])ositor. 

Certain  genera  such  as  Leiohunnm  (Fig.  2()o),  /*hahirnj!inn, 
and  OpUio,  are  exceedingly  common,  and  to  them  the  terms 
Harvest-men,  Harvest-spidt»rs,  or  Daddy  Longlegs  are  j)opu- 
larly  applied.  Other  forms,  such  as  (ronyJcptufi,  with  sj)inose 
})edipalps,  are  tr()i)ical  in  habitat,  while  Cyphophthalimis  and 
(rihhocclhnn  have  a  limited  distribution,  and  on  account  of  the 
many  differences  of  structure  which  they  i)resent  wh«»n  com- 
pared with  other  forms  are  sonu'times  grouped  together  to 
form  a  separate  order.  It  is  to  be  noted  es})ocially  that  these 
two  forms  possess  upon  tlie  seccmd  abdominal  segment  a  pair 
of  wartlike  elevations  at  the  summit  of  which  the  ducts  of 
numerous  spinning-glands  oj)eu. 


G.  Order  Araneee. 

The    order   Arane>o   includes   a    large    number   of   forms 
possessing  very  definite  characteristics.     The  ce])halothorax 


TYPE  AUACIINIDA. 


449 


»se 
1(1 
le 
n- 
to 

lir 
of 


is  uusegmeuteil,  as  is  also  the  alxloinen,  whicli  is  an  oval, 
spherical,  or  sometimes  irref^ulariy-sliapetl  region  which 
narrows  suddenly  auteri(jrly  so  as  to  \m\  much  narrower  than 
the  ce[)halothorax.  I'he  chelicera'  j)roject  somewhat  in  front 
of  the  cephalothorax  and  each  consists  of  a  broad  basal  joint 
and  a  terminal  strong  claw  which  may  be  Hexed  upon  the 
basal  joint,  and  has  opening  at  its  tip  the  duct  of  a  ]K)ison- 
gland  (Fig.  20(),  />j/)  which  lies  iu   the  cephalothorax.     The 

ht,  AR 


Fid.  30«5.  — DlAdUAM  OK  STUt'CTUKK    OF   A    SlMPKFt  'nftfr  Lkitkaut). 


ao  =  aorta. 

ee  =  cerebral  j;anj;lloii. 

ch  =  clielicera. 

*lff  -  iligestive  ^land. 

BP  =  )?«'"'li^  pore. 

ht  =  heart. 

lb  =  liiiig-hook. 

iHt  =  Malpi^liian  tubule. 

oe  =  eye. 

ov  =  ovary. 


pe  =  pcdipalp. 
Pf)  -  pi»is()n  jrlaiul. 
rb  —  Mrctal  bladder. 
r«  -  n'ci'ptaeuluiu  .semiiii.H. 
»  =  stuiiiacli. 
Hd  =  .slcunach  diverliiiiluiu. 
up  -  ."^piiiiicrfl. 
itpfl       spiaiiiiijf  ulands. 
tfi   -  Mioracic  nanjjlion. 
tr  —  liacbea 


ns 


podipalps  of  the  females  are  loglike  structures  usually  with  a 
terminal  unguis,  but  in  the  male  are  moic  or  less  swollen  to 
serve  as  accessory  organs  in  copulation.  'I'he  four  j)airs  oi 
s'>V(Ui-jointed  legs  are  all  similar  in  stiiu^ture  ami  serve  for 
walking,  <litVeiing  in  relative  lengtl'  lo  tlitlerent  gtuiera.  In 
the  embryo  the  abdomen  is  «listinctly  segmented  and  bears 
tiv((  or  six  pairs  of  rudimentary  appendag(^s,  the  nioie  ante- 
rior of  which  lat"r  disappear,  whih'  the  two  or  three  posterior 
pMiirs  j)ersist  as  tho  spinnerets  ("/>),  so  called  from  tlu^  occur- 
rence on  them  of  the  openings  of  the  ducts  of  tlie  spinning- 
glands  {sp(f). 

These  *re  very  uruerous  and  open  at  the  apices  of  the 
spinnerets,  each  glantl  prodiu'ing  a  tlnid  secretion  which 
quickly   hardens  on   c.vposurc   to   tlic   air    to   form    a    silken 


it 


!fi 


450 


IN VKUrEBHA  TE  MOliPUOLOG  Y. 


tlireail.  The  thickuess  of  the  thread  iimy  be  modified  by 
uuitiii^  together  the  secretions  of  a  greater  or  less  n umber  of 
the  glauds,  which,  moreover,  ditl'er  amoug  themselves,  some 
producing,  for  iustauce,  a  sticky  socretiou  with  which  certain 
of  the  threads  may  be  covennl.  lu  some  forms  there  is  situ- 
ated upon  the  abdomen  just  in  front  of  the  swimmerots  a 
c'litinous  plate,  the  criheUum,  which  is  perforated,  like  the 
spinnerets,  by  the  ducts  of  numerous  spinning-glands.  Its 
presence  is  associated  with  that  of  a  cahtmisti'iim,  a  })eculiar 
modification  of  the  i.ietatarsus  of  tlnj  last  pair  of  legs,  it  being 
furnished  with  a  double  row  of  bristles  which  are  rapiilly 
waved  over  the  cribellum  anil  draw  from  its  glantls  their 
secretion.  The  threads  are  used  for  several  purposes,  as,  for 
example,  to  fasten  tne  ova  to  the  body  of  the  i>arent  or  to 
form  a  cocoon  for  them,  or  else  to  form  a  snare  by  which 
insects  may  be  caught  to  serve  as  food.  These  snares  in 
some  cases  are  composed  of  an  irregular  network  of  threads 
arranged  without  any  dcHnite  jiattern,  as  in  TJu'rulium,  but 
some  othiu'  forms  show  a  trertain  aniount  of  architectural 
skill,  weaving  a  platform  of  felted  threads  whit'h  terminat«»s 
in  a  tubelikt;  place  of  concealment  for  the  spider  (e.g.,  Atjc- 
leiKi,  Tfijcmtrut)  or  webs  comijosed  of  threads  radiatiii^  from 
a  central  point  and  united  l)v  other  threads  arrangtd  :a  a 
spiral  or  in  concentric  circh^s  (e.g.,  hpini,  the  common  garden- 
spider),  or  else  using  the  threads  to  foiin  a  hinged  tra})-door 
covering  in  a  burrow  in  the  earth  which  serves  as  a  domicile 
as  in  the  Trap-door  Spider. 

The  digestive  tract  »'xpands  in  the  thoracic  region  into  a 
saclike  strui'ture  (.v)  fntni  each  side  of  which  three  or  more 
usually  rive  cacal  diverticula  {sd)  urise,  the  anterior  pair 
sometimes  anastomosing  so  as  to  foiin  a  ring,  while  in  some 
cases  ih'fuifd)  sin-ondary  iliverticulu  extend  from  the  more 
postiiri«)r  ones  into  the  coxal  joints  of  the  legs.  In  the  abdo- 
intMi  the  intestine  is  more  cylindrical,  giving  ris»^  to  much- 
bianchiMl  lateral  diverticula  whi«'h  tog(>ther  form  the  so-cnlied 
livtir  {(i[f),  and  having  connected  with  it,  just  as  it  ynun  the 
end-gut,  two  elongated  Malpighian  tubuh's  imf).  The  end- 
gut  itself  tiilates  into  a  large  rectal  bladder  (/•/;)  which  a  short 


TYl^h'  AliAVUNWA. 


451 


recti! in  connects  with  the  auus  situated  at  tlio  posterior  ex- 
tremity of  the  body. 

In  the  ^euus  J/ytjale  and  allied  forms  two  pairs  of  sti;^- 
niata  are  found  near  the  anterior  portion  of  the  alxlonu  n, 
both  of  which  lead  into  cavities  containing  bm^-books. 
In  the  majority  of  forms,  however,  but  one  pair  of  lun^-books 
(//>)  occurs,  the  second  pair«>f  stigmata  opening  into  a  tracheal 
tube  (tr)  extending  into  the  cephalothorax  and  t(>rminating  in 
a  bunch  of  unliranched  trachea*,  a  similar  bunch  arising  near 
its  base  and  extending  backwarils  into  the  abdomen  (Styt-s- 
tria).  In  some  forms  the  second  or  tracheal  stigmata  may 
be  situated  far  back  upon  the  abdomen,  and  may  be  united 
to  a  single  median  transvers(dy-elongated  cleft,  from  which  a 
bunch  of  uubranched  {Attns)  or  branched  tracheal  arises. 

The  heart  (/</),  which  lies  in  the  abdomen,  is  enclosed 
within  a  so-called  pericardium  and  p<jssesses  three  pairs  of 
ostia.  It  is  continuetl  anteriorly  and  posteriorly  into  aorta', 
and  gives  otT  also  lateral  arteries,  all  of  which  open  after  rel- 
atively short  courses  into  the  lacunar  spaces.  Tln^  blood  is 
returneil  to  the  pericardial  cavity,  whence  it  passes  into  the 
heart,  the  greater  portion  on  its  way  to  the  pericardium  pass- 
ing through  the  lung-books. 

The  nervous  system  consists  of  a  sync(U'ebrum  (cc)  and  a 
large  cephalothoracic  ganglionic  mass  (/</).  In  addition  to 
the  nerves  to  the  appendag«'s,  a  posterior  nerve  arises  from 
this  mass  and  pass«»s  backwards  towards  the  abdomen,  in 
Mi/(f(ih'  dilating  at  tlie  junction  «)f  that  region  with  tln^  cephalo- 
thorax into  a  pair  of  small  ganglia  A  sympathetic  or  visccial 
system,  consisting  of  a  nerve  arising  by  paired  tiunks  from 
the  brain,  is  distributed  to  the  anterior  portion  of  the  diges- 
tive tract.  The  eyes  are  i  snally  numerous,  three  or  four  pairs 
occurring  on  the  anterioi'  portion  of  tin!  cephalothorax,  their 
arrangement  varying  in  ditVereiit  genera. 

Ct»xal  glands  have  been  found  in  several  forms  in  connec- 
tiou  with  the  third  paii'  of  legs,  but  have  not  be<'n  fouml  to 
open  to  the  exterior  in  the  adult.  The  rej)roductive  organs 
open  iu  both  sexes  by  a  single  opening  situated  near  the 
anterior  end  of  i\n\  abdomen  between  the  anterior  stigmata. 
The  ovaries  (ov)  are  paired,  or  may  unite  to  form  a  ring,  and 


tiH 


■!J 


I! 

i  ■ 


11    i^ 

'    ■I'       .i 


I       \'\ 


452 


INVERTEBRA  TE  MORPHOLOG  T. 


the  two  short  oviducts  unite  to  form  a  vagiua  with  which 
ma}'  be  associated  receptacuhi  seiniuis  {rs),  though  more  usu- 
ally these  structures  open  iudepeudeutly  in  frout  of  the 
genital  orifice  and  may  be  single,  or  paired,  or  in  some  cases 

even  three  in  number.  The  testes  are  cy- 
lindrical structures  whose  long,  slender,  and 
frocpieutly-coutorted  vasa  deferentia  unite 
just  before  opening  to  the  exterior.  A 
remarkable  copulatory  organ  is  formed  by 
tlie  terminal  joint  of  the  pedipalp  of  the 
male  (Fig.  207),  which  bears  upon  its  inner 
surface  a  process  containing  a  spirally- 
coiled  tube.     This   tube    opens   at  the  ex- 

„  „  tremity  of  the  process,  and  is  filled  by  the 

Fro   207 Pedipalp       .  . 

OF  ~Mai,k    Spidek  spider  with  spermatozoa,  and  during  copu- 

(after  hkutkau).  latiou  is  inserted  into  the  receptacula  semi- 

nis  of  the  female. 

The  males  are  usually  smaller  than  the  females,  and  their 
approaches  are  frequently  resisted  by  the  latter,  who  en- 
deavor to  capture  and  destroy  the  persistent  swains.  In  the 
AttidsB  a  process  of  courtshij)  has  been  observed  to  occur,  the 
male  posturing  before  the  female  and  displaying  to  their  best 
advantage  the  highly-colored  hairs  with  which  the  body  is 
covered.  The  ova  are  in  many  forms  {Lycosa)  attached  to 
the  under  surface  of  the  abdomen,  while  in  others  thej'  are 
enclosed  in  a  silken  cocoon  which  may  either  be  carried 
about  by  the  female  or  suspended  in  the  webs  or  deposited  in 
protected  situations.  V. 

Two  suborders  are  recognized,  according  as  there  are  two 
pairs  of  lung-books  or  only  one.  The  TetrapneHmones  in- 
clude the  forms  with  two  pairs  of  lung-books,  anu)ng  which 
are  the  Tra])-door  8i)iders,  Cteima,  already  mentioned,  and  the 
Tarantula,  Mmjnle,  the  largest  of  all  the  s])iders  antl  re])uted 
to  attack  even  small  birds.  The  Dipneiitnones  have  but  a 
single  pair  of  lung-books,  the  majority  of  living  spiders  be- 
longing to  the  suborder.  Some,  such  as  Epeira,  Agelena,  Tege- 
nnria,  Theridium,  and  Segcstrio,  spin  webs,  while  others  catch 
their  prey  by  their  rapid  movements  {Lycosa)  or  by  suddenly 
springing  upon  it  {Attun). 


TYPE  AUACUNIDA. 


463 


7.  Order  Acarina. 
The  Acarina  are  for  the  most  part  small  forms,  many 
being  almost  microscopic,  while  the  largest,  the  Ticks  {Ixodes), 
do  not  when  at  their  greatest  size  exceed  a  centimeter  in 
length,  the  males  being  much  smaller.  Some  forms,  such  as 
(h'ihates  and  Nothriis,  live  among  moss  and  in  similar  situa- 
tions, while  others,  such  as  Uydrachna  and  Atax,  are  aquatic. 
Many  forms  are,  however,  parasitic  either  upon  plants  ( Te- 
tranychus  and  Phytoptus)  or  on  animals,  the  genus  Sarcoptes 
being  the  cause  of  the  disease  termed  the  Itch  in  man,  the 
symptoms  being  produced  by  the  Mites  burrowing  beneath 
the  skin.  Other  forms  affect  various  animals  and  birds,  the 
genera  DermaleichiiSf  ATialgea,  etc.,  feeding  upon  the  feathers 


Fig.   208. — A,   Sarcoptes  scnbiei ;   B,   Demodex  phyllot'den  (after  Csokor  from 

Wrkiht). 

of  various  birds,  while  others,  such  as  Democlex  (Fig.  '208,  7?), 
live  in  the  hair-foUicles  or  sebaceous  glands  of  the  skin,  i)ro- 
ducing  acnelike  pustules.  The  larvro  of  many  forms  wliiih 
are  non-p  jfa^itic  in  adult  life  have  a  parasitic  habit,  as  for 
instance  the  larvfo  of  nianv  of  the  Water-mites  and  of  the 
Harvest-mites  [Trombidium),  while  other  forms  live  upon 
orgai'ic  matter  of  various  kinds,  as  does  the  Cheese-mite, 
Tyrog/yp/iii.s. 

A.  distinguishing  characteristic  of  the  Acarina  is  the  ab- 
sence of  any  segmentation  and  the  fusion  of  head-thorax  and 
abdomen  to  a  siugh)  mass  (Fig.  208,  ^1).  The  form  of  the 
appendages  varies  greatly  in  different  genera  according  to  the 
use  to  which  they  are  put.     The  chelicerro  (Fig.  209,  Md)  are 


454 


IN  VEliTEBliA  TE  MOliPIIOLOO  Y. 


!      I 


I    h 


frequently  chelate,  but  in  parasitic  forms  are  reduced  to 
Btylets  eucioHed  by  the  fused  basal  joints  uf  the  pedipalps,  a 
])robuscis  beiuj^  thus  produced  which  can  pierce  the  integu- 
ment and  thus  reuder  the  juices  of  the  host  available  as  food. 
The  pedipalps  {Mxp)  undergo  various  modifications,  being 
sometimes  long  and  limblike,  sometimes  chelate,  while  their 
basal  joints  may  or  may  not  be  fused.  The  four  pairs  of  legs 
are  generally  adapted  for  walking,  and  terminate  in  ungues  or 
bunches  of  hairs  or,  in  some  parasitic  forms,  in  suctorial 
disks,  while  in  the  Water-mites  they  are  provided  with  usually 
long  bristles  along  the  sides,  serviceable  swimming-organs 
being  thus  produced.  In  the  genus  Demodex  the  four  legs 
are  reduced  to  short  uujointed  structures  each  provided  with 
four  ungues,  while  in  the  Leaf-mites,  J*hytoptun,  which  pro- 
duce galls  on  the  leaves  of  various  plants,  the  two  pairs  of 
posterior  limbs  are  reduced  to  wartlike  elevations  bearing 
bristles,  the  two  anterior  pairs  being  on  the  other  hand  five- 
jointed. 

The  chitinous  covering  of  the  body  is  usually  thick  and 
delicately  wrinkled.  It  usually  bears  numerous  setji'  and 
oceasiouiilly  also  plates  or  lateral  prolongations,  as  in  Ori- 
btites  and  its  allies.  Dermal  glands  alst>  frequenth*  occur, 
])r()duciug  oily  Huids  and  sometimes  odoriferous  secretions. 
Spinning  glands  opening  on  the  pedipalps  occur  in  Tetrtnuj- 
ehnsy  frequently  parasitic  on  the  leaves  of  the  Rose,  but  as  a 
rule  they  are  not  ileveloped.  / 

A  ])air  of  stigmata  (Fig.  209,  st)  occurs  in  many  forms, 
situateil  usually  near  the  coxje  of  the  last  pair  of  legs,  but 
not  unfrequrntly  tht^v  are  niiieh  further  forward,  lying  near 
the  basal  joints  of  tiie  pedipalps  or  even  of  the  chelicera'. 
They  opiMi  into  tiaclicje  which  branch  once,  bunches  of  lateral 
trachea'  being  sitnatetl  at  intervals  upon  the  two  brancln's. 
Frecpiently,  however,  especially  in  parasitic  and  aquatic  forms, 
both  trac'lit'ie  and  stigmata  are  wanting,  as  is  usually  also  the 
heart.  When  present  [(idnidsiui,  /jumIcs)  this  latter  structure 
is  small,  with  but  a  single  i)air  of  ostia,  and  is  prolonged  an- 
teriorly into  a  slender  aorta. 

The  digestive  tract  is  frequently  provided  with  glands 
opening  into  its  anterior  p(»rtion  and  supposed  to  be  salivary. 


TYPE  AliACUNIDA. 


465 


The  mid-gut  usually  seuds  off  u  uumber  of  ceecal  diverticula 
which  may  brauch  at  the  euds,  aud  Malpighiau  vessels, 
sometimes  oue,  sometimes  a  pair,  aud  sometimes  mauy  are 
usually  present,  while  iu  addition,  in  some  forms,  a  rectal 
bladder,  similar  to  that  occurring  iu  the  Aranete,  is  found. 


mg 


Fifl.  200. — Malk  of  GnmajtuH  tergipe»  (afu*r  Winklkhi. 

an  =  anus.  »(  =  >iiniiiu. 

<^e  =  (;j|}(!iil  pourhf.sof  iutesiiiic.  «/''  =  stiiriim-cuiiiil. 

(/o  =  ^:t'niliil  oritice.  T  —  it'.siis. 

Atd  =  clii'liccrii.  t  =  loniruc 

mg  =  iMaipijjIiiiiii  tubules.  Vd  =  viis  (Jt-ferens. 

Mxp  =  |H'(lipiil|»s.  I-[V  =  lin»l)N. 

Tho  novvons  Hystem,  as  nii<;lit  \v&  su]»|)os«-,l  fn»iu  the  con- 
centration of  t)i*»  body  regions,  i.s  coniito^u^d  of  ;i  supr:ui'So])]i. 
goal  syncerehi  tim  .md  a  lurgor  sui)<»'sophjig«'ul  iraiiglionic 
mass  from  wJiicii  ntuuerous  iierv«*>-  in-  giv«Mi  off.  Eyes  an^ 
usuully  wanting,  oi  may  he  j)n'H-^it  in  the  form  of  oue 
(Ixodes)  or  two  pairs  of  small  appar  -itlv  Himpie  oenlli. 

Coxal  glands  hav<*  been  dese-ii— <  as  "curriiug  at  the 
bases  of  the  sec«>ud   pair  of   legs     •  >ril)atid«pK     The   repro- 


M 


41)6 


IN  \  KliTKIinA TE  MOHPUOLOU  Y. 


tluctive  orj^ans  show  much  variety  iu  their  arruugement, 
beiiif^  sometimes  paired  ami  souietimes  iiuited  to  a  siugle 
mass.  The  single  ji^enital  oritice  is  situated  far  forward,  iu 
some  cases  eveu  betweeu  the  basal  joints  of  the  secoiitl  pair 
of  legs.  Numerous  accessory  structures  may  be  associateil 
with  the*  ducts,  the  receptacula  semiuis  iu  some  forms  open- 
ing to  the  exterior  quite  independently  of  the  oviduct,  and 
protrusible  organs  serving  for  copulation  in  the  male  anil  lor 
oviposition  in  tin'  female  may  occur.  The  Acarina  are  as  a 
rule  ovij)an)us,  though  a  few  forms  are  viviparous. 


i. 


J 


Development  of  the  Acarina. — Most  of  the  Acarina  whoso  developuient 
has  Ih'cii  traced  pass  throiigli  a  scries  of  larval  stages.  While  the  young 
enil)i',vt>  is  still  witiiin  the  egg  and  sometimes  l)eforo  the  appendages  have 
developed,  a  cuticidar  membrane  is  secreted  aronnd  it  lying  betweeu  the 
cinbrvit  and  the  egg-shell.  This  is  the  tlentopn m,  mul  within  it  farther 
development  proceeds.  In  those  forms  in  which  it  does  not  appear  until 
after  the  appendages  are  formed  a  degeneration  of  these  structures  takes 
place,  and  the  egg-shell  may  also  be  thrown  otT  leaving  the  embryo  sur- 
roiuided  oidy  by  the  deutovum  {Tiomhidinm).  New  appendages  now 
appear,  and  the  larva  hatches  out  from  the  deutovum  as  a  six-legged 
I'orm,  sometimes  showing  traces  of  .segmentation  either  iu  the  thoracic 
region  or  in  the  abdomen.  After  a  certain  time  a  certain  amount  of  de- 
generation t)f  the  tisiies  occurs  (histolysis)  and  the  appendages  again  dis- 
appear, a  chitinons  membrane  forming  around  the  now  almost  spherical 
body  of  the  larva.  A  regeneration  of  the  limbs  and  tissues  takes  place 
within  this  larval  membrane,  and  the  nymph  is  formed,  r^jsembling  tho 
adult  in  the  nund)er  of  appendages,  but  lacking  fulIy-<leveIoped  repro- 
ductive organs.  A  period  of  rest,  and  histolysis  again  occurs,  accum- 
paiiied  by  the  formation  of  a  third  cuticular  membrane  within  which  the 
nymph  becomes  transformed  into  the  fidly-developed  and  se.vually-nuiture 
adult  or  iin!ig(»,  which  finally  issues  from  the  membrane. 

This  complicated  process,  it  is  needless  to  say,  has  no  phylogenetic  sig- 
nilifaiic<>,  the  deutovum  indeed  being  absent  iu  certain  forms  (2V/;y///y- 
iliiis),  nor  does  it  seem  likely  that  even  the  six-legged  larva  is  anything 
but  a  secondary  stage  which  has  been  developed  within  the  group  of  tlu; 
Acarina.  There  is  no  «iueslion  but  that  the  order  represents  the  culmina- 
tion of  a  divergent  line  of  evolution,  perhaps  from  the  Pseudoscorpionida, 
and  since  the  .separation  numy  of  the  peculiarities  characteristic  of  the 
group  have  been  developed. 

Plnjlo<i<  nij  of  the  Araehn;<xa. — There  .seems  little  room  for  doubt  but 
that  the  Scorpions  among  living  forms  represent  most  closely  tho  ancestral 
Arachnoids,  their  segmentation  being  most  perfect  and  their  appendages 
more  uiunerous  than  those  of  other  forms.     It  is  through  the  Scorpions 


!; 


TYPE  AHAVIINIDA. 


457 


Ibut 
Itral 

L'OS 

Ions 


accordiugly,  that  relationships  to  olhor  fornis  must  bo  looked  for,  and  a 
conii)arison  of  tlieni  willi  Limtdu.s  reveals  similarities  of  slriiclures  so 
nnnieroiis  and  so  detailed  that  the  eoneliisi«)n  is  nnavoidahle  tlial  holli  are 
to  he  traced  back  t«»  a  eonnnon  ancestor.  Thus  (In*  eephitlollioracif  jip- 
pendages  in  both  are  identical  in  number,  and,  so  far  its  the  lirsl  two  pairs 
are  concerned,  in  general  structure  also,  while  the  genital  opereida  of  the 
Scorpions  are  comparable  in  their  relation  to  the  genital  orihces  to  the 
opereula  of  /w'//iw/».v,  and  the  pectines  to  the  first  pair  of  abdominal  ap- 
pendages. The  remaiinng  abdomiiud  appendages  of  Liniulns,  which  are 
branchiate,  seem  at  first  sight  to  be  unrepresented,  btjt  the  embryo- 
logical  investigation  of  the  Scorpions  appears  to  indicate  that  they  are 
represented  by  the  hnig-books,  which  bear  no  little  resemblance  to  the 
branchial  lamella'  of  himulus.  and  the 
conversion  of  oia^  set  of  organs  into  the 
other  may  be  supposed  U\  have  been  brought 
al)out  by  tla^  formation  behind  each  pair 
of  abdominal  appendages  of  an  invagina- 
tion, which,  deepening,  has  carried  in  with 
it  th(^  branchial  lamella*,  the  origiinvl  an- 
terior surfaces  of  the  appendage  forming 
the  ventral  wall  of  the  body  beneath  the 
lung-sac,  while  the  lamella^  project  into  the 
sac  for  its  ventral  surface  (Fig.  210).  In 
the  geiu'ral  form  of  the  body  IJnmtu.s 
corresi)onds  fairly  well  with  the  Scorpions, 
the  cephalothoraeic  regions  being  strictly  p  q.^^ 
comparable,  as  is  also  ila^  terminal  s|)ine 
with  the  sting  ;  the  abdomen,  however,  in 
the  branchiate  form  has  a  smaller  iniinber 
of  segments  which  are  all  fused,  a  difference 
readily  explained  by  the  probable  derivation 


-DiAdiiA.M  ofOuioin 
i^>v   LuNd-nooKS  uifit'i-  KiNoa- 

LKY). 

/  =  indifferent  stage. 
L  =  Limuhis  stage. 
A  —  Arachnidan  stage. 
of  both  forms   from   KitriipttrnsAWn^  ancestors  in   whi(!h   the  abdomen 
possessed  a  relatively  large  number  of  distinct  segments,  and  even  showed 
indications  of  a  diflVrentiation  into  a  pra'alxlomen  and  a  postabdoinen 
(see  Fig.  IU8). 

Intht!  internal  structure  (juite  as  striking  similarities  are  to  be  found 
in  the  presence  of  an  endosternite  in  botii  groups  and  of  co.val  glands  in 
connection  with  the  fifth  pair  of  apjuMidages,  in  the  tendency  towards  the 
concentration  of  the  postoral  ganglia,  and  in  the  invaginat(^  origin  of  the 
median  eyes,  to  mention  but  a  few  points. 

The  Arachnida  are  accordingly  to  hv.  traced  back  to  Lhniilus  or  Enryp- 
fer«A'-like  ancestors,  and  through  these  finally  to  the  Entomosfraca,  perhajjs, 
a  Crustacean  ancestry  being  clearly  indicated.  As  to  the  rtilationships  of 
the  varions  ordera  little  that  is  definite  can  be  .said,  differentiations  having 
taken  place  along  diflferent  lines  in  the  various  orders,  so  that  while  the 


! 


458 


IN VEHTblDHA TE  MOIWUOLOG  Y. 


ii' 

r;  1,. 


M 


i  r 

'}     t  ■ 


r  ^ 

Ii 


h 


!       ?! 


PudipHlpi  arc  inoru  priiiiitivu  u.s  rugurdH  tliu  iiuinbci'  of  abdominal  sog- 
luontH  and  tliuir  distiiictiiuHs  than  tliu  Aranea*,  yet  the  latt(>r  and  cspt'cially 
th(!  Ttitrapnuuinonos  show  a  much  moro  primitive  conilition  of  Iho  respira- 
tory orj^ans.  With  rej^ard  to  thesu  orj^ans  it  may  Im  slated  that  the  ct)n- 
dition  in  whicli  thuy  aru  rcprosentml  by  bunuhu.s  of  unbranchcd  traolitw 
is  more  primitive  than  that  in  whicli  they  are  branching  tubes,  the 
bunched  condition  Ijeing  pnjbably  derived  by  a  mudiiicution  of  original 
lung-books. 

TYPK  AKACHNIDA. 

1.  Ordor  Scoi'pionida. — Alxlomen  segmented  and  diflferentiated  into  praB^ 

abdomen  and  postiibdomen  ;  postab(h)men  terminating  in  poison- 
spine  ;  pcdipalps  chelate  ;  two  pairs  of  abdominal  ap]>en(lages  ; 
four  of  stigmata  and  lung-books.     Enscurpius,  Btithus. 

2.  Order  l^sendoscorpionida. — Abdomen  .segmented  but  not  ditferentiated  ; 

no  terminal  spine  ;  pedipalps  chelate  ;  no  abdominal  appen- 
dages ;  two  pairs  of  stigmata  opening  into  trachea' ;  first  pair 
of  legs  adapted  for  locomotion.  Cheli/er,  (Jbmiim,  Chenien. 
8.  Order /Sfo//y>/f/<c.— Head  .separated  from  thorax  with  three  segnients ; 
abdomen  segmented  ))ut  unditTerentiated ;  no  terminal  spine ; 
pcdipalps  palplike  ;  three  pairs  of  stigmata  leading  into  trachea'. 
Gah'inles,  Sofjnn/a. 

4.  Order  Pedifui/pL — No  distinction  of  head  and  thorax  ;  abdonien  seg- 

mented, and  either  undiirerentiated  or  with  three  small  .segments 
terminated  by  a  multiarticulate  fliigt'llum  ;  pedipalps  leglike  or 
subclielatc  ;  two  pairs  of  stigmata  and  lung-books  ;  first  pair  of 
legs  elongated  and  palplike.     Phrynus,  Thelyphonns. 

5.  Order   I'/taf(ui;/i(/(i.—A\Hh)mvii    .segmented    but   uiidiHeivntiated   and 

witliout  appendages  or  terminal  spine;  pculipalps  leglik(! ;  one 
pair  of  stigmata  leading  into  trachea^;  no  s])inning-giands. 
J,tiol)iiinn)i,  Phahnnjlum,  Ojnlio,  iiunyleptan,  Vifphophtluilmus, 
GibboceUuin. 

6.  Order  Animtv. — AI)domen  unsegmentcd  and  with  two  or  three  pairs  of 

rudiuK^ntary  papl  '.  dike  appendages  bearing  the  openings  of 
duels  of  lunnerous  spinning-glands ;  abdomen  not  fuscid  with 
eephalolhorax  ;  pedipalps  long  and  palplike  or  leglike. 

1.  Suborder   Tetraptieumoncs.  —  With   four   stigmata  opening   into 

.sacs  containing  lung-books.     Myi/die,  Chniza. 

2.  Suborder  Dijnuitmones. — With  lour  or  three  stigmata,  thcsmterior 

pair  opening  into  sacs  with  lung-books,  tlu^  posterior  one  or 
two  with  trachea'.  Epeira,  Ayelena,  Teyenari(\,  Thcridium, 
ISei/estria,  Attus,  Lycosa. 

7.  Order  Acarina.  —  Abdomen  un.segmented,   without  appendages,  and 

fu.sed  with  tlie  cephalothorax  ;  pedipalps  .sometimes  long  and 
leglike,  sometimes  chelate ;  stigmata  wanting  or  present  as  a 


TYPK  A  It  A  CUM  DA. 


459 


fs  of 

kvith 

liiito 

[rior 
or 
iati, 


singh'  pair  loading  into  truchoa';    many  forms  parasitic;  fre- 

(puMitly  with  coniplieatod  in(>taiuorplio.s(>s. 
Nonparasitic,  or  parasitic!  only  in  larval  stage;  terroslrial.     (hi- 

bntts,  Nnthnis,  Troiithhlinm. 
A(piatic'.     Uytlnifhiut,  Atti.r. 
Living  on  organic  matter.     Ti/ntifiy/thus. 
Parasitica  on  animals.     Dcmm/tx,  Sarmptes,  Dennaleichits,  Anal- 

Parasitic  on  plants.     TttrnnyrJtns,  I'/ii/tojUtin. 
IJTEItATUHE. 

OKNKUAL. 

H.  Orenaoher.  Untersuchtingen  iiber  Uaa  tiehorgan  der  Arthrojwden.  (UMtingcu, 
1H71). 

£.  B.  Lankester.  LiniuluH  an  Arachnoid.  Quurlcrly  Juurn.  Miuroscop.  8ci- 
«n<c,  XXI.  1881. 

J.  Maoleod.  Jiec/ierche»  sur  la  Htnictnre  et  la  slf/niflrntion  de  I'tippareit  respirn- 
toii'i-  des  Aidcfinulen.     Aicliivcs  «lc  Biologic,  v,  1884. 

R.  Sturany.  Die  Voxaldrimen  di  r  AraeUuoiilen.  Arltciten  a.  d.  Zoolog.  Inst. 
Witn,  IX,  181)1. 

J.  8.  Kingsley.  The  Kmhryology  of  Limulun.  Part  II.  Jourii.  of  Mor- 
phology, VIII,  181)3. 

KCOHPIONIDA. 

L.    Dufour.     Ilintoire   anatotniyue   et  phyaiologiqiui  dea    Scorpions.     Memoirs 

Acad.  Sciences.     Paris,  xiv,  18.j(j. 
£.  R.  Lankester.     On   the  Cowal  li lands  of  Scorpio,  etc.,  and  the  lirickred 

Glundti  of  Limnlus.     I'roceediiigs  of  the  Uoyai  Society,  xxxiv,  1884. 
0.  H.  Parker.     'J'he  h'yi-s  in  Scorj»ioii.s.     iJulletiu  of  the  Museum  of  C'ompar- 

ative  Zoology,  Xlii,  1879. 
W.  Patten      The  Origin  of  Vertebrates  from  Arachnoids.    Quarterly  Jouniul 

of  Microscop.  Science,  xxxi,  181)0. 
M.  Laurie.     The  Embryology  of  a  Scorpion  (Euscorpius  italicus).     Quarterly 

Jouru.  Microscop.  Science,  xxxi,  181)0. 

rSKlDOSCOliriONIDA. 

A.  Croneberg.  lieitrag  tiir  Kenntniss  des  Danes  der  I*seudoscorpione.  Ijulletin 
Soc.  Imp.  Nttluralisles  Moscou,  ii,  1888. 

SOIJKUO.K. 

L.  Dufour.  Anatomic,  physiologic,  et  histoire  naturHle  des  Galeodea.  Me- 
nioiros  Acad.  Sciences,  Paris,  xvii,  1858. 


it 

V 


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II 


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land 
md 


niAI.ANCIDA. 

R.    ROnler.      Beitriige   zur    Anatomic    der    Phalungiden.       Zeltschrlfl    ftir 
wissensch.  Zool.,  xxxvi,  1882. 


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IMAGE  EVALUATION 
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INVEHTEBRATE  MOIiPIIOLOOY. 


C.  K.  Weed.     ^  Descriptive  Catalogue  of  the  Harvest-spiders  (Phnlangiidce)  of 
Ohio.    Proceedings  United  States  National  Museum,  xvi,  1893. 

AUANEiS. 

E.  Keyserling.     Die  Spinneii  Amerikas.    Nliruberg,  1880-91. 

0.  W.  and  £.  G.  Feokhatn.     North  American  Spiders  of  the  Family  Attida. 

Tnuisaclious  of  the  Wisconsin  Acad.  Sciences,  1888. 
H.  C.  McCook.     American  Spiders  and  their  Spinning  Work.    Philadelphia, 

1889-90. 
J.  H.  Emerton.     Papers  in  the  Transactions  of  the  Connecticut  Academy,  vii, 

1889,  and  viii,  1891. 
G.  Marx.     Papers  in  the  Proceedings  of  the  Entomological  Society  of  Wash- 
ington, 1891.  and  in  tlie  Proceedings  of  the  U.  S.  National  Museum,  xii, 

1890. 
W.  Sehimkewitsch.     J^tude  sur  l'anat07nie  de  I'Epeire.    Annates  des  Sciences 

Naturelies,  (Jme  ser.,  xvii,  1884. 
Ph.  Bertkaa.     Ueber  den   Verdauungsapparat  bei  Spinnen.    Archiv.  fUr  mi- 

kroskop.  Anatomic,  xxiv,  1885 
Beitrnge  eur  Kenntniss  der  Sinnesorgane  bei  Spinnen.    I.  Die  Augen. 

Arcliiv  fUr  mikroskop.  Anatomic,  xxvii,  1886. 
E.  L.  Mark.     Simple  Eyes  in  Arthropods.     Bulletin  Museum  of  Comparative 

Zoology,  XIII,  1887. 
A.    T.   Brace.     Observations  on    the  Embryology   of  Insects  and  Arachnids. 

Baltimore,  1887.  . 

ACARINA. 

0.  Haller.    Ztir  Kenntniss  der  I'yroglyphen  und  Verwandten.    Zeitschr.  fUr 

wissensch.  Zoologie,  xxxiv,  1880. 
H.  Henkfng.     Beitrdge  zur  Atiatomie,  Entwicklungsgeschichte  und  Biologie  von 

Trombidium  fuliginosum.     Zeitschr.   fllr  wissensch.   Zoologie,   xxxvn, 

1882. 
A.  D.  Michael.     British  Oribatidte.     London,  1884. 
A  Nalepa.     Anatomie  der  Phytopten.    Sitzungsber.  Akad.  wissensch.  Wien, 

xcv,  1887. 
W.  Winkler.     Das  Ilerz  der  Acarinen  nebst  vergleichenden  Bemerkungen  uber 

das  Herz  der  Phalangiden  und  Chernetiden.    Arbeiten  a.  d.  zool.  Ins. 

Wien,  vn,  1888. 
Anatomie  der  Oatnasiden.    Arbeiten  a.  d.  zoolog.   Inst.  Wien,  vii, 

1888. 


APPENDIX  TO  THE  AEACHNIDA. 

There  are  three  orders  which  show  a  certain  amount  of 
affinity  to  the  Arachnida,  but  which  are  not  so  closely  related 
as  to  warrant  the  actual  association  of  them  with  the  orders 
which  have  been  assigned  to  that  type.     They  will  be  de- 


TYPM  ARACUNlDA. 


461 


it  of 

ated 

ders 

de- 


scribed here,  and  are   the  orders  of   the  Pentastomidc?,  the 
Pycnogonida,  and  the  Tardigrada. 

Order  Pentastomidse. 

The  PentastomidaB  are  all  parasitic,  living  in  the  adult 
stage  in  the  lungs  or  nasal  cavities  of  various  animals,  one 
species,  Pentastomum  tcenioides,  occurring  in  the  nasal  cavities 
or  sinuses  of  dogs  and  wolves,  while  several  species  have  been 
found  in  the  lungs  of  different  species  of  snakes  (Fig.  211). 
They  are  all  elongated  wormlike  forms,  some- 
times slightly  flattened  and  usually  distinctly 
aunulated,  the  annuli,  however,  not  repre- 
senting a  metamerism.  The  anterior  end  of 
the  body  is  rounded  and  bears  on  the  ventral 
surface  the  mouth,  upon  each  side  of  which 
is  situated  a  pair  of  strongly-recurved  hooks 
(h)  supplied  with  special  muscles  and  serving 
for  the  attachment  of  the  animal  to  the  tissues 
of  the  host.  With  the  exception  of  these 
hooks  no  appendages  are  present. 

The  body  is  covered  by  a  cuticle  secreted 
by  the  ectodermal  cells  (hypodermis),  be- 
neath which  lies  a  layer  of  circular  muscle- 
flbres,  and  beneath  these  again  a  layer  of 
longitudinal  muscles.  Tbe  ccelom  is  ample 
and  is  traversed  by  dorso-ventral  muscle- 
bands,  which  divide  it  into  a  central  com- 
partment containing  the  various  organs,  sus- 
pended by  mesenteries,  and  two  lateral  ones. 
There  is  no  heart  or  circulatory  apparatus, 
and  trachesB  or  other  respiratory  organs  are 
also  wanting. 

The  digestive  tract  is  a  straight  tube  extending  through 
the  body  from  the  mouth  to  the  terminal  anus,  giving  off  no 
Lateral  diverticula  throughout  its  course.  The  nervous  sys- 
tem (Fig.  212,  ng)  consists  of  a  ganglionic  mass  lying  below 
the  oesophagus,  a  comparatively  small  commissural  ring 
passing  round  that  portion  of  the  digestive   tract,  without, 


Fig.  2,n.  — Penta- 
stomum teretiuHcu- 
lum,  Femalk  (after 

SPENCEIlt. 

h  =  hooks. 
go  =  genitul  orifice. 


462 


INVERTEBRATE  MORPHOLOGY. 


I 


ft 


however,  possessing  any  ganglionic  enlargement  which  can 
be  termed  a  cerebrum.  Various  nerves  are  given  oflf  from 
the  mass,  two  of  which  extend  backwards  throughout  nearly 
the  entire  length  of  the  body.  The  only  sense-organs  pres- 
ent are  a  number  of  small  papillae  on  the  anterior  portion  of 
the  body,  which  are  probably  tactile  in  function. 

Glandular  organs  are  highly  developed.  Scattered  over 
the  surface  of  the  body  are  numerous  flask-shaped  glands, 
apparently  ectodermal  in  origin,  while  lying  in  the  coelom  on 
each  side  of  the  mid-gut  and  extending  back  almost  to  the 
posterior  end  of  the  body  are  two  long  cajcal  tubes,  a  glan- 
dular  structure  being  also  connected  with  them  anteriorly. 
These  glands  open  in  the  vicinity  of  the  hooks  and  have 
hence  been  termed  the  hook-glands  (Fig.  211,  hg),  and  it  has 
been  suggested  that  they  secrete  a  fluid  which  serves  to  keep 
the  blood  which  the  parasite  ingests  from  coagulating,  being 
thus  similar  to  the  glands  in  the  pharynx  of   the  Leeches 


Fia.  212. — Diagram  op  STnucTURE  of  Pemat-e  Pentnstomum  (after  spknkcb). 
go  =  genital  orifice.  od  =  oviduct. 

hg  =  hooli-glaud.  ov  =  ovary. 

1  =  intestine.  rs  =  seminal  receptacle. 

ng  =  nerve-ganglion.  ut  =  uterus. 

which  serve  the  same  purpose.  Unless  the  ectodermal  glands 
are  excretory,  no  special  organs  for  the  carrying  on  of  that 
function  occur. 

The  PentastomidsB  are  bisexual,  the  male  being  smaller 
than  the  female,  and  recognizable  by  the  situation  of  the  geni- 
tal orifice  (Fig.  211,  go),  which  is  near  the  anterior  end  of  the 
body,  while  in  the  female  it  is  near  the  posterior  end.  The 
ovary  and  testis  are  both  unpaired  organs  situated  beneath 
the  dorsal  surface  of  the  body  and  extending  almost  its  entire 
length.  Anteriorly  a  pair  of  oviducts  (Fig.  212,  od)  arise 
from  the  extremity  of  the  ovary  (ov)  and  pass  downwards  and 


TYPE  ARACHNID  A. 


463 


bh 
id 


forwards  towards  the  ventral  surface,  ou  nejiring  which  they 
uuite  to  form  a  loujj;  coiled  tube,  the  uterus  Cut),  which  passes 
backwards  to  the  geuital  oritice,  aud  just  where  the  two  ducts 
uuite  they  have  opeuiug  iuto  tlieiu  a  pair  of  pyriform  seniiual 
receptacles  (z's).  The  vasa  defereutia  are  also  paired,  aud 
arise  at  the  auterior  end  of  the  testis,  passiuj^  veutrally 
towards  the  geuital  pore,  uniting  before  they  reach  it  aud 
dilating  to  form  a  complicated  iutromittent  organ,  from  which 
two  tubes  with  muscular  walls  and  containing  spermatozoa 
project  backwards  aud  serve  as  ejaculatory  ducts  for  the  ex- 
pulsion of  the  spermatozoa  through  the  iutromittent  organ. 
The  only  genus  belonging  to  the  order  is  Pentastomum. 

Development  of  the  Fentastomidve. — Diirinj?  the  life-history,  of  a,  Pen- 
tastomum it  passes  through  a  marked  metamorphosis  associated  with  a 
change  of  hosts,  recalling  what  occurs  in  the  Cestoda.  The  ova  are  passed 
to  the  exterior  with  the  excreta  of  the  host,  or,  in  the  case  of  the  dog, 
with  the  mucous  discharge  from  the  nasal  passages,  and  the  embryo  which 
hatches  out  is  a  decidedly  Mitelike  form,  possessing,  however,  only  two 
pairs  of  legs  terminating  in  ungues.  No  other  appendages  are  present, 
but  the  embryo  is  provided  anteriorly  with  a  boring  apparatus.  If  this 
larva  of  P.  toenioides^  the  parasite  of  the  dog,  succeeds  in  gaining  entrance 
to  the  digestive  tract  of  a  rabbit  or  cat,  for  instance,  it  bores  through  the 
wall  of  the  intestine  and,  reaching  the  liver,  encysts  itself.  Within  the 
cyst  it  undergoes  several  moults,  finally  assuming  a  condition  similar  to 
the  adult  except  that  each  annulus  bears  a  circle  of  hooks.  Leaving  the 
cyst,  then,  it  wanders  through  the  tissues  of  the  host,  and  if  while  it  is  in 
this  condition  the  host  is  eaten  by  a  dog,  it  adheres  to  the  mucous  mem- 
brane of  the  mouth  of  the  latter,  and  makes  its  way  into  the  nasal  passages, 
there  moulting  again,  losing  the  ring  of  hooks  and  assuming  the  adult  form. 

The  principal  reason  for  supposing  Pentastomum  to  be  related  to  the 
Arachnids  is  the  occurrence  of  the  four-legged  larva,  which  resembles,  so 
far  as  its  external  form  is  concerned,  a  Mite.  The  internal  structure  is 
very  different,  however,  although  certain  Arachnid  features  arc  indicated  ; 
but  it  is  evident  that  these  forms  must  have  undergone  an  enormous  de- 
parture from  the  ancestral  form  during  which  the  remaikable  life-history 
And  peculiar  structure  have  been  acquired.  The  parasitic  habits  of  many 
Mites,  and  the  general  similarity  of  the  body  form  of  Demodex  to  that  of 
Pentastomum,  suggest  the  Mites  as  the  ancestors  of  the  latter,  a  theory 
which  is  as  plausible  as  any  other  which  can  at  present  be  suggested. 

Order  Fycnogonida. 
The  Pycnogonida  are  exclusively  marine  in  habitat,  and 
Tary  considerably  in  size,  the  smaller  forms,  such  as  Tanysty- 


464 


IN  VERTEBRA  TE  MORPIIOLOG  Y. 


I.   ^-^ 


I 


m 


lum,  being  only  about  a  millimetre  in  breadth,  while  f;he 
purple  Phoxichilidium  measures  over  three  millimetres  from 
tip  to  tip  of  the  legs,  and  the  deep-sea  form  Collossendeis  has 
a  span  of  over  sixty  centimetres.  The  body  proper  is  compar- 
atively small,  the  four  pairs  of  long  legs  which  arise  from  the 
thorax  being  exceedingly  conspicuous,  a  feature  which  has 


Fig.  213. — Phonchilidium  mnxillare  (after  Morgan). 

suggested  the  term  Pantopoda  sometimes  applied  to  the 
group.  Anteriorly  there  is  a  well-marked  proboscis  carrying 
the  mouth  at  its  anterior  end.  and  at  the  base  of  this  there 
arise  the  chelicerae,  which  are  rather  short  chelate  limbs. 
The  next  segment  of  the  body  succeeding  that  which  bears 
the  chelicersB  bears  upon  its  dorsal  surface  the  eyes,  and  may 
be  regarded  as  a  fusion  of  three  segments  since  it  bears  three 
pairs  of  appendages.     The  most  anterior  of  these  are  slender 


TYPE  ARACHNID  A. 


465 


bhe 

ing 

jre 

lbs. 

ll'S 

I'ee 
ler 


]\)inted  palps;  the  second  pair,  wautiug  iu  the  females  of 
some  species,  but  always  present  in  the  males,  arise  from  the 
ventral  surface  of  the  segment,  and  are  curved  jointed  struc- 
tures serving  to  carry  the  ova ;  while  the  third  pair  are  ex- 
ceeding long  jointed  walking-legs.  The  next  three  segments 
also  bear  long  walking-legs,  the  last  one  having  attached  to 
it  the  usually  unsegmented  rudimentary  abdomen. 

The  body  and  the  appendages  are  encased  in  a  well-de- 
veloped chitinous  cuticle,  and  there  are  no  indications  of 
special  respiratory  organs.  The  heart  lies  immediately  be- 
neath the  dorsal  integument  and  is  a  simple  tubular  organ 
with  from  two  to  three  pairs  of  ostia. 

The  portion  of  the  digestive  tract  which  lies  within  the 
proboscis  is  lined  with  chitin  a*nd  opens  behind  into  an 
elongated  mid-gut,  from  which  long  diverticula  extend  out  into 
the  chelicerae  and  the  proboscis  and  into  the  walking-legs, 
sometimes  reaching  even  into  the  terminal  joints  of  the  latter. 
A  short  hind-gut  leads  to  the  anus  at  the  tip  of  the  abdomen. 

The  nervous  system  consists  of  a  supraoesophageal  gan- 
glionic mass,  from  which  arise  the  optic  nerves  and  those  for 
the  chelicerse,  as  well  as  certain  nerves  passing  to  the  pro- 
boscis. Connected  with  this  brain  by  circumoesophageal  com- 
missures is  a  ventral  chain  consisting  of  five  pairs  of  ganglia, 
the  first  pair  of  which  is  really  formed  by  the  fusion  of  two 
pairs,  distinct  in  the  embryo,  and  innervates  the  palps  and  the 
ovigerous  legs,  while  the  four  pairs  of  walking-legs  are  sup- 
plied by  the  remaining  four  pairs.  Finally  one  or  two  small 
ganglia  also  occur,  innervating  the  abdomen.  The  eyes  are 
four  in  number,  situated  at  equal  intervals  upon  a  small 
domelike  elevation  on  the  dorsum  of  the  first  thoracic  seg- 
ment, which,  it  is  to  be  remembered,  is  compound.  Each  eye 
is  covered  by  cuticle,  sometimes  thickened  so  as  to  form  a 
lens,  below  which  is  a  layer  of  cells  forming  the  corneal  or 
cuticular  hypodermis.  Below  this  comes  a  thick  layer  com- 
posed of  retinal  elements  with  nuclei  in  their  outer  portions 
and  rodlike  bodies  towards  the  inner  ends  where  they  rest 
upon  a  layer  of  pigment. 

These  eyes  recall  the  postbacilhir  eyes  of  the  Arachnids  by  their  struct- 
ure, but  show  one  remarkable  peculiarity,  i.e.,  a  distinctly  bilateral  ar- 


1^ 


II 


466 


INVERTEBRATE  MORPHOLOGY. 


rangement  l)oth  of  the  corneal  hypodermis  and  of  the  retinal  elements,  a 
distinct  raphe  being  observable  upon  the  inner  surface  of  the  eye,  the  reti- 
nal elenaents  being  arranged  on  either  side  of  it.  Sucli  a  condition  as  this 
cannot  readily  be  explained  by  a  simple  unilateral  inversion  such  as  was 
described  as  probably  occurring  in  Arachnidan  eyes;  it  suggests  rather  an 
inversion  of  two  sides  of  a  primitive  optic  cup,  the  posterior  wall  at  the 
same  time  forming  the  pigmented  layer  of  the  eye.  Whether  the  Arachnid 
eye  is  not  also  traceable  to  sucli  an  arrangement,  all  traces  of  the  original 
raphe  being  lost,  is  a  question,  though  at  present  it  seems  more  probable 
tluit  it  has  been  produced  by  a  suppression  of  the  inversion  of  one  side  of 
the  cup. 

Glands,  occurring  in  the  palps  and  ovigerous  legs,  have 
been  regarded  as  excretory  in  function,  but  no  Malpighian 
tubules  or  coxal  glands  seem  to  exist,  though  an  homology  of 
the  excretory  glands  just  mentioned  and  of  glands  occurring  in 
the  walking-legs  of  the  males  with  the  latter  is  not  impossible. 
The  Pycnogonids  are  bisexual,  the  reproductive  organs  lying 
in  the  thorax  and  sending  out  branches  into  the  Walking-legs, 
on  the  fourth  joints  of  one  or  more  of  which  they  open.  As 
already  stated,  the  male  carries  the  eggs  upon  his  ovigerous 
legs,  fastening  them  as  they  are  extruded  by  the  female  by 
means  of  the  excretion  of  the  glands  occurring  upon  the 
walking-legs. 

Development  and  Affinities  of  the  Pycnogonida. — The  young  Pycno- 
gonid  leaves  the  egg  as  a  six-limbed  embryo,  which  recalls,  in  a  general  way, 
tiie  nauplius  of  the  Crustacea,  and  indeed  has  suggested  a  derivation  of 
tlie  Pycnogonids  from  that  group.  The  resemblance  is,  however,  but 
superficial,  important  differences  l)eing  found  in  the  structure  of  the  eyes 
and  in  the  absence  of  an  anus,  to  say  nothing  concerning  the  details  of  the 
early  development.  On  the  other  hand  these  last,  as  well  as  the  structure 
of  the  eyes,  recall  the  Arachnids,  and  it  seems  most  probable  that  the 
Pycnogonids  are  to  be  regarded  as  having  descended  from  ancestors  which 
might  have  been  included  in  the  type  Arachnida. 


Order  Tardigrada. 

The  Tardigrada  are  small  forms  not  exceeding  a  milli- 
metre in  length,  with  an  unsegmented  body  provided  with 
four  pairs  of  short  conical  appendages  tipped  with  claws,  the 
last  pair  being  situated  at  the  posterior  extremity  of  the  body. 
The  bddy  is  covered  b}'  a  cuticle  secreted  by  the  subjacent 
hypodermis,  below  which  and  traversing  the  ccelom  is  a  well- 


TYPE  ARACHNID  A. 


467 


h 
e 


developed  system  of   muscle-bauds.      There   are  no  special 
orgaus  either  for  respiratiou  or  circulatiou. 

The  mouth,  surrouuded  by  papilhc  (Fig.  214,  p),  lies  at  the 
anterior  extremity  of  the  bod}',  aud  leads  iuto  a  tubular 
mouth-cavity  coutaiuiug,  imbedded  in 
its  walls,  a  pair  of  chitinous  or  partly 
calcareous  teeth,  and  receiving  the 
ducts  of  two  glands  {sg)  which  have 
been  regarded  as  salivary  or  perhaps 
poisonous  in  function.  Behind,  this 
cavity  opens  into  a  muscular  pharynx 
{ph)  which  is  connected  by  a  short 
tesophagus  with  the  mid-gut.  At  the 
junction  of  this  with  the  rectum  or 
hind-gut  is  a  pair  of  crecal  diverticula 
(?</),  probably  Malpighian  tubules,  and 
into  the  hind-gut  there  also  open  the 
ducts  of  the  reproductive  organs,  the 
hind-gut  thus  serving  as  a  cloaca.  It 
opens  on  the  ventral  surface  of  the 
body  a  short  distance  from  the  pos- 
terior extremity  and  therefore  in  front 
of  the  last  pair  of  appendages. 

The  nervous  system  consists  of  a 
supracesophageal  ganglion  (ce)  united 
with  a  chain  of  four  pairs  of  ventral 
ganglia.  No  special  sense-organs  occur 
except  two  ej'es  situated  at  the  sides 
of  the  head.  The  sexes  are  distinct, 
the  reproductive  organ  being  unpaired 
and  opening  into  the  cloaca,  into  which 
opens  also  in  both  sexes  an  unpaired 
accessory  gland. 

The  Tardigrada  occur  in  water  usually,  especially  in  such 
locations  as  the  gutters  on  the  roofs  of  houses,  though  some- 
times found  also  among  moss.  The  group  contains  but  a 
small  number  of  genera,  of  which  Macrobiotm  is  perhaps  the 
most  common. 


Fig.  214. — Diagram  of 
Stuuctuue  op  Macro- 
Mot  m  Hufelandii  (com- 
bination of  tigures  by 
Plate). 
an  =  auus. 

ce  —  cerebral  ganglion. 
dg  =  dorsal  gland. 
Ig  =  lateral  gland  or  Mal- 
pighian tubule. 
m  =  muscle. 
ov  =  ovary. 
p  =  papillae. 
ph  =  pharynx. 
sg  =  sal  i  vary  (?)  glands. 


1^1 


468 


IN VEHTKIiHA  TK  MOliPIIOLOG  Y. 


Affinities  of  the  Tontit/nida.  The  omhryologloal  history  of  these 
forms  lias  not  yet  been  sutticiently  studied  to  allow  of  any  defluite  conclu- 
sions as  to  their  atlinifies.  The  presence  of  four  pairs  of  limbs  has  usually 
been  rej»arded  as  pointin;^  to  a  relationship  with  the  Acarina,  but  the  ab- 
sence of  all  uiouth-appendaKes,  the  stnu'ture  of  the  legs,  and  the  position 
of  the  last  pair  with  regard  to  the  anal  opening,  not  to  mention  the  peculi- 
arities of  the  internal  organization,  are  opposed  to  any  close  relationship 
with  the  Arachnida.  The  Tardigrada  must  be  considered  as  holding  an 
independent  position,  without  distinct  indications  of  relationship  with  any 
of  the  types,  until  further  information  as  to  their  developmentalphenomena 
has  been  secured. 

LITERATURE. 


PKNTASTOMIDA. 

R.  Leuokart.  Ban  tmd  Entwicklungsgeschichte  der  Pentaatomen.  Leipzig  und 
Heidelberg.  1860. 

C.  W.  Stiles.  Ban  und  Eiitioicktungageschichte  von  Pentastomum  proboaciileum 
uml  P.  subeyUndricHin.    Zeitschr.  flir  wissensch.  Zoologie,  Lki,  1891. 

A.  B.  Spencer.  T?ie  Anatomy  of  Pentastomum  teretiuaculum  (Baird).  Quar- 
terly Journal  of  Microscop.  Science,  xxxiv,  1892. 

PYCNOOONIDA. 

E.  B.  Wilson.  77ie  Pycnogonida  of  New  England  and  AdjoMtit  Waters.  Re- 
port of  the  U.  S.  Commissioner  of  Fish  uud  Fisheries  for  1878.  Wush- 
ington.  1880. 

A.  Dohrn.  Die  Pantopoden  des  6o{fes  von  Neapel.  Fauna  und  Flora  des 
Golfes  von  Neapel.     Monographic,  in,  1881. 

P.  P.  C.  Hoek.  Report  on  the  Pycnogonida.  Scientific  Results  of  the  Voyage 
of  H.M.S.  Challenger.    Zoology,  in,  1881. 

T.  H.  Morgan.  A  Contribution  to  t/ie  Embryology  and  Phytogeny  of  the  Pycno 
gonids.  Studies  for  the  Biol.  Laboratory,  Johns  Hopkins  University,  v, 
1891. 


TAKDIGllADA. 

L.  Plate.    Beitriige  zur  Naturgeschichte  der  I'ardigraden. 
Ajuitom.  Abtheilung,  in,  1888. 


Zoolog.  JahrbUcher, 


TYPE  TliAVUEATA, 


469 


CHAPTEK  XV. 


TYPK  TKACIJKATA. 


The  Tracheata  are,  like  the  Aniclinitlu,  esseutially  terres- 
trial forms,  for,  though  a  few  Insects  have  adapted  themselves 
to  au  aquatic  mode  of  life,  they  are  nevertheless  uir-breatliers, 
liviug  either  at  the  surface  of  the  water  or  coming  to  the  sur- 
face from  time  to  time  to  renew  the  air  contained  in  the 
tracheoD  which  ramify  through  the  body  and  serve  as  respira- 
tory organs.  However,  a  few  Insect-larvic  have  acquired  the 
power  of  extracting  oxygen  from  the  water  by  branchia-like 
processes  of  the  body,  but,  eveu  in  these  cases,  tracheae  form 
the  organs  by  which  the  respiration  is  carried  on,  the  branchiio 
being  richly  supplied  with  them. 

The  body  is  distinctly  segmented  (except  in  Peripaftis,) 
and  is  covered  by  a  chitinous  cuticle  secreted  by  the  ecto- 
dermal cells,  which  constitute  the  so-called  hypodermis.  The 
appendages  are  usually  uniramous,  and  with  few  exceptions 
(Peripatm)  are  jointed.  The  anterior  pair  in  all  cases  are 
more  or  less  elongated  multiarticulate  structures  provided 
with  seuse-hairs,  and  are  situated  prjeorally,  while  of  the 
remaining  pairs,  varying  in  number  in  diflfereut  groups,  the 
most  anterior  pair  is  specialized  to  serve  as  mandibles,  while 
the  succeeding  one  or  two  pairs  usually  form  maxilhe.  Numer- 
ous glands  of  varying  function  are  developed  in  the  hy})o- 
dermis,  the  most  interesting  of  which  are  the  crural  glamls, 
well  developed  in  Peripahis,  and  represented  more  or  less 
perfectly  in  certain  other  forms.  In  addition  to  these,  glands 
which  secrete  an  acrid  or  offensive  fluid  {repugnatorial  glands) 
are  frequently  present,  as  well  as  others  which  secrete  waxy 
substances,  or  even  in  some  cases  silk. 

The  coelom  except  in  Peripatus  is  lacunar  throughout,  pos- 
sessing no  definite  walls,  and  is  traversed  in  various  directions 
by  muscles,  serving  to  flex  or  extend  the  body  and  to  move 


i 


I 


M 


470 


IN  VERTEBRA  TE  MORPUOLOU  Y. 


! 


n\ 


ir  .:*;- 


i* 


I 


the  appeiida^es.  A  marked  difference  between  the  Traclieata 
and  the  Arachuida  is  the  universal  absence  of  an  eudu* 
sternite,  a  structure  of  ct)usiderable  phylogenetic  significance 
in  the  latter  group.  A  heart  is  invariably  present,  lying 
above  the  intestine,  and  situated  iu  a  pericardial  sinus  incom- 
pletely partitioned  off.  In  the  majority  of  forms  the  parti- 
tion is  composed  of  a  varying  number  of  triangular  muscles, 
the  alar  muscles,  which  are  attached  by  their  bases  to  the 
walls  of  the  heart,  and  by  their  apices  to  the  body-wall. 
While  at  rest  they  are  somewhat  vaulted,  the  convexity  being 
dorsal,  and  on  contraction  flatten  down,  thus  enlarging  the 
sinus  and  causing  a  flow  of  blood  into  it.  The  heart  (Fig. 
227,  h)  is  elongated  and  imperfectly  divided  into  a  series  of 
chambers,  separated  by  pairs  of  valves  which  allow  the  blood 
to  flow  from  behind  forwards  but  not  in  the  reverse  direction, 
the  heart  being  closed  behind.  Ostia  are  present  in  the 
lateral  walls  to  allow  of  the  entrance  of  blood  into  the  heart- 
chambers,  whence  it  is  propelled  through  very  short  arteries 
which  open  widely  into  the  lacunar  spaces  of  the  ccelom.  In 
many  forms  a  ventral  sinus  surrounds  the  ventral  ganglionic 
nerve-chain,  the  blood  flowing  in  it  from  before  backwards, 
but  with  this  exception  definite  vessels  are  wanting.  This  is 
compensated  for  by  the  rich  branching  of  the  trachero,  which, 
as  stated,  serve  as  respiratory  organs  and  convey  air  to  all 
parts  of  the  body  ;  the  air  is  iu  fact  brought  directly  to  the 
tissues,  instead  of  being  carried  to  them  by  the  blood  from 
limited  portions  of  the  surface  of  the  body.  The  blood  is 
usually  colorless,  but  in  some  cases  is  of  a  bright  yellow  or 
green  color,  owing  to  pigment  contained  in  the  plasma,  and  it 
contains  in  all  cases  colorless  amceboid  corpuscles. 

The  tracheae  (Fig.  215,  tr)  commr  icate  with  the  exterior 
along  the  sides  of  the  body  by  a  varying  number  of  pairs  of 
stigmata  {st\  and  may  either  consist  of  bunches  of  unbranched 
tubes  connected  with  each  stigma,  or  of  a  number  of  richly- 
branching  tubes,  each  one  arising  from  a  separate  stigma  and 
anastomosing  in  some  cases  through  some  of  its  branches 
with  the  tubes  from  other  stigmata.  Each  stigma  is  usually 
provided  with  an  apparatus  by  which  it  may  be  closed,  and 
in  the  Insects  the  air  is  expired  from  the  tracheae  by  the  con- 


TYPK  THACUKATA. 


471 


traction  of  certuiu  duruo-veutrul  miiHcles  t)f  the  ubilomen, 
which  caune  a  coinpreusiuu  of  the  organs  iu  that  region  of  the 
body,  inspiration  foih)wing  on  their  relaxation  and  the  conse- 
quent re-expansion  of  the  abdomen.  In  structure  the  tracheie 
are  simply  to  be  regarded  as  invaginations  of  the  body-wall, 
and  consist  of  a  single  layer  «)f  cells  continuous  with  the  hypo- 
dermis  of  the  body,  lined  within 
— that  is  to  say,  on  the  surface 
with  which  the  air  is  in  contact 
— with  chitin,  which  is  thick- 
ened in  such  a  way  as  to  form 
a  sjiiral  band  extending  along 
the  tube  and  serving  as  a  spring 
to  keep  its  walls  apart. 

The  digestive  tract  is  in 
most  groups  a  straight  tube, 
but  iu  Insects  (Fig.  227)  it  may 
be  coiled  in  a  more  or  less  com- 
plex manner  and  differentiated 
into  several  parts.  Glands  of 
various  kinds  are  usually  asso- 
ciated with  it,  salivary  glands 
(Fig.  227,  sg)  opening  into  the 
anterior  portion  and  Malpighian 
tubules  (m?j),  in  connection  with 
the  posterior  portion,  being 
the  most  constant  iu  occur- 
rence. It  is  to  be  noted  that 
the  fore-gut  and  hind  gut  are 
ectodermal  in  origin,  and  that 
the  Malpighian  bodies  arising 
as  outgrowths  from  the  hind- 
gut  are  also  ectodermal,  differing 
thus  in  origin  from  the  similarly- 
named  organs  of  the  Arachnida,  which  are  apparently  of 
endodermal  origin,  arising  from  the  mid-gut.  In  function 
both  organs  are  similar,  the  Malpighian  bodies  of  Tracheates 
being  excretory. 

The  nervous  system  in  the  less-differentiated  members  of 


Fm.  215.  —  FiGUUE    snowiNO   the 

DiSTIUBUTION     OF     TUACIIE/K     IN 

Aphh  ptlargonii  (after  Witlaczil). 
At  =  antennae. 
g  =  gland-duct. 
at  =  stigma. 
tr  =  trachcii. 
1,  2,  3  =  thoracic  appendages. 


472 


INVEliTEBRATE  MOliPUOLOG Y. 


tile  tjpe  cousists  of  a  aupraoesoplmj^eal  gauj^liouic  mass,  con- 
uected  by  ciicumu)sopIia<j;eal  coiiiinissures  Avith  a  cliaiu  of 
veutral  f^aii^lia,  a  pair  of  ganglia  correspoiidiug  typically  with 
each  segment,  lu  the  lusecta  (Fig.  228)  more  especially, 
however,  c()iisi«lerable  coiiceutratiou  occurs,  a  uumber  of  the 
postoral  ganglia,  or,  in  some  cases,  all  of  them,  fusing  to  a 
single  mass.  A  well-cleveh)ped  stomatogastric  or  sympathetic 
nervous  system  occurs  in  all  forms,  arising  from  the  supra- 
u3sophageal  ganglionic  mass  by  two  trunks,  which  unite  to 
form  a  single  nerve,  passing  to  the  digestive  tract,  and  iu 
some  cases  provided  with  ganglionic  enlargements  both 
paired  and  unpaired. 

Sense-organs  of  various  kinds  are  well  developed  in  the 
Tracheata,  with  the  exception  of  Peripatus,  in  which  the 
only  definite  organs  of  special  sense  are  the  eyes.  In  other 
forms  the  antennas  and  other  })()rtious  of  the  body  are  pro- 
vided with  hairs  connected  with  nerves  and  serving  as  tactile 
organs,  and  seta)  situated  uj)<)n  the  mouth-parts  and  associated 
with  peculiar  nerve-endings  have  been  su[)[)osed  to  re[)res(mt 
organs  of  taste,  and  others  again,  on  the  antennae,  olfactory 
organs.  Eyes  are  very  generally  i)resent.  In  Peripatus  and 
most  Myriapoda  simple  eyes  or  ocelli  are  alone  [jresent ;  in 
I*erip((fns  they  resemble  closely  in  structure  the  eyes  of  the 
Annelids  or  Mollusca  (e.g.  //(diotls,  see  Fig.  127),  but  in  the 
INIyriapods  and  Insects  they  are  usually  more  complicated. 
Thus  in  a  young  larva  of  Aciliiifi  (Fig.  21(5,  A),  a  water- 
beetle,  the  chitin  is  thickened  to  form  a  cornea  (l)  which  lies 
over  a  depression  of  the  lupodermis,  the  cells  at  the  bottom 
of  which  are  modified  to  form  a  retina,  each  being  continuous 
at  its  inner  end  with  tlie  optic  nerve  {n),  while  at  its  outer 
end  it  bears  a  layer  of  chitin  (r).  The  cells  of  the  lip  of  the 
de|)ression  have  converged  together  so  as  to  meet  beneath  the 
cornea,  which  is  indeed  formed  by  these  cells,  and  a  cavity  is 
thus  enclosed  into  which  there  protrude  from  among  the 
retinal  cells  large  cells  (rm/c)  with  chitin  dejjosited  on  their 
adjacent  surfaces.  In  a  later  stage  (Fig.  210,  />)  the  lips  (jf 
the  depression  hiive  united,  a  continuous  corneal  hypodermis 
{vb)  being  thus  produced ;  pigment  has  been  deposited  in  the 
lateral  cells,  and  the  retinal  cells,  pigmented  near  their  outer 


.'.j^ii^\iii...i»*'isiA'>&iiafciism 


TYPE  THAVllEATA. 


473 


euds  and  in  Ciiutiuuatiuu  with  the  optic  uerve,  have  developed 
distiuct  rods  \^r)  at  their  outer  euds. 

lu  tlie  liiseeta  and  occasionally  in   the  Myriapoda  [Scuti- 
gera)  tlnuc^  are  in  aiUlition  to  these  sim[)le  ocelli  compound 


«} 


es,  situated  at  the  sides  of  tlie  head  and  similar  in  structur( 


to  the  compountl  eyes  of  the  Crustacea.      Each  of  the  om- 
mutidia  of  which  the   eye   is   composed,  and  there    may  be 


in 
the 
-ho 
id. 
ir- 
lies 
bm 
•us 
ter 
he 
Ihe 
is 
Ihe 
iir 
of 
liis 
he 
ler 


V 


Fig.  316.— Suctions  THitoiTdn  an  Ocri.lus  of  a  Lauva  ok  Aeilius  in  {A)  a 

VkkY    YoUNO   and   in   (/)')  AN   OlDEK   Sl'KCIMKN    (afl.T  I'attkn). 

ir  —  inverted  rods.  n  —  luirve. 

I  —  coriiwi.  pg  =  piiriucnt. 

mgc  =  inudian  jjiimf  cells,  r  =  rmls. 
rb  =  vitreous  body. 

several  thousand  of  tliem  in  each  eye,  consists  of  an  external 
cornea  (Fifj;.  217,  co),  usually  more  or  less  hexai^onal  in  outline, 
j^iviu}.^  the  eye  a  faceted  appearance.  ]?eneath  the  cornea 
are  two  cells  which  secrete  it  and  form  the  corneal  hypodei- 
mis,  and  below  these  aj^ain  come  four  cells,  the  crystalliiK'- 
cone  cells,  which  may  {cHconoiis  eyes)  or  nniy  not  {aconons 
eyes)  manufacture  a  crystalline  cone  (c),  and  finally  beneath 
these  is  a  circle  of  seven  retinular  cells  (four  in  /A'pi,s)H(i\ 
each  one  of  which  is  ])i<,5meuted  and  manufactures  a  portion 
of  the  chitinlike  rhabdom  {rh)  which  they  surround  ;  these 
cells  are  i)robably  continuous  at  their  inner  ends  with  tln^ 
optic   uerve.      Additional    pij^ment-cells   {pg)    separate    the 


474 


JNVEUTEBliATE  MORPF    ^00 Y. 


various  ommatidia. 


CO  - 


_-Rh 


Pg*^ 


Ommatidfum 
Musca  (after 


T 

Fig.  217 
OP  Eye  of 

HlCKSONM. 

c  =  cr>stiilliuu  coue. 
CO  =  cornea. 
pg  =  pigment  cells. 

r  =  retiuula-cells. 

lih  =  rlmbcloni. 

1'  =  trachea. 

tv  =  tracheal  dilatatiou 


Other  sense-orgaus  occur  in  the  various 
groups,  but  may  more  satisfactorily  be 
considered  in  the  special  descriptions 
of  these  groups. 

True  nephridia  similar  to  those  of 
the  Annelids  occur  in  Peripatus^  but  in 
the  Myriapods  and  Insects  they  are  en- 
tirely wanting,  their  place  as  excretory 
organs  being  taken  by  the  Malpighian 
tubules.  The  Tracheata  are  bisexual, 
the  reproductive  organs  being  typically 
paired  and  opening  to  the  exterior  by 
ducts,  which  may  unite  befoil-e  reaching 
the  genital  orifice.  Accessory  struc- 
tures, such  as  a  bursa  copulatrix  for 
the  reception  of  the  penis  and  a  recep- 
taculum  seminis  occur  in  the  female,  and 
vesicula3  seminales  and  accessory  glands 
in  the  male.  The  region  of  the  body- 
wall  in  the  vicinity  of  the  reproductive 
orifice  is  in  the  Insects  frequently  in- 
vaginated,  adding  a  still  greater  com- 
plication, and  furthermore  the  terminal 
portion  of  the  duct  in  the  male  is 
frequently  capable  of  being  evaginated 
and  thus  serving  as  a  penis,  while 
integumentary  elevations  or  processes 
of  the  last  abdominal  segment  form 
ovipositors  in  the  females. 


I.  Class  Protracheata. 

This  interesting  group  contains  but  a 
single  genus,  Peripatus,  which  has,  how- 
ever, a  wide  distribution,  species  being 
found  in  the  West  Indies  and  South  America,  at  the  Cape  of 
Good  Hope  and  in  New  Zealand,  thus  indicating  an  original 
wide  distribution  of  the  genus  which  has  become  extinct 
except  in  these  few  widely-separated  regions. 

Peripat/iis  is  an  elongated  cjliudrical  form,  measuring  in 


TYPE  THACHEATA. 


475 


the  Cape  species  (Fig.  218)  from  about  five  to  six  and  a  lialf 
centimetres  in  length,  and  is  found  beneath  stones  or  bark  <.)r 
amongst  decaying  wood.  The  body- wall  is  finely  annulated, 
the  annuli  not,  however,  corresponding  to  segments,  and  the 
cuticle  is  thin,  small  papilhe  being  scattered  all  over  the 
surface  of  the  body,  each  terminating  in  .a  short  bristle. 
The  head  is  but  poorly  marked  off  from  the  rest  of  the  body 
and  bears  a  pair  of  many-jointed  anteiinse,  at  the  base  of  each 
of  which  towards  the  sides  of  the  head  is  situated  an  eye.  The 
mouth  lies  in  the  middle  of  the  ventral  side  of  the  head  and 
is  surrounded  by  numerous  papilhe,  and  within  its  cavity  is 
situated  a  pair  of  jaws  furnished  with  strong  chitinous  sickle- 
shaped  teeth.     These  jaws  really  represent  the  second  pair  of 


Fig.  318. — Peripatus  capensis  (after  Moseley  from  Balfour), 

appendages,  the  third  pair  being  represented  by  two  short 
papillae  lying  at  the  sides  of  the  head  and  lumng  at  their  tii)s 
the  openiugs  of  a  pair  of  glands  which  extend  far  back  into 
the  bodj^-cavity  and  from  which,  when  the  animal  is  irritatevl, 
there  is  violently  emitted  a  sticky  fluid,  whence  the  glands 
have  been  termed  the  slime-glands  (Fig.  219,  s<^).  There  is 
no  division  of  the  trunk  into  thoracic  and  abdominal  regions, 
and  it  bears  a  number,  varj'ing  according  to  the  species  from 
seven  to  twenty-one  pairs,  of  ambulator}'  appendages,  each  of 
which  consists  of  a  proximal  stouter  and  soniowhat  conical 
pt>rtion  bearing  rings  of  papilla)  and  .1  more  slender  short 
distal  portion  which  bears  at  its  tip  a  pair  of  claws  (unguefi). 
These  limbs  are  unsegmented,  differing  in  this  respect  from 
those  of  the  Myriapods  and  Insects,  and  are  also  soft  owing 
to  the  thinness  of  the  cuticle,  a  feature  wdiich  has  suggested 
the  name  Malacopoda  formerly  ap[)lied  to  the  type,  as  the 
presence  of  the  terminal  ungues  has  suggested  the  term 
Onyehophora.  The  anus  is  situated  at  the  posterior  extremity 
of  the  body  and  has  on  either  side  of  it  the  anal  papillae 
which  represent  the  last  pair  of  limbs. 


476 


IN  VERTEBRA  TE  MOBPUOLOG  Y. 


Beneath  the  thin  cuticle  is  situated  the  hypotlerniis,  and 

beneath  this  a  well-devek)ped 
dermal  muscular  system  re- 
calling that  of  the  Annelida, 
being  composed  of  an  outer 
layer  of  circular  muscles,  be- 
low which  are  diagonal  fibres, 
and  below  these  again  strongly- 
devek)ped  longitudinal  muscles 
arranged  in  bundles,  two  of 
which  are  situated  dorsally, 
two  laterally,  and  three  ven- 
trally.  In  addition  to  these, 
dorso  -  ventral  bands  occur 
passing  across  the  ccelom  and 
dividing  it  into  three  chambers, 
one  median  and  two  lateral, 
and  special  muscles  are  also 
present  for  moving  the  limbs. 

The  coelom  is  tolerably 
capacious  and  consists  of  two 
portions.  The  larger  portion 
is   divided    by  partitions   into 

■Bw     n<ft      Tn-  -„  ™  several     subordinate     cavities 

Fig.  219. —Figure   showing  the  t  .   ,.      ,    ,  , 

SxnucTuuE  OF  A  Female  Pmpa^Ms  (^i^^-  220),  and  is  lined  through- 

out  by  a  peritoneal  epithelium 
which  covers  the  various  or- 
gans. From  it  are,  however, 
separated  certain  cavities  with 
definite  walls,  which  stand  in 
relation  to  the  nephridia  and 
the  reproductive  organs  and 
will  be  spoken  of  in  connection 
with  these  organs.  A  heart 
(Fig.  220,  h)  of  a  tubular  form 
extends  throughout  almost  the 
entire  length  of  the  body,  lying 
in  a  pericardial  space  (Fig.  220,  pc)  incompletely  separated 
from  the  rest  of  the  coelom  by  a  fenestrated  transverse  par- 


(from  Hertwio). 

a  =  anus. 
at  =  auteunse. 
bin  =  ventral  nerve-cord. 

d  =  intestine. 
go  =  opening  of  reproductive  organ. 

o  =  ovary. 
og  =  brain. 

p  =  pharynx. 
ad  = 'slime-glands. 
so  =  nephridia. 
sp  =  salivary  glands. 

tr  =  tracheoB. 

u  =  uterus. 


TYPE  TRAVllEATA. 


All 


titiou.  A  pair  of  ostia  are  situated  ou  the  dorsal  surface  of 
the  heart  iu  each  metamere,  aud  pass  the  blood  iuto  the  heart 
from  the  pericardial  space.  liespiration  is  performed  by 
tracheie  (Fig.  211),  tr)  consisting  of  sleuder  uubraiiched  tubes 
which  arise  iu  buuches  from  stigmata,  either  scattered  ir- 
regularly over  the  surface  of  the  body  iu  considerable 
numbers  or  else  arranged,  as  in  P.  capensis,  somewhat  imper- 
fectly in  two  rows  upon  the  dorsal  and  two  on  the  ventral 
surface  of  the  body. 

The  mouth  opens  into  the  mouth-cavity  containing  the 
mandibles,  and  this  communicates  posteriorly  with  a  muscu- 
lar pharj'ux,  and  has  opening  into  it  tlie  ducts  of  two  Jong 
tubular  salivary  glands  (sp)  which  extend  through  more  tlum 
half  the  length  of  the  body.  The  pharynx  {p)  communicates 
by  a  short  cesophagus  with  the  stomach,  which  extends  as  a 
straight  tube  almost  to  the  extremity  of  the  bod}',  where  a 
short  rectum  places  it  in  connection  with  the  anus.  The 
pharynx  and  oesophagus  and  the  rectum  are  lined  with  chitin 
aud  represent  the  fore-gut  and  hind-gut  of  other  Tracheates, 
the  stomach  being  the  mid-gut.  No  Malpighian  tubules  or 
other  diverticula  of  the  intestine  occur. 

The  nervous  system  shows  several  highly-interesting 
features.  There  is,  as  is  usual  in  metameric  animals,  a  supra- 
oesophageal  ganglion-mass  (Fig.  219,  07)  composed  of  at  least 
two  aud  probably  three  pairs  of  ganglia,  of  which  the  first 
supplies  the  antennae  and  the  second  the  mandibles,  while  a 
third  pair  lies  at  the  sides  in  close  contact  with  the  second 
pair  and  sends  nerves  to  the  oral  papilhe.  These  latter  are, 
however,  postoral  and  ventral  in  position,  and  from  them 
there  extend  back  two  ventral  cords  (bm)  which  in  each 
metamere  dilate  into  a  ganglionic  swelling.  The  two  ventral 
cords  are,  however,  widely  separated,  Ijnng  in  the  lateral 
chambers  of  the  ccelom  (Fig.  220),  and  are  connected  by  a 
large  number  of  cross-commissures — a  condition  which  recalls 
the  arrangement  iu  the  Amphineurous  MoUusca  (see  Fig. 
124),  the  similarity  being  further  increased  by  the  facts  that 
the  two  cords  unite  behind  aud  above  the  rectum,  as  in  the 
Solenogastres,  and  that  the  ganglion-cells  are  not  confined  to 
the  enlargements  but  are  scattered  all  along  the  cords.     The 


p!     i 


1      i 


478 


M  VERTEBRA TE  MORPUOLOO  T. 


eyes  are  the  only  special  organs  of  sense ;  their  structure  has 
been  already  indicated  (p.  472). 

One  of  the  most  interesting  features  of  Peripatus  is  the 
bccurrence  in  it  of  typical  nephridia  (Fig.  219,  so).  Upon  the 
under  surface  of  the  proximal  portion  of  each  limb,  with  the 
exception  of  the  penultimate  or  last  pair,  there  is  a  slitlike 
opening  which  leads  into  a  more  or  less  coiled  tube  lying  in 
the  coelomic  compartment,  which  extends  into  the  limb  and 


Fig.  230. — Tkansveusb  Section  op  a  Peripatus  (after  Sbdgwick). 
c  =  central  compartment  of  ccBlom.     mg  =  slinie-glauds. 


g  =  reproductive  orgau. 
h  =  heart. 
/  =  iutestine. 

'  =  lateral  compartment  of  coelom. 
VI  =  muscles. 


N  =  ventral  nerve-cords. 
7ie  =  uephridia. 
p  =  compartment  of  coelom   which 

extends  into  the  limb. 
pe  =  pericardial  compartment  of  the 

coelom. 


terminates  in  a  thin-walled  vesicle.  These  tubes  are  ne- 
phridia, the  terminal  vesicles  (Tig.  220,  we)  representing  por- 
tions of  the  coelom  into  which  the  uephridia  open — a  fact 
indicated  by  their  embryological  history.  The  nephridia  ,are 
thus  exactly  comparable  in  every  respect  with  the  nephridia 
of  Annelids,  communicating  at  one  extremity  with  the  exterior, 
and  at  the  other  with  the  ccelomic  cavity.  It  is  interesting  to 
note  that  the  development  of  the  salivary  glands  shows  that 


jim^jf»4'it'^%*M^lilJi^-i^iiJtail^ 


TYPE  TRACHEA'" A. 


479 


they  are  the  modified  uephridia  of  the  third  segment  of  the 
body,  that  which  bears  the  oral  papillte,  and  furthermore  it  is 
to  be  noted  that  in  the  last  or  next  to  last  (according  to  the 
species)  limb-bearing  segment,  in  which  nephridia  are  wantiug, 
are  found  the  ducts  of  the  reproductive  organs — a  fact  which 
suggests  that  these  are  also  modified  nephridia.  This  idea  is 
confirmed  by  the  development  of  the  genital  ducts,  and  car- 
ries Avitli  it  the  corollary  that  the  cavities  of  the  reproductive 
organs  (Fig.  220,  g)  are  portions  of  the  cceloin,  just  as  they 
were  shown  to  be  in  the  MoUusca  (see  p.  288). 

In  addition  to  the  uephridia  there  are  associated  with  cer- 
tain of  the  appendages  glands  which  open  on  the  under  sur- 
face of  their  basal  moiety  and  are  termed  the  crural  glands. 
In  P.  capensis  they  are  present  in  all  the  appendages  except 
the  more  anterior  one,  and  the  slime-glands  are  simply  the 
highly-modified  crural  glands  of  the  oral  papilljB,  those  of 
the  last  pair  of  appendages  in  the  males  of  this  species  being 
similarly  elongated  though  possessing  a  difierent  function. 
In  P.  Edioardsiiy  however,  crural  glands  occur  only  in  the 
males,  and  in  these  only  in  a  few  segments  immediately  in 
front  of  that  bearing  the  reproductive  opening. 

The  Protracheata  are  bisexual,  the  female  usually  being 
somewhat  larger  than  the  male.  The  ovaries  are  paired, 
though  included  within  a  common  capsule,  and  lie  in  the  pos- 
terior part  of  the  coelom.  They  are  continuous  with  two 
uteri,  which  immediately  at  their  origin  are  united  by  a  trans- 
verse tube,  and  each  bears  a  receptaculum  ovorum  and  a 
receptaculum  seminis.  Beyond  this  each  continues  its  course 
along  the  side  of  the  body,  passing  backwards  to  finally 
unite  at  the  common  orifice,  lying  a  short  distance  in  front  of 
the  anus  on  the  ventral  surface  of  the  body.  The  testes  are 
slender  paired  structures  which  are  continuous  Avith  a  slender 
vas  deferens.  This  dilates  a  short  distance  from  the  testis 
into  a  vesicula  seminalis  and  then  unites  with  its  fellow  of  the 
opposite  side  to  form  a  slender  somewhat  coiled  tube,  the 
ductus  ejaculatorius,  in  the  terminal  portion  of  which  the 
spermatozoa  are  united  together  into  a  spermatophore.  The 
Protracheata  are  viviparous. 


ii 


480 


IN  VEltTEBRA  TE  MOIiPIIOLOO  Y. 


Affinities  of  t/ie  Pr(»tracfieata.—Pertpatus  is  a  liighly-sugfjestive  form 
on  accoutit  of  possessing  both  Annelidaii  and  Tracheate  characteristics,  so 
that  it  has  been  generally  regarded  as  indicating  a  descent  of  the  Tracheate 
forms  from  tlie  Annelids.  Its  Annelidan  features  are,  first,  the  presence  of 
a  distinct  dermal  muscular  system;  second,  the  occurrence  of  crural  glands 
which  seem  to  be  homologiics  of  the  glands  which  secrete  the  seta?  in  the 
Annelida;  third,  the  possession  of  nephridia  corresponding  closely  to  those 
of  the  Annelids;  and  fourth,  the  structure  of  the  eyes.  On  the  other 
liand,  its  Tracheate  affinities  are  shown  by  the  claw-tipped  feet,  by  the 
adiiptution  of  the  feet  (mandibles)  for  masticatory  purposes,  by  the  ten- 
den(!y  towards  a  concentration  of  the  anterior  segments  to  form  a  head, 
and  by  the  occurrence  of  trachea?.  Both  these  sets  of  foatures  are  highly 
important,  and,  taken  with  the  wide  distribution  of  Peripatus,  point 
strongly  to  its  being  the  roi)resentative  of  a  connecting  link  between  Tra- 
cheates  and  Annelida,  a  phylogeny  which  may  be  considered  more  in  de- 
tail at  the  close  of  this  chapter. 


II.  Class  Myriapoda. 

The  Myriapoda  possess  a  distiuct  head  composed  of  a  num- 
ber of  fused  segments  end  followed  by  a  distinctly-segmented 
body  formed  of  a  varying  number  of  segments,  all  of  which 
are  mure  or  less  similar,  there  being  no  differentiation  of  a 
thorax  and  abdomen.  A  single  pair  of  appendages  as  a  rule 
is  borne  by  each  segment,  with  the  exception,  in  some  cases, 
of  the  last.  The  most  anterior  pair  are  usually  long  multi- 
articulate  anteunio,  the  second  pair  mandibles,  and  the  third 
and  fourth,  or  the  third  alone,  Jire  modified  to  form  maxilhi); 
the  succeeding  pairs,  with  one  or  two  exceptions,  are  ambula- 
tory, and  are  jointed  and  tipped  by  a  claw. 

The  chitinous  cuticle  is  generally  thick,  and  consequently 
no  definite  system  of  dermal  muscles  is  developed,  a  number 
of  separate  muscles  occurring  in  each  segment  for  moving  the 
appendages  and  the  various  segments  upon  one  another. 
Glands  of  various  kinds  opening  upon  the  surface  of  the  body 
occur,  the  most  important  being  glands  or  protrusible  gland- 
ular sacs  situated  upon  the  basal  joints  of  a  number  of  the 
appendages  and  apparently  homologous  with  the  crural 
glands  of  Peripatus. 

The  heart  is  in  all  forms  very  long,  extending  through  the 
entire  length  of  the  body  behind  the  head,  and  possessing 


TYPE  TRACIIEATA. 


481 


just  as  many  chambers  aud  pairs  oi  alar  muscles  as  there  are 
truuk-segmeuts.  The  uuuibor  of  stigmata  vary,  in  some 
forms  only  a  single  pair  occurring,  wliile  in  others  there  is  a 
pair  on  each  segment  of  the  trunk;  and  the  form  of  the 
trachete  varies  also,  as  they  are  sometimes  branched  and 
sometimes  arranged  in  bunches  composed  of  a  number  of  un- 
branched  tubes. 

The  digestive  tract  is  almost  always  a  straight  tube,  ex- 
tending through  the  body  to  the  terminal  anus.  The  mouth 
is  guarded  in  front  by  a  well-developed  upper  lip  or  labrum, 
while  the  fusion  of  the  maxilhe  behind  it  in  many  forms  pro- 
duces a  lower  lip.  It  leads  into  an  ectodermal  fore-gut,  and 
this  into  an  endodermal  mid-gut,  which  is  usually  provided 
with  a  number  of  unbranched  diverticula  termed  hepatic  cieca. 
One  or  two  pairs  of  Malpighiau  tubules  open  into  the  ante- 
rior end   of  the  ectodermal  rectum,  and  serve  as  excretory 


organs. 


The  nervous  system  except  in  the  head  region  shows  but 
little  trace  of  concentration,  there  being  as  a  rule  in  each 
segment  of  the  trunk  a  pair  of  ganglia.  The  auteunal  ganglia 
are  fused  with  the  supraoesophageal  ganglionic  mass  which 
sends  off  branches  to  the  ocelli ;  these  may  be  quite  numer- 
ous, though  compound  eyes  do  not  as  a  rule  occur.  A  sympa- 
thetic system  is  present  as  in  other  Tracheates. 

There  are  no  nephridia  so  far  as  known  in  the  group,  the 
excretion  being  performed  by  the  Malpighian  tubules.  The 
reproductive  organs  are  paired,  and  open  to  the  exterior  in 
some  cases  by  paired  orifices,  but  more  usually  by  a  single 
opening,  which  may  be  situated  either  far  forwards,  or  else 
in  other  cases  near  the  posterior  extremity  of  the  body. 


1.  Order  Fauropoda. 

The  order  Pauropoda  contains  a  few  small  forms  in  which 
the  trunk  possesses  but  ten  metameres  and  nine  pairs  of  ap- 
pendages, which,  with  the  exception  of  the  first  pair,  are  six- 
jointed  and  terminate  in  a  claw.  In  some  species,  when  viewed 
from  the  dorsal  surface,  the  segments  appear  to  be  less  nu- 
merous than  the  appendages,  a  condition  which  results  from 


482 


INVERTEBRATE  MORPHOLOGY. 


Fio 


Pauropus 


the  fu-siou  of  ceitiiiu  raetamereH  in  pairs,  so  that  two  pairs  of 
appendages  appear  to  belong  to  some  of  the  segments,  the 

double  nature  of  which  is  further  showf\  by 
the  occurrence  in  tiiem  of  two  pairs  of 
nerve-ganglia.  The  anteuuju  (Fig.  221)  are 
remarkable  in  form,  consisting  of  a  four- 
jointed  basal  portion  which  bifurcates  at 
the  tip,  one  of  the  bifurcations  bearing  two 
long  tiagella  and  a  peculiar  spherical 
stalked  body,  while  the  other  one  bears  a 
single  ilagellum.  Mandibles  are  present, 
and  there  is  also  a  single  pair  of  but  poorly- 
developed  maxilla). 

TracheiB  or  other  respiratory  organs  are 
Huxley UtToiu  lbunis).  not  yet  known  to  exist.     A  large  simple  eye 
occurs  on  each  side  of  the  head  near  the 
base  of  the  autennce,  and  the  reproductive  opening  is  situa- 
ted upon  the  second  trunk-segment. 

Further  study  of  this  group  is  much  needed  to  elucidate 
its  characteristics.  The  genus  Pauropus  has  ten  trunk-seg- 
ments, of  which  the  first  nine  bear  appendages ;  while  in 
Eurypauropus  there  are  only  six  trunk-segments,  and  eyes  are 
wanting. 

2.  Order  Diplopoda. 

The  Diplopoda,  sometimes  termed  the  Chiloguatha,  are 
popularly  kn(j  an  as  the  Millipedes  on  account  of  the  commoner 
forms  possessing  an  unusually  large  number  of  appendages. 
The  body  is  usually  cylindrical  and  provided  with  a  hard 
cuticle,  and  many  forms  are  in  the  habit  of  rolling  themselves 
when  disturbed  into  a  ball  or  a  helixlike  coil,  thus  protect- 
ing the  more  delicate  ventral  surface  of  the  body.  The  an- 
tenna (Fig.  222,  at)  are  generally  seven-jointed  and  are  never 
very  long,  and  the  mandibles  are  strong  jaws  without  palps. 
In  front  of  the  mouth  is  a  well-developed  upper  lip  {ul\  while 
behind  it  is  a  lower  lip  formed  by  a  fusion  of  the  maxilla)  (ma?). 
According  to  some  authors  this  lower  lip  represents  two  pairs 
of  appendages,  but  its  innervation  and  embryological  history 
seem  to  be  opposed  to  this  view.     The  segments  behind  the 


TYPB  TliACHKATA. 


483 


liead  vary  iu  uuniber  in  diflfereut  genera  from  eleven  {Glo- 
merin)  to  over  one  liuiulred,  and  the  number  of  appendages  is 
much  greater  still,  since  the  majority  of  the  segments  bear 
two  pairs  of  limbs  and  in  reality  represent  each  two  meta- 
meres.  The  four  or  five  anterior  trunk-segments  are,  however, 
single,  bearing  but  a  single  pair  of  limbs  (Fig.  222),  and  one 
of  them — in  some  cases  the 
first,  in  others  the  second,  but 
more  usually  the  third  {lulus) — 
is  entirely  destitute  of  appen- 
dages. The  last  few  segments 
also  carry  but  a  single  pair  of 
appendages,  as  does  also  the 
seventh  segment  in  the  males, 
the  appendages  of  which  are 
usually  modified  to  serve  as 
copulatory  organs. 

Stigmata  occur  on  each  of 
the  trunk-segments,  the  double 
segments  bearing  two  pairs, 
situated  on  the  ventral  surface 
near  the  coxal  joints  of  the 
limbs.  Each  one  has  in  connec- 
tion with  it  a  bunch  of  un- 
branched  tracheae,  a  condition 
recalling  somewhat  that  of  Peri- 
patus,  although  the  location  of 
the  stigmata  is  much  more  regular  and  definite.  Upon  the 
dorsal  surface  of  the  body  there  is  in  most  species  a  row  of 
pores  which  have  been  mistaken  for  stigmata,  but  are  really 
the  openings  of  glands  (gl,  repugnatoria)  secreting  an  oily 
evil-smelling  fluid  which  serves  as  a  means  of  defence.  In 
the  genus  Polydesmus  the  secretion  contains  hydrocyanic 
acid.  Crural  glands  do  not  as  a  rule  occur,  but  protrusible 
warts  occurring  on  the  coxal  joints  of  a  number  of  legs  iu 
some  genera  {Lysiopetalum)  have  been  regarded  as  homolo- 
gous structures. 

The  nervous  system  has  the  characteristic  Myriapodan 
arrangement,  each  of  the  double  segments  possessing  two 


Fig.  222.— Anteuiou  Portion  op  a 

DlPI.OPOD. 

at  =  antoniia. 
cop  =  copulatory  appendages. 
mx  =  maxilla. 

ul  =  upper  lip,  [segments. 

1,  2,  3,  4  =  the  four  anterior  truuk- 


i 

!    ! 

1    * 


484 


INVEIiTKBHATK  MOIWIIOLOO  Y. 


vJ^'^ 


pairs  of  ganglia.  Eyes  are  iisuallj  j)resoiit  aud  are  always 
Hiiiiple,  varyinj^  in  iiuinbtir  from  two  to  aH  many  as  oijjhty. 

The  Diiilopoda  are  l)is(>Mial,  and  tlie  ovaries  or  testes  form 
a  sinj:;le  mass  from  which  two  ducts,  or  oue  which  hiter 
divides  into  two,  arise  and  pass  forward  to  open  ou  tlie  ven- 
tral surface  of  the  body  between  the  second  and  third  trunk, 
segments.  The  emliryos  when  first  hatched  out  j)ossess  but 
three  pairs  of  legs,  situated  upon  the  first,  third,  and  fourth 
segments  in  Strontjylosoma,  and  on  the  first,  secon«l,  and  fourth 
in  Inlns,  oue  or  more  segments  without  appendages  lying 
behind  the  fourth  pair.  By  successive  moults  new  segments 
and  appendages  are  added  aud  the  form  of  the  adult  gradu- 
ally acquired. 

The  Diplopoda  live  for  the  moat  part  under  stones,  etc., 
or  among  dead  leaves,  and  find  their  food  in  decaying  vege- 
table matter,  though  some  forms  will  attack  living  vegetation 
aud  may  prove  thereby  injurious  to  gardens.  The  commonest 
form,  lulus,  may  readily  be  obtained  under  stones  or  boards 
all  through  the  summer. 


3.  Order  Chilopoda. 

The  Chilopoda,  or  Centipedes,  are  very  different  in  their 
habits  from  the  Millipedes,  being  carnivorous  and  provided 
with  poison-glands  which  render  the  larger  forms  of  8colo- 
pendra  dangerous  even  to  man.  The  body  is  as  a  rule  some- 
what flattened  and  less  hard  than  that  of  the  Diplopoda. 
The  autennte  (Fig.  223,  at)  are  usually  long,  with  at  least 
twelve  joints,  and  may  be  as  long  as  the  body,  while  the 
mouth-parts  are  much  more  complicated  than  in  the  Dip- 
lopoda. The  mandibles  and  upper  lip  resemble  the  corre- 
sponding parts  in  that  groijp,  but  the  raaxillre  (mx)  are  jaw- 
like, are  not  fused  together,  and  in  some  forms  {Geophilns) 
bear  a  palp.  Behind  the  maxilla3  comes  a  pair  of  second 
maxillae  {mx'),  which,  however,  do  not  serve  as  jaws  but  are 
reduced  to  a  pair  of  palplike  structures,  and  behind  these  again 
is  a  pair  of  maxillipeds  (mxp),  the  appendages  of  the  first 
trunk-segment,  with  their  basal  joints  fused  to  form  a  lower 
lip  supporting  a  four-jointed  palp,  the  last  joint  of  which  is 


TYPE  TliACUKATA. 


485 


Fig.  323.— Antkuiou  Pou- 

TION  OF  A  ClUliUl'OD. 

at  =  iiDtcnnoD. 
»«j'  =  uiuxilla. 
mx'  =  second  mnxillu. 
mxp  =  maxilliped. 


cluwlike  uud  is  perforated   by  the  tluct  of   a  poisfm-gland. 

Eacli  trnuk-se({iueiit,  of  which  tht'ie  may  be  over  a  huudred, 

bears  but  a  single  pair  of  .\,pi)eiidu^es, 

there  being  uo  conipouud  se^iueuts  as 

ill  the  Diplopoda.     Each  le^'   is  as   a 

rule  seven-joiuted,  the  coxal  joiiits  of 

those  of  the  same  segmeiit  beiug  widely 

separated,   and  there   is  uo   mod'tica- 

tiou  of  the    seventh  pair   to   serve  as 

copulatory  or<5aus.  though  the  pair  of 

the    penultimate    segment   are    much 

reduced  in  size  and  lie  at  the  sides  of 

the  reproductive  orifice 

Stigmata  are  usually  wanting  in  the 
first  three  trunk-segments,  but  occur  in 
a  certain  number  of  the  others,  lying 
usually  laterally  between  the  segments 
except  in  Sciitigei^a,  in  which  they  have  a  dorsal  position. 
They  open  into  branched  tracheal  trunks  which  usually  an- 
astomose with  one  another,  though  in  Scutigent  they  open 
into  sacs  from  which  a  large  number  of  simple  iiiibranched 
tracheal  tubes  arise  arranged  in  a  bunch  as  in  the  Diplopoda. 
Crural  glands  occur  on  the  coxal  joints  of  several  of  the 
posterior  appendages. 

The  nervous  system  is  arranged  as  in  other  Myriapods,  and 
simple  eyes  are  usually  present,  in  Scutigera  only  being  closely 
aggregated  together  to  form  a  faceted  eye.  This,  however,  is 
not  a  compound  eye  exactly  similar  to  that  of  the  Insects,  but 
is  to  be  regarded  simply  as  a  close  aggregation  of  simple 
eyes. 

The  reproductive  organs  are  usually  paired,  and  the  sexes 
separate.  The  ducts  unite  before  opening  to  the  exterior,  so 
that  there  is  but  a  single  opening  situated  on  the  antepenulti- 
mate segment  of  the  trunk,  the  appendages  of  which  are 
greatly  reduced  in  size.  The  embryos  of  Scolopendra  and 
GeopJdlus  leave  the  egg  with  almost  the  same  number  of  ap- 
pendages as  the  adult,  while  those  of  Scutigera  and  FAthohius 
possess  but  seven  pairs  of  legs  (in  addition  to  the  maxillipeds) 
and  gradually  acquire  others  by  successive  moults. 


i:i 


486 


IN VEKTEBHA TE  MORPIIOLOG  Y. 


s 


lu  Scutujera,  a  form  which  frequents  the  wanner  parts  of 
the  world,  the  dorsal  surface  of  the  bod}'  is  covered  in  by 
eight  shieldlike  folds  which  conceal  a  certain  number  of  the 
segments,  which  are  about  lifteen  in  number.  fAtkolnus  has 
the  same  number  of  segments  and  is  common  under  stones, 
etc.,  as  is  also  Geophihis  aud  Scolopemira,  both  elongated  forms, 
the  former  usuall}'  without  ejes,  while  the  latter  usually 
possesses  them  but  has  only  some  nine  or  ten  pairs  of  stig- 
mata. Some  of  the  species  of  /Scolopendra,  especially  those 
living  in  warm  countries,  grow  to  a  considerable  size  and  are 
capable  of  inflicting  a  dangerous  wound. 


4.  Order  Symphyla. 

The  order  Symphyla  contains  a  number  of  small  forms 
referable  to  one  or  two  genera,  of  which  the  best  known  is  the 
genus  Scolopendrella  (Fig.  224).  Unfortunately  the  details  of 
the  structure  of  the  members  of  the  group  are  by  no  means 
well  known,  a  circumstance  all  the  more  to  be  regretted  since 
/Scolopendrella  seems  to  possess  certain  Insect-like  features. 

The  body  is  ehmgated,  and  on  the  dorsal 
surface  possesses  a  number  of  plates 
which  overlap  slightly,  but  which  do 
not  correspond  in  number  Avith  the  ap- 
pendages. The  head  bears  a  pair  of 
long  many-jointed  antenujiB,  and  behind 
these,  in  the  region  of  the  mouth,  is  a 
pair  of  mandibles  and  a  single  pair  of 
maxilhe,  both  these  last-named  ap- 
pendages being  deeply  imbedded  as  it 
were  in  the  tissues  of  the   head,   their 

Fig.  234.'-  Scolopendrella  *iP«  ""l-^'  projecting.     The  first  pair  of 
immaculata  (from  lkunw).   trunk  appendages  is  not  transformed  into 

maxilli})eds  as  in  the  Ohilopoda,  but  is 
ambulatory  in  function,  and  most,  but  not  all,  of  the  succeed- 
ing segments,  of  which  there  are  apparently  fourteen,  bear  a 
pair  of  five-jointed  legs  terminated  by  two  claws.  Attached 
to  the  coxal  joints  of  most  of  these  appendages  is  a  peculiar 
spurlike  process,  internal  to  which  is  situated  a  protrusible 


'M%.:'  ^^-'i^iiaaitii^tfAuwkiiJafWtt^^i^iiiKt 


TYPE  THACUEATA. 


487 


glandular  sac  which  is  probably  to  be  regarded  as  a  crural 
gland.  The  last  pair  of  appendages  may  be  unjointed,  each 
bearing  a  tactile  seta,  and  attached  to  the  last  segment  is  a 
pair  of  conical  processes  each  of  which  has  opening  at  its  tip 
the  duct  of  a  spinning-gland. 

Two  stigmata,  situated  at  the  base  of  the  antennjc,  are  the 
only  ones  which  occur,  their  position  being  very  remarkable. 
They  open  into  bunches  of  branched  trachesv  which  extend 
throughout  the  greater  portion  of  the  body,  leaving  only  the 
appendages  and  the  posterior  part  of  the  trunk  destitute  of 
an  air-supply.  The  Malpighian  tubules  attached  to  the 
hind-gut  are  very  long,  and  salivary  glands  opening  upon  the 
maxilliD  are  present. 

Eyes  do  not  occur.  The  reproductive  organs  are  situated 
in  the  fourth  trunk-segment  and  are  paired.  The  oviducts 
and  vas  deferens  unite  together  and  open  to  the  exterior  by  a 
single  pore  situated  also  upon  the  fourth  segment,  though  it 
has  been  described  by  some  authors  as  situated  at  the  pos- 
terior extremity  of  the  body.  Very  little  is  as  yet  known 
concerning  the  development  of  Scolopendrella,  whose  similarity 
to  the  Insects  is  shown  in  the  antennt©  and  the  mouth-parts  ; 
indeed  it  has  been  advocated  by  some  writers  that  the  genus 
should  be  associated  with  the  Thysanura  among  the  lusecta, 
and  the  possibility  of  its  being  a  connecting  link  between  that 
group  and  Myriapodous  forms  is  indicated  in  the  name  applied 
to  the  order. 


III.  Class  Insecta. 

The  class  Insecta  is  far  richer  in  species  than  any  other 
class  of  animals,  some  two  hundred  thousand  species  belonging 
to  it  being  known  to  exist,  and  of  these  about  eighty  thoiisand 
are  beetles.  A  very  large  number  are  provided  with  organs 
of  flight  and  may  be  termed  aerial ;  others  are  terrestrial, 
living  either  upon  the  surface  of  the  earth  or  excavating  bur- 
rows beneath  its  surface  ;  while  some  have  ada})ted  them- 
selves to  an  aquatic  mode  of  life,  and  others  are  even  marine, 
members  of  the  genus  Hahhates  being  found  on  the  surface 
of  the  ocean  many  miles  from  land.     Many  species,  living  as 


•? 


488 


INVERTEBliATE  MOIiPUOLOQY. 


they  do  upou  vegetable  food  either  in  the  adult  or  larval 
stages,  and  occurring  occasionally  in  enormous  numbers,  form 
powerful  enemies  to  the  horticulturist  and  agriculturist,  the 
llocky  Mountain  Locust,  for  example,  devastating  at  times 
the  crops,  while  fruit  and  forest  trees  are  injured  by  the  at- 
tacks of  other  forms. 

The  lusecta  ditt'er  from  other  Tracheata  in  having  the 
body  divided  into  three  well-marked  regions.  The  most  an- 
terior of  these  is  the  unsegmeuted  head,  bearing  the  antennaB 
and  the  masticatory  appendages,  and  immediately  following 
it  is  the  thorax,  composed  of  three  segments,  the  prothorax, 
mesothorax,  and  metathorax,  each  of  the  last  two  bearing 
usually  n  pair  of  wings  upou  its  dorsal  surface,  while  pos- 
teriorly is  the  segmented  abdomen  composed  typically  of  ten 
segments,  sometimes  as  broad  as  the  thorax  at  the  junction 
with  that  region,  sometimes  contracted  to  a  narrow  stalk.  In 
many  cases,  however,  the  apparent  number  of  segments  falls 
below  ten  owing  to  the  fusion  of  certain  of  the  posterior  seg- 
ments or  the  union  of  the  anterior  segment  with  the  thorax, 
and  in  the  Butterflies  aud  two-winged  Flies  the  thoracic  seg- 
ments seem  to  be  reduced  to  two  owing  to  the  close  associa- 
tion of  the  metathorax  with  the  first  abdominal  segment. 

Four  pairs  of  appendages  are  borne  by  the  head.  The 
anteume,  and  indeed  all  the  appendages,  vary  greatly  in 
shape  in  the  various  groups,  but  are  usually  long  slender 
multiarticulate  structures  provided  with  sensory  hairs.  The 
masticatory  appendages  are  a  pair  of  mandibles  aud  two 
pairs  of  maxilhe,  which  are  variously  specialized  for  biting, 
piercing,  or  sucking.  The  most  typical  condition  is  that  in 
which  the  entire  apparatus  is  adapted  for  biting  and  that  may 
be  described  here,  leaving  special  modifications  to  be  con- 
sidered in  connection  with  the  orders  in  which  they  occur. 
The  mandibles  (Fig.  225,  C)  are  strong  uujointed  toothed 
plates  which  meet  together  in  the  middle  line  and  are  pro- 
vided with  strong  muscles.  The  first  maxillae,  or,  as  they  are 
usually  termed,  the  maxillne  (Fig.  225,  B),  on  the  other  hand 
are  distinctly  jointed,  and  consist  of  a  basal  joint,  or  cardo, 
succeeded  by  a  second  joint,  or  stipes,  which  bears  on  its 
outer  side  a  multiarticulate  palpus  (p)  and  terminates  in  one 


w,  ,tfii»»iwrjitiiiw»]iaiaifei>fe.'  ■'•• 


. . .  t  -  ^i.'-^^-,-*u»i*iiM(j ; 


TYPE  TliACUEATA. 


489 


Fi«i. 


or  two  uuseguieuted  plates  of  which  the  iuuermost  is  usually 
toothed.  The  second  maxillae  (Fig.  225,  A)  are  also  jointed 
and  are  fused  together  to  form  a  lower  lip,  or  labium.  The 
fused  basal  joints  form  the 
suhmenlum  {sni),  the  second 
joints  the  mentum  (m),  which 
bears,  as  does  the  stipes  of  the 
ni;ixill«3,  a  jointed  palp  (p)  and 
terminates  frequently  in  two 
uujointed  plate-like  processes. 
The  three  pairs  of  appendages 
of  the  thorax  are  typically 
ambulatory,  but  are  modified 
for  clasping,  swimming,  digging, 
etc.,  according  to  the  habits  of 
the  insect.  They  typically  con- 
sist of  a  basal  joint,  the  coxa, 
succeeded  by  one  or  two  small 
joints,  the  trochanter,  upon 
which  follow  a  femur,  a  tibia, 
and  a  tarsus,  the  last  consisting 

of  live  (occasionally  four)  short  joints,  the  terminal  one  bear- 
ing two  claws  or  ungues.  The  abdomen  in  the  adult  forms  is 
as  a  rule,  destitute  of  appendages,  except  in  the  Thysanuraus 
(Fig.  231),  the  lowest  of  all  the  orders  of  Insects.  In  these  a 
number  of  the  segments  are  provided  with  a  pair  of  spurlike 
processes  which  recall  the  spurs  upon  the  basal  joints  of  the 
trunk  appendages  of  Scolopend reiki,  and  are  apparently  ho- 
mologous with  them.  In  the  embryos  of  probably  all  forms 
rudimentary  appendages  are  found  on  some  of  the  abdominal 
segments,  but  tliej'  later  disappear,  showing,  however,  a  de- 
scent of  the  Insecta  from  forms  in  which  abdominal  appen- 
dages Avere  functional  in  the  adult.  Processes  of  various 
kinds,  such  as  cerci,  ovipositors,  and  copulatory  organs,  are 
frequently  borne  by  the  ])osterior  abdominal  segments,  but 
these  do  not  seem  to  be  equivalent  to  appendages. 

As  stated,  a  pair  of  wings  is  usually  borne  by  the  meso- 
and  metathoracic  segments.  These  structures  are  entirely 
wanting  in  the  lowest  insects,  the  Thysanura  and  Collembola, 


S25.— MouTn-PAKTS  of  a  Bee- 
tle, MonohaminuH. 
A  =  lubiuin. 
B  =  maxilla. 
G  =  mandible. 


490 


INTBRTBBHATE  MORPHOLOGY. 


as  well  as  in  certain  forms  belonging  to  other  groups  which 
have  lost  them  through  parasitism  (Mallophaga,  Pvlex,  Mdo- 
phagus)  or  other  causes  (Worker  and  Soldier  Termites,  Neuter 
Ants,  the  females  of  some  Moths).  They  are,  when  their 
possessors  are  first  hatched  out,  saclike  structures,  pro- 
cesses of  the  body-wall,  trachese  enclosed  within  blood-lacunse 
extending  from  the  body  into  their  cavities.  Later,  however, 
the  walls  of  the  sac  come  into  contact,  the  cavity  being  ob-x 
literated,  the  tracheae  with  the  blood-lacunro  in  which  they 
are  situated  remaining  enclosed  within  the  flat  plates  so 
formed  and  constituting  the  so-called  veins  of  the  wing,  which 
have  in  most  species  a  characteristic  and  constant  arrange- 
ment. In  certain  forms  the  anterior  wings  become  more 
or  less  thickened  by  the  deposition  in  them  of  additional 
chitin,  and  may  form  hard  plates  {dytra,  Fig.  239)  which 
serve  as  a  cover  and  protection  for  the  posterior  v;ings,  which 
in  such  cases  are  alone  used  in  flight.  In  the  two-winged 
Flies  (Fig.  244)  the  posterior  wings  are  very  much  reduced, 
being  represented  only  by  two  small  club-shaped  structures 
termed  "  balancers,"  attached  to  the  sides  of  the  metathorax. 

The  body  is  enclosed  in  a  chitinous  cuticle,  usually  of 
some  firmness  and  frequently  bearing  numerous  hairlike  pro- 
cesses, certain  of  which  serve  as  sense-organs.  Glands  open- 
ing on  the  surface  of  the  body  also  occur  in  connection  with 
the  integument ;  for  example,  peculiar  protrusible  glandular 
sacs  are  situated,  two  or  four  on  each  segment,  on  the  ab- 
dominal segments  of  the  Thysanura  and  Collembola  close  to 
the  spurlike  abdominal  appendages  present  in  those  forms, 
and  are  in  all  probability  homologous  with  the  similar  struc- 
tures of  Scolopendrella  and  therefore  presumably  represent 
crural  glands.  These  glands  appear,  however,  to  be  wanting 
in  other  insects.  Many  genera  of  Hemiptera  possess  glands 
which  produce  a  malodorous  secretion,  and  wax-glands  occur 
in  the  Plant-lice  (Coccidce)  and  Bees,  the  latter  also  possess- 
ing poison-glands  in  connection  Avith  a  complicated  stinging- 
apparatus,  which  is  a  modified  ovipositor. 

The  respiratory  stigmata  vary  greatly  in  number  in  dif- 
ferent groups  of  Insects.  In  the  wingless  Thysanura  and 
Collembola  there  are  usually  ten  stigmata  on  each  side  of  the 


.io«;'«4fe.'iia<<<Mi<afe»*''^^ 


TYPE  TRACnSATA. 


491 


body,  two  being  situated  on  the  sides  of  the  thorax  and  eight 
on  the  abdomen,  but  in  Campodea  the  number  is  reduced  to 
three  pairs,  which  occur  ui)ou  the  thorax.     In  the  winged 
forms  the  number  also  varies  somewhat,  but  there  are  again 
typically  ten  pairs,  arranged  as  in  the  Thysanura.     They  lead 
into  short  trunks,  which,  in   Campodea,  ramify  through  the 
body  without  anastomosing,  but  more  usually  they  are  united 
on  each  side  of  the  body  by  a  longitudinal  tube,  from  which 
pass  off  numerous  branches  penetrating  to  all  parts  of  the 
body,  and  transverse  connecting  tubes  passing  between  the 
systems  of  the  tAvo  sides  (see  Fig.   215).     In  certain  forms 
which  are  active  flyers  the  longitudinal  tubes  are  frequently 
dilated    to    form    air-sacs,  as  in   the 
Bees,  or  numerous  air-sacs  may  occur 
which  may  be   more  or   less  emptied 
or  expanded  according  to  the  will  of 
the    insect,  the  specific  gravity  of  the 
body    being    thus    altered.       In    the 
aquatic   larvie   which    occur  in   some 
forms,  such  as  the  Maj-flies  (Epheme- 
ridse,  Fig.  226),  adaptations  occur   for 
the  breathing  of  air  dissolved  in  the 
water,  the  sides   of  the   body  in  the 
abdominal  region  being  prolonged  into 
a  number  of  pairs  of  platelike  process- 
es, into  which  branches  of  the  traehet© 
project,  an   interchange   of   the   gases 
contained   in   the    tracheae   for    those 
dissolved    in   the    water    taking  place 

throuyrh  the  walls  of  the  plates,  simi- 

,     ,     f        ,     ,  •       .1      1  1  •     Fig-    226.— Lauva 

larly  to  what  occurs  in   the  brauchi*     ephemeuiu  (f.on, 

of  the  Crustacea,  though  in  these  forms 

the  exchange  is  directly  with  the  gases 

of   the   blood.      These   structures  are   consequently  termed 

tracheal  branchisB,  and  while  they  are  functional,  the  stigmata 

are  closed,  only  opening  when  the  adult  stage  is  reached  and  a 

terrestrial  life  adopted.     As  a  rule  the  tracheal  branchiie  are 

thrown  off  at  the  moult  by  which  the  adult  form  is  reached, 

but  in  a  few  forms  they  persist  throughout  life. 


OF     AN 

.KINIS). 

a  =  truclieul  bmucbioi. 


492 


INVERTEBRATE  MORPHOLOOY. 


A  dermal  muscular  system  does  not  exist,  but  complicated 
and  well-developed  muscles  are  present  for  the  movement  of 
the  various  parts  of  the  body,  those  occurring  in  the  thorax 
being  especially  well  developed  and  serving  for  the  movement 
of  the  limbs  and  wings.  As  in  other  Tracheates  the  ccelom 
is  lacunar,  and  the  heart  lies  in  a  pericardial  sinus  below  the 
dorsal  surface  of  the  body,  alar  muscles  extending  from  it  to 
the  walls  of  the  body  and  partly  dividing  the  sinus  into  a  dor- 
sal and  a  ventral  chamber.  In  the  Thysanura  the  heart 
extends  from  the  posterior  thoracic  region  throughout  the 
greater  part  of  the  abdomen,  and  consists  of  nine  chambers 
separated  by  valves  and  each  provided  with  a  pair  of  ostia  and 
a  pair  of  alar  muscles  In  the  majority  of  forms  (Fig.  227,  h), 
however,  the  heart  is  entirely  confined  to  the  abdominal 
region,  and  the  number  of  chambers,  though  frequently  as 
high  as  eight,  may  be  greatly  reduced.  An  aorta  extends 
forwards  from  the  anterior  chamber  into  the  head,  in  the 
Butterflies  (Fig.  227)  dilating  in  the  thorax  to  form  a  second- 
ary heart  {aJi),  and  sends  off  branches  which  quickly  empty 
into  the  lacunar  spaces. 

The  greater  portion  of  the  abdomen  is  ^cupied  by  a 
peculiar  tissue,  termed  the  fat-body,  in  which  the  various 
organs  are  more  or  less  imbedded,  and  which  receives  its 
name  from  the  fact  that  its  cells  contain  globules  of  fatty 
matter,  and  in  the  adult  insect  usually  also  concretions  of  uric 
acid.  It  is  not  necessarily  confined  to  the  abdomen,  but  may 
extend  into  the  thorax  or  even  into  the  head.  In  certain 
Beetles — the  Fireflies  (Lampyridae)  and  Pyrophorus  of  the 
West  Indies — certain  regions  of  the  body,  especially  the 
abdomen,  and,  in  Pyrophorus,  two  spots  upon  the  thorax,  give 
out  under  certain  conditions,  apparently  under  control  of  the 
animal,  a  very  bright  light,  usually  spoken  of  as  a  phosphores- 
cence. The  tissue  which  produces  the  light  is  the  fat-body, 
or  special  portions  of  it  abundantly  supplied  by  trachesB,  and 
the  process  seems  to  be  one  of  oxidation  of  phosphorus- 
containing  substances.  The  exact  nature  of  the  phenomenon 
is  but  poorly  understood  at  present,  and  it  is  not  possible  by 
any  means  at  our  disposal  to  produce  in  the  laboratory  a 


■■^i>«rrmtim**iri^:u 


TYPE  TRACHEATA. 


493 


light  equal  in  iuteusity  to  that  of  the  Firefly  with  the  expeuili- 
ture  of  as  little  energy. 

The  digestive  tract  is  as  a  rule  much  more  complicated 
than  iu  other  classes  of  Tracheates  and  is  generally  more  or 
less  twisted  or  contorted  in  the  abdominal  region,  so  that 
usually  it  is  longer  than  the  body.  T-ie  mouth  is  bounded 
in  front  by  a  usually  large  upper  lip  or  labrum,  generally 
described  with  the  mouth-parts,  but  distinguished  from  them 
in  not  representing  a  pair  of  appendages.  The  anterior 
portion  of  the  intestine,  the  fore-gut,  is  ectodermal  in  origin 


ag" 


Fig.  227. — Structure  op  Butterfly,  Danais  arehippus  (iifter  ruroess). 


a  •—  anteuna. 
ag  =  accessory  glnnd. 
ah  =  accessory  heart. 
an  =  abdomiual  ganglion. 

be  =  bursa  copiilalrix. 
c  =  crop. 

ec  —  canal  uniting  be  and  oviduct. 

ce  —  cerebral  ganglion. 

7i  =  heart. 
I  =  thoracic  limbs. 


mv  =  Malpighian  tubules. 
od  =  oviduct. 
ov  =  ovary. 
ph  =  pharynx. 
pi  =  palp. 
8  =  stomach. 
ag  =  salivary  gland. 
tg  =  thoracic  ganglion. 
I-III  =  thonicic  segments. 
1-9  =  abdomiual  segments. 


as  in  other  Tracheates  and  consists  of  a  mouth-cavity 
into  which,  or  in  its  neighborhood,  the  ducts  of  one  or 
more  generally  well-developed  salivary  glands  (Fig.  227,  sf/) 
open.  The  secretion  of  these  glands  varies  considerably  in 
diflferent  forms,  one  of  the  pairs  present  in  the  larvje  of  the 
Butterflies  and  certain  Moths  being  transformed  into  silk- 
spinning  glands,  the  silk  of  the  Silkworm  being  a  product  of 
their  activity.  When  digestive  the  secretion  seems  to  have 
a  peptonizing  eflfect  as  well  as  the  power  of  transforming 
starch  into  sugar,  and  is  consequently  of  considerable  diges- 
tive importance.     The  mouth-cavity  opens  behind  into  an 


494 


INVERTEBRATE  MORPHOLOGY. 


oesophagus,  whose  posterior  region  is  frequently  dilated  into  a 
crop  (Fig.  227,  c)  which  in  some  Beetles  is  lined  with  chitinous 
teeth  or  bars  and  whose  walls  are  muscular,  the  apparatus 
probably  serving  for  a  further  mastication  of  the  food.  The 
mid-gut  which  succeeds  the  crop  is  usually  dilated  into 
a  stomach  (s),  lined  in  some  cases  by  glandular  cells,  or, 
in  others,  having  opening  into  it  numerous  glandular  diver- 
ticula, the  so-called  liver-pouches.  The  hindgut,  like  the 
fore-gut  of  ectodermal  origin,  has  opening  into  its  anterior 
extremity  the  Malpighian  tubules  {mv),  which  vary  consider- 
ably in  number,  amounting  to  nearly  one  hundred  m  some 
Hymenopterans,  though  more  usually  limited  to  from  four  to 
eight.   They  are  excretory  in  function,  and  are  apparently  the 


Pig.  228.— Different  Arrangements  of  the  Nervous  System  in  Insects 
(from  Gegknbaur).    A,  Termes  ;  B,  Dytiscus  ;  C,  a  fly. 

only  excretory  organs  which  occur.  The  anus  is  situated  at 
the  extremity  of  the  body,  and  in  close  proximity  to  it  odor- 
iferous glands  frequently  open  into  the  hind-gut,  serving  as 
organs  of  defence.  In  some  cases  they  secrete  an  acrid  fluid 
which,  as  in  the  Bombardier  beetle  {Brcichimis),  can  be  ex- 
pelled with  almost  explosive  force. 

The  nervous  system  in  forms  where  it  shows  the  least 
amount  of  modification  (Fig.  228,  A)  consists  (1)  of  a  supra- 
CBSophageal  mass  composed  apparently  of  three  pairs  of 
ganglia  and  supplying  the  eyes  and  the  antennre  ;  (2)  of  a  sub- 
oesophageal  mass  composed  also  of  three  pairs  of  ganglia 
supplying  the   segments  indicated    by   the   mandibles,   the 


.»H1»U<A.>'>.>.M'< 


;.,4i«,aj^y((S*W«tafc«W*fcw^ 


TYPE  TRACHKATA. 


495 


0  a 
>us 
bus 
'he 
ito 
or, 
er- 
;lie 
ior 
er- 
me 
to 
,Le 


T8 


it 


t 


maxillae,  and  the  labium  ;  (3)  of  three  pairs  of  ganglia  in  the 
corresponding  thoracic  segments  ;  and  (4)  of  a  chain  of  ab- 
dominal ganglia,  a  pair  occurring  in  each  segment  except 
usually  the  last  two  or  three,  in  which  a  compound  ganglion 
occurs.  Frequently,  however,  this  typical  condition  is  modi- 
fied by  a  greater  or  less  concentration  of  the  various  ganglia, 
the  thoracic  ganglia  fusing  to  a  single  mass,  as  may  also,  more 
or  less  perfectly,  the  ganglia  of  the  abdominal  chain  (Fig.  228, 
B),  and  the  latter  may  even  unite  Avith  the  thoracic  ganglia  to 
form  a  single  mass  situated  iu  the  thorax,  as  in  certain  two- 
winged  flies  (Fig.  228,  C).  A  visceral  system  is  usually  pres- 
ent arising  from  the  supraoesophageal  (cerebral)  mass  and 
being  distributed  to  the  walls  of  the  digestive  tract. 

The  antennae  of  insects  seem  to  act  as  sense-organs,  and 
serve  apparently  to  control  the  flight,  since  when  removed  the 
insect  is  not  able  to  fly  with  its  accustomed  ability.  So  too 
it  seems  probable  that  in  the  Ants  and  Roaches  these  appen- 
dages are  the  seat  of  the  olfactory  sense,  and  in  the  Mosquito 
it  seems  that  certain  hairs  upon  them  may  be  auditory  in 
function.  Compound  eyes,  frequently  consisting  of  several 
thousand  ommatidia,  are  usually  present,  as  well  as  a  small 
number  of  simple  eyes  (ocelli)  situated  upon  the  dorsal  sur- 
face of  the  head.  Special  organs,  which  have  usually  been 
considered  auditory,  also  occur  in  many  forms,  virying  con- 
siderably in  complexity.  In  its  simple  form  such  an  organ 
consists  of  a  single  nerve-fibre  which  dilates  into  a  ganglion- 
cell,  prolonged  into  a  terminal  hair  which  is  enclosed  within 
a  sheath  fastened  at  one  end  to  the  wall  of  the  body.  This 
whole  apparatus  is  termed  a  chordotonal  organ,  and  there  is 
usually  attached  to  the  sheath  just  where  the  hair  arises  from 
the  ganglion  a  ligament,  which  is  also  inserted  into  the  body- 
wall.  In  the  majority  of  cases  a  number  of  gaugliou-cells  and 
hairs  are  associated  to  form  a  chordotonal  organ  (Fig.  229), 
the  various  hairs  sometimes  being  grouped  within  a  single 
sheath, — sometimes,  however,  spreading  out  in  a  fanlike  man- 
ner, each  possessing  its  own  sheath.  These  organs  occur  in  vari 
ous  parts  of  the  body,  on  the  antennae  or  on  the  limbs.  In 
the  grasshoppers  (Acridiidae)  the  first  abdominal  segment 
bears  on  either  side  a  thin  tense  membrane,  a  thinned  portion 


496 


mVEJiTKBRATE  MORPHOLOGY. 


/^x.^ 


of  the  cuticle,  recalliu^  the  tympauic  membraue  of  the  human 
ear,  beneath  and  iu  couuectiou  with  which  is  a  chordotoual 
apparatus,  further  improved  by  the  occurrence  iu  close  prox- 
imity to  it  of  a  saclike  enlargement  of  a  trachea  which  serves 
as  a  resonator.  Similar  organs  occur  in  pairs  on  opposite 
sides  of  the  tibiiu  of  the  first  pair  of  legs  in  the  Crickets 
(Gryllida)),  and   seem  from    their   structure   to  be  auditory 


Fig.  229.— SuBGENUAii  Ciioudotonal  Ouqan  of  the  Tibia  of  the  Second 
Thouacic  Appendage  of  Isopteryx  (after  Graber  from  Lang). 
bk  =  blood-corpuscles.  gz  =  uerve-cells. 

c  —  integment.  tr  =  tracheii. 

es  =  terminiil  ligiiment.  sc  =  termiuiil  hairs  and  sheaths. 

organs,  whence  the  conclusion  that  the  more  simple  chordo- 
tonal  organs  also  subserve  this  function. 

It  is  interesting  to  note  that  the  males  of  the  forms  provided  with  a 
tympanal  organ  possess  the  power  of  making  a  harsh  or  sharp  chirping 
noise,  produced  in  the  Grasshopper  by  rubbing  the  femora  of  the  hind  legs, 
which  are  furnished  on  their  inner  surfaces  with  a  row  of  fine  teeth,  over 
the  strong  marginal  veins  of  the  anterior  pair  of  wings  ;  and  in  the  Crickets 
and  Locustidae  by  rubbing  together  the  two  anterior  wings,  a  row  of  teeth 
upon  a  vein  of  one  wing  working  upon  a  projecting  smooth  vein  of  the 
other.  The  m.ale  Cicadas  also  make  a  similar  noise,  the  stridulating  appa- 
ratus resembling  that  of  the  Grasshoppers,  and  in  all  cases  it  seems  to  be  a 
sexual  characteristic  serving  to  attract  the  females. 

The  sexes  are  separate,  and  a  more  or  less  distinct  sexual 
dimorphism  occurs,  the  males  being  usually  smaller  and  more 
slender  than  the  females.  In  some  cases,  as  in  the  Tussock- 
moth  (Orgyia),  the  female  lacks  wings  and  has  a  very  different 


TYPE  TRACUEATA. 


497 


appearance  from  the  males,  and  iu  many  Beetles  the  male  is 
adorned  with  spines  and  tubercles  upon  the  head  which  are 
but  rudimentary  or  absent  in  the  female.  Differences  in  the 
shape  of  the  anteunto  and  the  prenence  or  absence  of  stridu- 
lating  organs  also  serve  to  distinguish  the  two  sexes  in  some 
of  the  groups.  In  a  few  forms  a  polymorphism  is  produced 
by  the  failure  of  certain  individuals  to  reach  sexual  maturity 
and  by  the  assumption  by  them  of  certain  special  structural 
characteristics.  Examples  of  such  cases  are  afforded  by  the 
Bees,  Ants,  and  Termites,  the  workers  of  the  first  two  groups 
being  immature  females,  while  in  the  Termites  (Fig.  237)  the 
so-called  neuters  may  be  either  males  or  females,  always, 
however,  immature. 

The  ovaries  (Fig.  227,  ov)  are  paired  and  consist  of  a 
varying  but  usually  rather  large  number  of  tubes,  which  start 
from  a  common  basis.  At  the  extremity  of  each  tube  is  the 
germ-producing  region,  the  rest  of  the  tube  being  divided  into 
a  series  of  chambers  each  of  which  contains  an  ovum  sur- 
rounded by  a  layer  of  follicle-cells.  Not  uufrequently  the 
chambers  are  arranged  more  or  less  distinctly  in  pairs,  the 
lower  one  of  each  pair  containing  an  ovum,  while  the  upper 
contains  a  number  of  small  cells  similar  in  appearance  to  the 
primary  germ-cells,  but  which  serve  as  nutrition  for  the  ovum 
which  gradually  absorbs  them  (see  Fig.  20).  From  each 
ovary  an  oviduct  arises,  the  two,  however,  soon  uniting,  and 
receiving,  usually  not  far  from  the  unpaired  orifice,  the  ducts 
of  various  glands  {ag)  whose  secretion  serves  to  cause  an 
adhesion  of  the  ova  to  the  structures  on  which  they  are 
deposited.  A  receptaculum  semiuis  is  usually  present,  and 
there  is  frequently  a  large  pouch,  partially  separated  from  the 
oviducts,  which  receives  the  male  organ  during  copulation  and 
is  termed  the  bursa  copidatrix  {he).  The  genital  orifice  is  situ- 
ated on  the  ventral  surface  of  the  ninth  abdominal  segment 
and  is  usually  surrounded  bv  a  number  of  papilla?,  or  some- 
times by  long  processes,  which  serve  as  ovipositors  and  are 
to  be  regarded  simply  as  processes  of  the  segments  from  which 
they  arise  and  not  as  modified  limbs. 

The  testes  are  also  paired,  each  being  composed  of  a 
number  of  separate  spherical  or  tubular  portions.     The  ducts 


f 


r^r 


498 


INVERTEBRATE  MORPHOLOGY. 


from  these  various  portions  ou  each  side  uuite  to  form  a  vas 
defereus  which  may  dilate  iuto  a  vesicula  seminalis  and  then, 
uuitiu^  with  its  feUow  of  the  opposite  side,  forms  the  ductus 
ejjiculatorius.  Occasiouully  the  vesicula  is  unpaired  arisiug 
from  the  poiut  of  uiiiou  of  the  two  vasa  defereiitia,  aud  very 
frequently  accessory  glands  occur.  The  ductus  ejaculatorius 
opens  usually  ou  the  ventral  surface  of  the  tenth  abdominal 
segment,  and  projections  of  the  body-wall  in  the  vicinity  of 
the  orifice  form  a  groove  or  tube  through  which  the  sperma- 
tozoa, usually  uuited  iuto  spermatophores,  are  introduced 
into  the  bursa  copulatrix  of  the  female. 

Parthenogenesis   occurs  as  a  normal  process  in  certain 
Insects,  though  always  associated  with  true  sexual  reproduc- 


FiG.  230.— Aphis  mali,  Winged  and  Wingless  Forms  (from  Packard). 

tion.  Examples  of  it  are  found  in  certain  Coccidse  {Aspidio- 
his)  and  in  some  of  the  Gall-wasps  (Cynipidse),  the  fertilized 
ova  producing  both  males  and  females,  while  in  the  Bees,  for 
example,  in  which  both  fertilized  and  unfertilized  ova  are  de- 
posited, the  latter  give  rise  to  drones  or  males  alone,  while 
workers  or  queens,  i.e.  the  femaiss,  develop  from  the  fertilized 
ova.  Occasionally  heterogony  occurs,  as  in  the  Plant-lice 
(AphidsB).  These  forms  uiider  favorable  conditions  of  tem- 
perature and  food  produce  viviparously  usually  wingless  indi- 
viduals, not,  however,  from  true  ova,  but  by  a  process  which 
may  rather  be  compared  to  internal  budding,  as  in  the  RediaB 
of  certain  Trematoda.  Generation  after  generation  of  such 
individuals  may  be  produced  during  the  summer,  but  on  the 


■„iu^M'.  j.mU.«mmi<c.. 


TYPt!  TliACIIh'ATA. 


490 


approach  of  cold  weather  or  ou  the  exhaustion  of  the  food- 
Bupply  males  aud  females  appear  by  which  true  fertilized  ova 
are  produced,  aud  from  these,  surviving  the  winter,  viviparous 
heterogouous  females  develop. 

In  the  gonus  Phylloxera,  which  \\m  phiyed  such  havoc  on  gnipe-viiiuH 
in  France,  a  greater  complication  of  gencnitions  occui-s.  A  vvintor  vgg, 
which  has  BUivived  boneuth  tlie  bulk  of  the  vine,  gives  rise  to  wingless 
forms  which  migrate  to  the  roots,  and  there  produce  numerous  generii- 
tions.  After  a  time  winged  forms  appear  whicli  ascend  from  tht)  roots, 
and,  reproducing  parthenogenetically,  increase  rapidly  in  number  and  servo 
to  distribute  the  sjMicies  over  wider  areas.  Certain  of  these  produce  snuill 
ova  from  which  males  develop,  and  others  larger  ones  whicii  give  rise  to 
females,  both  sexes  being  destitute  of  both  wings  and  digestive  tract,  and 
by  these  forms  the  fertilized  winter  eggs  are  produced. 

In  certain  flies  {Miaator^  Cecidomyia)  predogenesis  occurs, 
the  female  reproductive  organs  becoming  mature  while  the 
insect  is  still  in  the  larval  stage,  aud  the  ova,  developing  par- 
thenogenetically within  the  body,  give  rise  to  another  gen- 
eration of  larvte.  This  process  may  be  repeated  several 
times,  the  last  generation  of  larvte  developing  into  the  adult 
form  (see  Fig.  29). 

The  more  primitive  Insects,  the  Thysanura  and  Collem- 
bola,  leave  the  egg  in  a  form  resembling  the  adult,  differing 
from  it  only  in  size  and  in  the  immaturity  of  the  reproduc- 
tive organs,  and  pass  through  no  marked  metamorphosis 
during  their  post-embryonic  development.  Such  forms  are 
termed  ametabolic.  A  similar  abrtence  of  metamorphosis  is 
found  in  certain  forms  degenerated  by  parasitism  and  lacking 
wings,  but  these  have  evidently  descended  from  winged  forms 
which  passed  through  a  certain  amount  of  metamorphisni, 
so  that  the  ametabolism  is  secondary  and  should  be  distin- 
guished from  the  primitive  ametabolism  of  the  Thysanura. 
In  the  majority  of  winged  forms,  however,  a  more  or  less 
pronounced  metamorphosis  occurs.  In  the  simpler  cases  the 
young  are  distinguishable  from  the  adults  by  the  absence  or 
but  slight  development  of  the  wings,  which  become  larger 
after  successive  moults,  the  adult  form  being  thus  gradually 
acquired.  In  these  cases  of  gradual  metamorphosis  the 
habits  of  the  adult  and  larva  are  similar,  but  where  they 


/'X. 


r^-^ 


600 


INVhmTKBUATE  MORPIIOLOOT. 


ili£fer  greater  changes  result,  leading  to  hemimetaholism.  This 
occurs,  for  instauce,  in  the  Fish-flies  (Ephenieridi©)  and 
Dragon-flies  {LibcUula),  in  which  the  larvjc  are  adapted  for  an 
aquatic  life  and  possess  tracheal  brauchiie  (Ephenieridflo)  and 
other  features  which  are  lost,  either  gradually  by  successive 
moults  or  suddenly  at  the  last  mouit,  the  adult  winged  Dragon- 
fly, for  instance,  issuing  from  a  peculiar  aquatic  larva  with 
the  merest  rudiments  of  wings. 

Finally,  a  large  number  of  forms  are  holometaholic.  lu 
such  cases  the  habits  of  the  larvae  are  difterent  from  those  of 
the  adults ;  for  instance,  the  larvae  of  the  Butterflies,  the 
caterpillars  (Fig.  231),  are  wormlike  creatures  with  power- 


Pio.  231. — Lauva.  Piii'A,  AND  IiMA(io  oK  I'icfin  oleiocua  (tiom  Rii^et). 

ful  jaws  feeding  on  plant-tissues,  while  in  the  adults  the  mouth- 
parts  are  adapted  for  sucking.  The  transformation  from  the 
I.irva  to  tlie  adult  is  accomplished  by  the  intervention  of  a 
resting  stage  or  pvpa,  during  which  no  nutrition  is  taken,  and 
when  the  transformation  takes  place  the  fully-developed 
insect  or  imago  issues  from  the  ruptured  skin  of  the  pupa. 
The  pupa  varies  consideral)ly  in  form  in  difl'erent  groups, 
in  some  being  enclosed  in  a  silken  case  manufactured  by 
the  larva  before  the  last  moult  and  termed  a  cocoon,  lu 
some  cases  the  adult  api)endages  project  from  the  body  of 
the  pu]ia  {pnpn  libera),  but  in  other  cases  they  are  united 
with  the  surface  of  the  body  and  but  indistinctly  visible 
{pupa  ohtccta),  an  arrangement  usually  found  in  the  Butter- 
flies, whose  pnpao,  owing  to  their  frequent  brilliant  colora- 
tion, are  termed  cJirysalids.  a  term  which  has  been  somewhat 
incorrectly  extended  to  the  mummy-like  pupo)  of  other  forms. 


TYPE  TRACnKATA. 


501 


I'iiJiiUy,  iu  some  of  the  two-winged  liios  the  pupa  is  ouclosetl 
witliiu  the  last  liiiviil  skiu,  possessing  then  a  evlindrietil  form 
without  any  indication  of  the  adult  limbs  {ptipa  coarctaf<f). 
A  metamorphosis  in  which  a  distinct  pupa-stage  occurs  is  said 
to  be  "complete"  iu  contradistinction  to  the  Iiemimetabolic 
form  frequently  spoken  of  as  "  incomplete." 

Mention  sliouUl  be  made  liere  of  tlie  (limorphisin  or  polymorpliism  whieli 
occiu's  in  certain  adult  Insects.  It  lias  aireudy  reeeived  passinj;  mention 
(p.  497),  but  in  addition  to  tlio  freciuently-oeeufriiif,'  se.\ual  dimorphism 
tliere  occurs  in  f«)rnjs  whieii  live  lojietlier  in  colonies  a  polymorphism  asso- 
ciated with  a  division  of  labor  on  the  parts  of  the  menib(U\s  of  the  colony. 
Thus  in  the  Bees  there  are  found  the  drones  or  males  with  heavy  bodies, 
the  queen  or  female,  as  larj^e  us  the  drones  but  with  a  nnich  more  slender 
body,  and  the  workers,  which  are  sterile  females  distinguished  by  their 
.smaller  size  and  by  other  features,  such  as  a  peculiar  uioditication  of  the 
tibias  of  the  last  pair  of  legs  which  adapt  tluwn  for  tlie  collection  of  pollen 
from  the  flowers  which  they  visit.  Among  the  Ants  a  similar  trimorphism 
occurs,  males,  females,  and  n«!Uters  or  workers  constituting  the  colony;  and 
in  some  tropical  forms  the  workers  are  of  two  kinds,  namely,  ordinary 
workers  with  small  heads  and  nnuidibh's,  and  soldiers  with  large  heads  and 
sti'ong  prominent  mandibles,  whose  functions  are  indicated  l)y  the  popular 
name  a.|>plied  to  them,  though  guards  would  perhaps  be  ;aore  appropriate. 
Finally,  among  the  Termites,  popularly  known  as  the  Wiiite  Ants,  four 
forms,  i.e.,  males,  females,  workers,  and  .soldiers,  also  occur. 

In  certain  ButttM'llies  a  pecidiar  form  of  dimorphism  or  trimorphism 
termed  "seasonal  dimorphism"  occurs,  an  excellent  tixample  of  it  being 
offered  by  the  Am(M'ican  Pupil  in  AJa.v,  of  which  there  hav»>  l)een  descrilx'd 
three  distinct  vari(^ties,  differing  markedly  in  coloration  both  in  the  males 
and  the  females,  and  distinguished  as  \\ni  varit'ties  }y(ilshii,  'J't/diiioiiitlis 
and  Marcellns.  From  chry.salids  which  have  passed  the  wilder  there  hatch 
out  in  the  early  days  of  .spring  foruis  li.donging  to  the  variety  Walsfiii,  and 
somewhat  later,  from  those  whose  (level«>pment  lias  Ixhmi  retarded,  the 
Tflamoniavs  form;.  During  the  <'arly  i)art  of  sumnuu'  the  Wa/s/iii  forms 
die  out  aiiit  ;t  little  lafi^r  the  Tela moti ides  also  disappear,  both  forms  pr(>- 
vioush,  however,  depositing  ova,  most  of  which  dcn'elop  iido  larva*  and 
chrysaiids  and  hatch  out  in  tin;  later  months  of  summer  as  flu>  Manr/ltui 
form,  wl"»si!ova,  again  developing  into  chrysalids.  pass  the  winter  in  that 
state,  and  give  rise  in  the  following  spring  successively  to  the  Wn/Nhii  and 
'l\'/(inioni(f(S  Un'ius.  The  three  varieti(>s  an>  (nidently  produced  l)y  itiflu- 
t'lices  acting  upon  the  chry.salis  and  dilTeriug  according  to  the  season,  per- 
haps according  to  tem])erature,  whence  the  distinguishing  name  applied  to 
this  form  of  dimorphism,  which  is  aN.)  said  ♦^o  occur  in  certain  Spiders. 


603 


INVKRTEBRATE  MORPHOLOGY. 


1.  Subclass  Apteeyoota. 

The  members  of  this  subclass  are  all  small  and  do  not 
possess  wiugs,  the  absence  of  these  structures  being  a  primi- 
tive feature  and  not  due  to  degeneration  resulting  from  para- 
sitism or  other  causes.  In  some  forms  rudiments  of  abdomi- 
nal limbs  are  present  in  the  adults,  and  there  is  no  meta- 
morphosis in  the  post  embryonic  development  (primary 
ametabolism). 

1.  Order  Thysanura. 

The  Thysanura  or  Bristle-tails  possess  ten  abdominal 
segments,  the  terminal  one  bearing  two-  or  three-jointed  hair- 
like processes,  whence  the   name  applied  to  the  order.     The 

body  in  some  forms  (Lepisma)  is  covered 
with  scalelike  hairs  giving  it  a  silvery-gray 
appearance,  but  in  other  cases  these  are 
wanting.  The  antennae  vary  in  length,  hm 
are  always  simple  cylindrical  structui'P, 
the  terminal  joint  in  some  iorms  (Campodea) 
bearing  a  peculiar  bilobed  structure  sup- 
posed to  be  sensory,  and  the  mouth-parts 
are  adapted  for  biting  purposes  and  are 
usually  well  developed.  The  first  abdom- 
inal segment  in  some  forms  bears  a  pair  of 
indistinctly-jointed  appendages,  probably 
rudimentary  limbs,  find  a  number  of  the 
succeeding  segments  in  Campodea  bear  spur- 
like processes,  also  supposed  to  be  limbs 
and  recalling  the  spurs  of  Svolopendt'ella, 
especially  as  protrusible  glandul.'ir  struc- 
tures, comparable  perhaps  to  crural  glands, 
Fig.  2S2.-Cumpo<lea  ^^^^^^.  ^^   association   with   them    in   some 

forms. 

The  nervous  system  shows  but  little 
concentration,  eight  abdominal  ganglia  occurring  in  Lepisma, 
and  eyes  are  usually  present,  being  in  some  cases  compound. 
The  stigmata  vary  in  number,  being  usually  ten,  though  in 
Campodea  they  are  reduced  to  three,  and  the  tracheoB  in  this 


8U(phylinU8  (after 
Lubbock  from  Hux- 
ley) 


TYPE  TR  ACHE  ATA. 


503 


s 


same  form  are  iuterestiug  iu  being  destitute  of  lougitucliual 
ami  trausverse  anastomoses. 

Lepisma  is  frequently  found  in  houses,  in  attics  and  similar 
places,  feeding  upon  woollen,  linen,  and  other  fabrics,  but  also 
on  meal  or  sugar.  Campodea  (Fig.  232),  on  the  other  hand,  is 
to  be  found  under  stones  or  dried  leaves  and  is  a  small  white 
form,  by  no  means  uncommon. 

2.  Order  CoUembola. 

The  CoUembola  are  distinguished  from  the  Thysanura  by 
the  abdomen  consisting  usually  of  but  six  segments,  and  in 
some  cases  the  number  is  even  smaller.  The  body  iu  Podura 
is  covered  with  scales,  and  the  terminal  segment  of  the  body 
is  usually  provided  with  two  processes  which  may  be  bent  up 
underneath  the  abdomen  and  then  suddenly  extended,  pro- 
pelling the  insect  to  a  considerable  distance.  These  structures 
are  absent  in  the  adult  Anurida,  but  occur  in  young  speci- 
mens, and  their  occurrence  and  function  have  suggested  the 
popular  name  of  Spring-tails  applied  to  the  order.  Neither 
abdominal  appendages  nor  coxal  glands  occur,  but  the  tirst 
segment  bears  a  peculiar  orgun,  having  in  Annrida  the  form 
of  a  saccular  p/rotrusion,  which  is  probabl}^  adhesive  in  func- 
tion. The  antennje  are  usually  short,  and  bear  in  some  forms 
an  antenual  sense-organ  similar  to  that  of  Campodea ;  the 
mouth-parts  are  biting,  but  frequently  much  reduced  in  size. 

The  nervous  system  is  usually  much  concentrated,  there 
being  in  Anurida  but  three  postoral  ganglia  situated  in  the 
thorax,  the  abdominal  ganglia  having  evidently  fused  with 
the  last  thoracic.  Simple  eyes  are  present  in  vary  ing  numbers, 
but  compound  eyes  never  occur.  A  peculiar  organ  lying  be- 
hind the  bases  of  the  auteunso,  and  hence  termed  the  post- 
antennal  organ,  occurs,  and  has  been  supposed  to  be  a  sense- 
organ,  but  further  information  is  required  concerning  it. 
Tracheje  are  usually  present,  though  quite  wanting  in  Anurida. 

The  genus  Podura  is  to  be  found,  sometimes  in  consider- 
able numbers  on  the  surface  of  standing  water  in  the  early 
spring,  while  other  forms  occur  in  damp  earth  or  under  bark. 
Anurida  is  found  upon  the  seashore  underneath  stones  just 
above  tide-mark. 


504 


INVERTEBRATE  MORPHOLOOT. 


II.  Subclass  Pteeyoota. 

The  members  of  the  subclass  Pterygota  are,  as  the  name 
indicates,  typically  provided  with  wings,  though  in  a  compar- 
atively few  cases  these  structures  may  have  disappeared 
through  degeneration  due  to  parasitic  habits,  or  through  special 
adaptation  to  certain  conditions  of  life,  as  in  the  neuters  of 
the  Ants  and  Termites.  In  nearly  all  cases  the  larvjc  differ 
in  form  from  the  adults,  and  various  grades  of  metamorphosis 
are  found. 

1.  Order  Dermaptera. 

The  Dermaptera  or  Earwigs  (Fig.  233)  are  usually  small 
insects  which  resemble  not  a  little  the  Thysanura.  The 
abdomen  terminates  in  a  pair  of  forceplike 
^cesses  termed  cerci,  their  shape  suggesting 
J  generic  name  Forjicida,  applied  to  certain 
members  of  the  order.  The  anterior  wings  are 
y^jIXjl  small  and  chitinous  and  serve  as  covers  for 
the  protectior  of  the  posterior  p^ir,  which  are 
larger,  membranous  and  veined,  and  when  at 
rest  are  folded  longitudinally  like  a  fan,  and  in 
addition  twice  transversely,  so  that  they  are 
almost  completely  hidden  by  the  scalelike  an- 
terior pair.  The  antennse  are  long  and  filiform, 
and  the  mouth-parts  adapted  for  biting.  The  Earwigs  are 
terrestrial  forms  and  pass  through  a  gradual  metamorphosis. 
In  many  respects  tliey  approach  nearer  the  Thysanura  than 
any  other  insects,  and  are  related  rather  closely  to  the  suc- 
ceeding order. 

3.  Order  Orthoptera. 

In  this  order,  which  includes  the  Locusts,  Grasshoppers, 
{Caloptenus)y  Crickets  {Gryllus),  Cockroaches  (Periplaneta), 
and  other  forms,  the  mouth-parts  are  adapted  for  biting  and 
the  last  segment  of  the  abdomen  bears  two-jointed  cerci.  The 
anterior  wings  form,  as  in  the  Dermaptera,  covers  for  the 
posterior  pair  and  are  chitinous  plates;  the  posterior  ones  are, 


Fro.  2SS.— Labia 
minor  (after  Leu- 

NT8). 


TYPE  TUACUEATA. 


605 


on  the  other  haud,  membrauous  and  the  veins  are  for  the 
most  part  arranged  longitudinally,  so  that  when  at  rest  the 
wings  are  folded  like  a  fan,  though  in  some  forms,  such  as 
the  Crickets,  in  which  the  anterior  wings  are  short,  a  trans- 
verse fold  also  occurs.  In  the  female  Cockroaches  the  ante- 
rior wings  are  very  small,  and  the  posterior  ones  wanting,  and 
in  the  Walking  Stick  {Uiapheromera) — so  named  from  its  resem- 
blance to  a  green  or  dead  twig — both  pairs  are  entirely  want- 
ing. 

The  anteunsB  are  usually  long  and  filiform,  and  the  legs 
strong  and  adapted  to  a  terrestrial  life,  some  forms,  such  as 
the  Cockroach,  being  exceedingly  active.  In  the  Grasshop- 
pers, Locusts,  and  Crickets  the  femora  of  the  last  pair  of  legs 
are  greatly  enlarged  and  very  muscular,  serving  for  jumping, 
while  in  the  Mole-cricket  {GryUotalpu),  which  burrows  in  the 
ground,  the  anterior  pair  is  greatly  enlarged  and  adapted  for 

As  in  the  Earwigs,  the  metamorphosis  is  gradual. 


3.  Order  Ephemeridee. 

The  EphemeridaB,  or  May-flies  (Fig.  234),  are  characterized 
b}^  the  remarkable  brevity  of  their  existence  in  the  imago- 
stage,  some  forms  existing  but  for  a  few 
hours,  while  others  live  for  several  days, 
the  existence  being  merely  long  enough 
to  ensure  the  accomplishment  of  the  re- 
productive acts.  The  body  is  elongated 
and  terminates  in  two  or  three  elongated 
hairlike  cerci,  and  on  the  thorax  there  are 
borne  usually  two  pairs  of  wings,  of  which 
the  anterior  pair  is  considerably  larger  Fig.  2M.—Potamanthus 
than  the  posterior.  The  antennse  are  7nnrginatu8(tromVACK' 
short,  and  the  mouth-parts  adapted  for  ^^^^' 
biting,  though  usually  much  reduced,  since  the  imago  takes 
no  nutrition  during  its  short  existence.  The  first  pair  of  legs 
is  usually  slender  and  directed  forwards,  being  of  little  use  in 
locomotion.  An  interesting  structural  peculiarity  is  the  oc- 
currence of  paired  reproductive  ducts  which  open  by  separate 


r 


xJ-^^'^ 


506 


INVERTEBRATE  MORPHOLOGY. 


pores  instead  cf  unitiug  as  they  do  in  the  majority  of  In- 
sects. 

The  larvsB  are  aquatic  and  provided  with  tracheal  bran- 
chiae (see  Fig.  226),  recalling,  except  for  these  structures,  the 
Thysauura.  By  a  series  of  moults  the  adult  stage  is  gradu- 
ally acquired,  the  wings  appearing  in  what  is  termed  the  sub- 
imago  stage,  a  final  moult  being  necessary  before  maturity  is 
reached.     The  metamorphosis  is  thus  incomplete. 

The  genus  Ephemera  is  of  frequent  occurrence  in  the 
neighborhood  of  lakes  and  ponds,  sometimes  occurring  in 
enormous  numbers. 


4.  Order  Odonata. 

The  members  of  this  order,  the  Dragon-flies,  are  elongated 
forms  with  two  pairs  of  nearly  equal,  abundantly-veined  wings 
of  usually  large  size,  all  the  forms  being  excellent  filers  and 
seeking  their  prey  in  the  air.  The  head  is  united  to  the 
thorax  by  a  ^narrow  stalk  which  permits  extensive  rotation  of 
the  head,  and  the  abdomen,  terminating  in  two  unsegmented 
platelike  cevci,  is  long,  and  in  the  large  Dragou-fiies,  jEschna 
and  Diplax  (Fig.  235),  and  in  the  brightly-colored  Agrion  very 
slender,  though  somewhat  stouter  in  the  genus  Libellula.  The 
antennae  are  very  small  and  the  mouth-parts  adapted  for  biting, 
while  the  legs  are  slender,  the  anterior  pair  being  directed 
somewhat  forwards  so  as  to  serve  for  grasping  the  prey. 
The  lateral  comp^and  eyes  are  very  large,  meeting  on  the 

dorsum  of  the  head,  and 
in  front  of  them  are  situ- 
ated a  pair  of  small  ocelli. 
The  larvae  are  aquatic 
and  are  characterized  by 
the  remarkable  develop- 
ment of  the  labium,  which 
is  very  much  enlarged, 
terminating  in  two  power- 
Fio.  235.-2)tpto  elisa  (from  Packard).       f^^j  jg_^g  ^^^  provided  with 

a  hinge,  so  that  it  can  be  flexed  so  as  to  lie  beneath  the  head 
or  suddenly  thrust  out  to  capture  the  unwary  prey.     This 


TYPE  TRACHEATA. 


607 


apparatus  is  termed  the  "  mask."  Bespiration  is  carried  on 
by  tracheal  gills,  cousisting  iu  Agrion  of  three  leaflike  pro- 
cesses situated  at  the  posterior  eud  of  the  body,  and  also  by 
the  termi  lal  portion  of  the  intestine,  into  wliich  water  is 
taken  and  which  is  abundantly  supplied  with  tracheie.  The 
water  can  be  forcibly  expelled  from  the  intestine,  serving  to 
propel  the  insect  through  the  water  if  it  so  desires.  The 
metamorphosis  is  incomplete. 


5.  Order  Flecoptera. 

The  Plecoptera,  or  Stone-flies  (Fig.  236),  are  found  in  the 
vicinity  of  water  and  have  a  somewhat  elongated  body,  fre- 
quently terminating  in  two  long 
cerci  {Perla).  The  antennae  are  long 
and  flliform  and  the  mouth-parts 
adapted  for  biting,  while  the  legs 
are  strong  and  used  for  walking. 
Two  pairs  of  wings  occur  almost 
equal  in  size,  but  lacking  the  com- 
plicated venation  found  in  the  Odo- 
nata,  and  when  at  rest  lie  flat  upon  F^«-  236.-Stone-flv.  Perla. 
the  abdomen,  completely  concealing  it.  The  larvso  are 
aquatic,  and  are  usually  to  be  found  in  considerable  numbers 
under  stones  in  swiftly-running  streams.  They  recall  the 
Thysanura  in  their  appearance,  and  possess  tracheal  brauchifi) 
on  the  under  surface  of  the  thorax,  which  in  some  forms  are 
retained  in  the  adult.  The  metamorphosis  is  gradual  or  in- 
complete according  as  these  structures  are  or  are  not  retained 
in  the  imago. 

6.  Order  Corrodentia. 

The  members  of  this  group  possess  biting  mouth-parts 
and  are  sometimes  destitute  of  wings.  The  Termites,  or 
White  Ants,  live  in  colonies  and  show  a  polymorphism.  The 
males  and  females,  termed  kings  and  queens  (Fig.  237,  A,  B), 
are  at  first  provided  with  large  wings  resembling  those  of  the 
Plecoptera,  but  after  the  marriage  flight  settle  to  the  ground 
and  become  wingless.   The  workers  select  from  the  many  pairs 


/ 


608 


INVERTEBRATE  MORPHOLOOY. 


one  for  each  nest,  the  remaining  iiu selected  ones  soon  flying. 
The  neuters  are  of  two  sorts :  the  workers  (Fig.  237,  6'),  pale  in 
color  and  with  comparatively  small  heads  and  mandibles,  and 
the  soldiers  (Fig.  237,  />),  in  which  the  head  is  very  large  and 
dark  colored  and  carries  a  pair  of  large  mandibles.  Both 
these  forms  are  destitute  of  eyes,  and  are  to  be  regarded  as  in- 
dividuals which  have  not  passed  beyond  the  larval  stage,  being 
potentially  either  males  or  females  with  the  reproductive 
organs,  however,  undeveloped.  The  young  larvin  resemble 
Thysanura  in  their  general  form  and  are  cared  for  and  fed  by 


A  B  C 

Fig.  237. —  Termes  lucifugua  (from  Leunis). 
A,  winged  male;  B,  female  after  loss  of  wings;  C,  worker;  D,  soldier. 

the  workers.  Those  forms  which  are  destined  to  become 
kings  and  queens  are  nursed  for  a  longer  time  than  the  others, 
and  progress  further  in  their  development,  being  really  the 
only  members  of  the  colony  which  reach  the  imago  state. 

The  Termites  shun  the  light,  and  the  American  species  are 
chiefly  found  in  rotten  wood,  upon  which  they  feed,  excavating 
burrows  within  it.  In  some  cases  they  prove  very  destructive 
to  the  woodwork  in  houses,  eating  away  the  interior  of  the 
wood  and  leaving  eventually  only  a  thin  shell  in  place  of  the 
originally  solid  beam.  The  African  species  builds  large  clay 
mounds  from  three  to  iour  metres  in  height,  tunnelled  by  a 


tdt^iStSBii 


TYPE  TliA  CUE  A  TA . 


609 


somewhat  complicated  system  of  chambers,  galleries,  ami 
storehouses. 

To  this  group  beloug  also  the  Paocidcv  uud  the  Mallophmja. 
The  former  are  small  forms  fouud  u})()U  the  leaves  of  various 
trees  and  occasionally  in  houses.  They  do  uot  show  pt^ly- 
morphism  aud  are  usually  provided  with  wiugs,  though  the 
geuus  Atropos,  uot  uucomuiou  iu  books  which  have  remained 
long  undisturbed,  lacks  them.  The  Mallopliaga  are  all 
destitute  of  wings  and  are  parasitic,  living  upon  the  bodies  of 
birds  {lAotheum),  whence  they  fire  usually  termed  the  Bird-lice. 
They  feed  upon  the  feathers  and  are  comparatively  active  iu 
their  movements.  A  few  forms  occur  on  mammals,  e.g. 
Tricliodectes  on  the  dog. 

The  larvfB  of  all  these  f  rms  resemble  the  adults  excei)t  in 
size  and  in  the  absence  of  wings,  and  the  metamorphosis  is 
gradual.  Since  the  Mallopliaga  are  destitute  of  wings  in  the 
adult  condition  they  may  properly  be  said  to  be  secondarily 
ametabolic. 


7.  Order  Thysanoptera. 

The  Thysanoptera  are  small  Insects  which  live  upon  the 
leaves  of  various  plants,  which  they  pierce  for  the  })urpose  of 
obtaining  nutrition,  and  sometimes  are  very  injurious  to 
wheat,  clover,  and  other  cultivated  plants.  The  wings  are 
narrow,  but  imperfectly  veined,  and  with  the  edges  fringed 
with  numerous  slender  hairs  ;  they  are,  however,  occasionally 
wanting.  The  antenna)  are  short  and  filiform  and  the  mouth- 
parts  intermediate  between  the  biting  and  the  sucking  type. 
The  mandibles  are  reduced  to  styletlike  piercing-organs  and 
are  enclosed  within  a  tubular  proboscis  formed  by  the  fusion 
of  the  labrum  with  the  raaxillas  and  labium,  both  of  these  last 
appendages  retaining  their  palps  and  showing  usually  their 
typical  parts.  The  legs  are  adapted  for  rajud  locomotion  and 
are  peculiar  in  that  the  terminal  joint  of  the  tarsus,  instead  of 
bearing  ungues,  is  provided  with  a  protrusible  sac  which 
serves  for  adhesion  ;  on  account  of  this  peculiarity  the  order 
is  sometimes  known  as  the  Physnpoda. 

The  larvsB  except  for   the  absence  of  wings   are   closely 


'^ 


510 


IN VEliTEBIiA TE  MORPHOLOO  7. 


similar  to  the  adults  aud  the  metamorphosis  is  gradual, 
though  the  tendency  towards  the  development  of  a  distinct 
pupal  stage  is  shown  by  the  fact  that  the  last  larval  stage 
takes  no  nourishment.  The  genus  Phln'othrips  is  character- 
ized by  the  last  abdominal  segment  being  tubular  in  form, 
while  Thrips  possesses  in  the  female  forms  an  ovipositor  com- 
posed of  four  valvelike  pieces. 


8.  Order  Rhynchota. 

The  .members  of  this  order  are  divisible  into  two  groups, 
the  Heteroptera  and  //omoptera,  according  to  the  character  of 
the  anterior  wings.  In  the  Heteroptera,  which  includes  the 
majority  of  forms  popularly  known  as  Bugs,  the  basal  partions 
of  the  anterior  wings  are  chitinous,  while  the  tips  are  mem- 
brauoui!,  the  posterior  wings  being  entirely  membranous.  A 
tyjucal  member  of  this  group  is  the  common  Squash-bug 
{Anasa,  Fig.  238,  A),  and  other  examples  are  the  Water-boat- 
man  {Notonecta),  the  large  Water-scorpion  {Belostoma),  and  the 
slender  Water-scorpion  {lianatra),  all  of  which  are  of  frequent 
occurrence  in  ponds,  swimming  powerfully  beneath  the  Avater 
by  means  of  the  flattened  posterior  legs  which  serve  as  oars, 
the  anterior  pair  being  directed  forwards  aud  serving  for 
grasping  the  prey.  The  Water- measurer  or  Water-spider 
(Hydrometra)  is  also  very  coMimon  in  ponds,  darting  about 
upon  the  surface  in  search  of  pre^,  a  habit  which  also  char- 
acterizes the  genus  Hnlohates,  which  lives  upon  the  surface  of 
the  ocean  and  is  found  many  miles  from  land.  Some  mem- 
bers of  the  group  are  entirely  destitute  of  wings,  as  for  ex- 
ample the  Bedbug  (Gimex)  and  the  Louse  {Pediculuti). 

In  the  Homoptera  the  wings  are  both  membranous,  the 
anterior  pair  being  larger  than  the  posterior,  and,  as  in  the 
other  group,  are  sometimes  wanting.  The  Cicada  is  a  member 
of  this  group,  as  are  also  the  Aphidce,  or  Plant-lice  (Fig.  230), 
so  frequent  in  green-houses  and  upon  various  uncultivated 
plants  whose  juices  they  suck,  a  habit  also  shared  by  the 
nearly-allied  Coccidte,  including  the  scale-insects  {Aspidiohts) 
and  the  Mealy-bugs  (Dactylopius),  both  of  frequent  occurrence 
on  cultivated  plants,  the  former  sometimes  doing  no  little 


TYPE  THAC1IKATA. 


611 


Fig.  2'i8.—A,  Anaaa  tristin;  B,  Mouth- 
I'AUTS  OF  JVt'/ja  cinerea  (afier  Saviony 
from  OwKN). 

lb  — ■  Iiibiiim.  m  =  mandibles. 

li  =  tongue.  ma  =  maxilluj. 


damage  to  apple-trees.     The  remarkable  lieterogony  of  these 
forms  has  already  beeu  described  (p.  498). 

lu  both  the  suborders  the  moutii-parts  are  adapted  for 
piercing  aud  sucking.  The 
labium  (Fig.  238,  B,  Ih)  is 
prolonged  into  a  slender, 
usually  four-jointed  process, 
grooved  upon  its  upper  sur- 
face, the  groove  being  con- 
vertible into  a  tube  by  the 
closure  over  it  of  the  long 
slender  mandibles  (m)  and 
maxilla?  {mx)  which  form  long 
slender  needlelike  piercers. 
The  antenna3  are  usually 
short  and  filiform,  though 
in  some  of  the  Heteroptera  they  may  be  almost  as  long  as 
the  body. 

Many  of  the  Rhynchota  are  provided  with  glands  which 
secrete  an  offensive  fluid,  e.g.  in  Chnex  and  Anasa,  and  in  the 
CoccidiTB  wax-glands  are  also  abundantly  present,  producing 
a  secretion  which  may  cover  the  body  with  Avaxen  scales,  or 
in  some  cases  form  a  wool-like  mass  covering  the  greater 
part  of  the  abdomen  {Pemphigtis).  The  Aphidje  also  possess 
as  a  rule  upon  the  antepenultimate  abdominal  segment  a  ])air 
of  tubular  elevations  or  papillse  from  which  a  sweet  secretion 
issues,  the  so-called  "Honey-dew,"  which  covers  the  leaves 
and  stems  of  the  plants  upon  which  the  Insects  live,  and  is 
eagerly  sought  for  by  various  Insects,  more  especially  by 
Ants. 

The  larvsB  of  the  Rhynchota  as  a  rule  resemble  the  adults 
even  to  the  structure  of  the  mouth-parts,  and  the  metamor- 
phosis is  consequently  gradual.  The  Cicada  forms,  however, 
an  exception  to  this  rule,  the  lar, ,;,  occurring  beneath  the 
surface  of  the  ground  and  living  upon  the  rets  of  trees. 
It  becomes  transformed  into  a  pupa,  which,  however,  con- 
tinues to  lead  an  active  existence,  becoming  quiescent  only  a 
short  time  before  the  moult  which  results  in  the  formation  of 


512 


IN  VERTEBHA  TE  MOIWIIOLOG  Y. 


the  imago,  very  diflferent  iu  appearauce  from  the  pupa.     The 
metamorphosis  here  approaches  the  complete  type. 

0.  Order  Coleoptera. 

The  order  Coleoptera  includes  the  Beetles  aud  is  richer 
in  species  thau  auy  other  order  of  auimals.  The  members  of 
the  group  are  characterized  by  the  anterior  wings  being  con- 
verted into  hard  chitiuous  plates,  the  elytra,  which  cover  in 
and  protect  the  posterior  membranous  wings  and  the  abdo- 
den,  being  short  only  in  a  few  forms,  such  as  the  Uu*"mi^. 
beetles  {Necrophorm),  in  which  the  tip  of  the  abdomen  n  mh 
exposed,  aud  the  Staphylinida3,  or  llove-beetles,  and  Melo'e,  in 
which  they  cover  only  the  more  anterior  portions  of  the  ab- 
domen, the  posterior  wings  in  the  last-named  form  being 
wanting,  as  they  may  also  be  iu  some  of  the  Weevils,  Occa- 
sionally, as  iu  the  Fireflies  {Lumpt/ris),  the  elytra  are  but 
slightly  thickened,  and  in  some  forms  they  may  be  completely 
fused  together. 

The  antennte  vary  greatly  iu  shape,  being  usually  filiform 
and  sometimes  very  long,  as  in  the  Boring-beetles  {Jlonoham- 


FlG.  239.  —  CoUtlpa  lanigera  and  its  Lauva  (a)  (from  Packard). 

m?<s,  Clytus,  Saperday  etc.),  though  occasionally,  as  in  the 
Lamellicorn  beetles  {Melolontha — the  June  Bugs  and  Cotalpa^ 
Fig.  239),  the  terminal  joints  are  fl.itteued  and  folded  together 
like  the  leaves  of  a  book.  The  mouth-parts  (Fig.  225)  are  iu 
all  cases  adapted  for  biting,  aud  the  legs  for  locomotion.  In 
the  Lady-bugs  [Coccinella)  the  tarsus  consists  of  but  four 
joints,  one  of  which  is  rudimentary,  while  in  the  Weevils 
(Curculionidse),  in  which  the  anterior  part  of  the  head  is  pro- 


TYPK  TllACJIKATA. 


013 


louged  into  a  cyliudiicjil  suoutlike  ])roi'OHs  jit  tlio  oxtreniity 
of  which  is  the  mouth,  in  the  lioriii<^-l)«'otlL's,  and  in  the  Po- 
tato-beetle {Doryphoni)  it  is  fornietl  of  five  joints,  ono  of  which 
is  oxceeilingl}'  snudl.  In  other  forms,  sucli  as  J/cIoi  and  the 
Blister-beetles  (Lyfta),  the  tarsi  of  the  two  anterii>i  puirs  of 
legs  are  five-jointed  and  those  of  the  last  pair  four-jointed, 
and  in  others  again,  such  as  the  Fireflies,  the  Click-beetles, 
(Elateridie),  the  Lamellicornes,  the  Burying-beetles  and 
Staphylinida',  the  Water-beetles  {Gi/ri)iii.s,  HyilrophUuH^  etc.), 
the  Carabidie  {CttIosom<t,  CiWdhus,  //xrpalua,  /h'ltchuiiis,  etc.), 
and  the  Tiger-beetles  {C'iciiuh'Ja),  all  the  tarsi  are  five-jointed, 
and  all  the  joints  approximately  equall}^  developed. 

The  larva>  vary  greatly  in  form  in  the  dift'ereut  genera. 
lu  the  Lady-bugs  and  some  other  forms  they  are  Thysauuri- 
form,  the  three  anterior  trunk-segments  (corresponding  to  the 
thoracic  segments  of  the  imago)  possessing  each  a  pair  of  limbs, 
while  the  abdomen  terminates  in  a  pair  of  cerci.  In  some 
Water-beetles  (Gyriniis)  tracheal  gills  are  present,  and  the 
larval  of  the  Lamellicorns  (Fig.  23t),  <i)  are  soft-bodied  eyeless 
white  forms,  characterized  by  a  saclike  dilatation  of  the  last 
abdominal  segment,  and  live  beneath  the  surface  of  the 
ground  feeding  upon  the  roots  of  grasses.  In  the  Click-bee- 
tles (Elateridie)  the  body  of  the  larva  is  elongated  and  slen- 
der and  very  hard,  these  forms  being  known  as  the  wire- 
worms  and  feeding,  like  the  Lamellicorn  larvte,  upon  the  roots 
of  plants.  In  the  Boring-beetles,  the  larvae,  which  excavate 
burrows  beneath  the  bark  or  in  the  wood  of  vari(.>us  trees, 
have  the  limbs  almost  or  quite  rudimentary,  while  maggot- 
like larvtB  are  characteristic  of  the  Weevils. 

The  larva,  whose  life  may  be  prolonged  through  several 
years,  passes  finally  into  a  resting  pupa  stage  of  the  Uhern 
form,  resembling  in  the  body  form  and  the  mouth-parts  the 
imago  which  sooner  or  later  issues  from  it.  The  metamor- 
phosis is  thus  complete. 

In  the  peculiar  Meloid  form  Sitaris  an  interesting  phenomenon  known 
as  hyper  metamorphosis  occurs.  The  first  larva  is  Thysanuriforra,  and  is 
parasitic  upon  the  males  of  certain  bees,  passing  to  tlie  female  bee  during 
copulation,  and  then,  during  tlie  deposition  of  tlie  ova  in  the  cells  filled 
with  honey,  the  parasite  slips  upon  the  egg,  which  it  consumes.     It  then 


r)i4 


INVElirEBRA TE  MORPUOLOO  Y. 


transforms  into  a  maggotlike  second  larva  which  lives  upon  the  honey  on 
the  surface  of  which  it  floats,  and  after  a  time  passes  into  a  resting  pseudf)- 
chrysalis  stage,  from  wiiich  a  larva  simihir  to  tlie  second  one  emerges,  and 
this  finally  transforms  into  a  pupa  which  gives  rise  to  the  adult. 

10.  Order  Neuroptera. 

The  Neuroptera  are  characterized  by  the  abuudaut  and 
rich  venation  of  their  wings,  in  which  numerous  cross-veins 
extend  between  the  longitudinal  ones.  The  mouth-parts  are 
adapted  for  biting,  the  mandibles  being  in  some  forms  {Cory- 
dalis)  very  large.  The  lace-winged  flies  {Chrysopa)  also  be- 
long to  this  group,  as  does  also  the  Ant-lion  {Myrmdeon,  Fig. 
240),  whose  larva  excavates  a  funnel  in  loose  sand,  and 
buries  itself  at  the  bottom  with  only  the  head  and  powerful 


Fig.  240. — Myrmeleon  obsoletus  (from  Packard). 

mandibles  projecting,  ready  to  snap  up  any  insect  whicli  slips 
down  the  yielding  sides  of  the  trap.  The  larvfo  are  usually 
Thysanuriform,  those  of  CJiri/sopa  attacking  Aphides,  whence 
they  are  frequently  termed  Aphis-lions,  while  those  of  Cory- 
dalis  are  aquatic  and  possess  tracheal  branchire  upon  the 
abdomen.  This  larva  is  familiar  to  anglers  as  the  Hell- 
gramite.     The  metamorphosis  is  complete. 


11.  Order  Panorpata. 

This  order  contains  a  small  number  of  forms,  the  majority 
of  whicli  possess  membranous  wings  resembling  those  of  the 
Neuroptera,  except  that  the  cross-veins  are  not  so  numerous. 
The  anterior  part  of  the  head  is  produced  into  a  downwardly 
projecting  snout,  at  the  extremity  of  which  are  the  small 
biting  mouth-parts,  the  arrangement  recalling  that  found  in 
the  Curculionidro  among  the  Coleoptera.     In  the  genus  Pii- 


-■11  I  'Mltlgrir^iiaiiji  itWllVV.. 


TYPE  TRACUEATA. 


515 


norpa,  the  Scorpion-fly,  the  abdomeu  terminates  in  a  pair  of 
forceplike  processes  similar  to  tliose  of  the  Dermaptera. 

The  metamorphosis  is  complete,  the  larvje  differing  from 
tliose  of  the  orders  already  described  in  possessing  in  addi- 
tion to  the  three  pairs  of  thoracic  legs  eight  pairs  of  abdom- 
inal proplike  appendages. 

13.  Order  Trichoptera. 

The  Trichoptera,  also  a  small  order,  includes  the  Caddis- 
flies  {Phrygaiieay  Anaholia).  They  possess  two  pairs  of  wings, 
the  anterior  pair  usually  differing  slightly  in  appearance 
from  the  posterior,  Avhich  are  larger  and  folded  when  at  rest 
in  a  fanlike  manner,  the  venation  consisting  principally  of 
longitudinal  veins,  with  but  few  transverse  ones.  The  body 
and  the  wings  are  generally  abundantly  covered  with  hairs, 
which  in  some  forms  are  scalelike.  The  antenme  are 
usually  long  and  filiform,  and  the  mandibles  rudimentary,  the 
maxilhe  and  labium  forming  a  short  sucking  proboscis. 

The  metamorphosis  is  complete,  the  larvio  being  aquatic 
and  provided  with  spinning-glands  with  which  they  bind  to- 
gether small  twigs  and  particles  of  sand  to  form  cases  within 
which  they  live.  They  possess  tracheal  branchijo  upon  the 
sides  of  the  abdominal  segments,  and  the  last  segment  bears 
a  pair  of  short  but  stout  processes  which  are  provided  with 
hooks.  The  pupa  is  formed  within  the  larval  case,  but  before 
transforming  into  the  imago  it  leaves  the  case  and  crawls  to 
land,  where  the  imago  emerges. 


13.  Order  Lepidoptera. 

This  is  a  large  order,  including  the  Butterflies  and  Moths, 
all  of  which,  with  the  exception  of  the  females  of  a  few  forms 
(Orgyta),  possess  two  pairs  of  wings  covered  with  overlai)i»iiig 
scalelike  hairs,  and  with  but  few  transverse  veins.  When  at 
rest  the  wings  are  rarely  folded,  but  are  either  held  erect,  as 
in  the  Butterflies,  or  lie  one  over  the  other,  resting  upon  the 
abdomen.  The  body,  like  the  wings,  is  covered  with  hairs  or 
scales. 

The   antennoB  differ   considerably   in   shape   in   difl'erent 


516 


INVERTEBKATR  MOHPHOLOOT. 


forms,  beiug  iu  the  Butterflies  usually  club-shaped,  while  in 
male  moths  they  are  frequently  featherlike,  though  more  sim- 
ple or  filiform  iu  the  females.  The  mouth-parts  are  adapted 
for  sucking,  forming  in  most  cases  a  long  tube,  which,  when 
not  in  use,  is  coiled  into  a  helix.  In  the  smaller  members  of 
the  group  (Microlepidoptera),  which  are  in  many  respects  the 

most  primitive  and  include  such 
forms  as  the  Clothes-moth  (Tinea), 
the  moth  of  the  Apple-maggot  {Oai'- 
pocapsa),  the  leaf-rollers  (Pyralidas), 
etc.,  the  sucking  arrangement  is  by 
no  means  perfect,  the  mandibles  being 
present,  and  the  maxillae  and  labium 
resembling  in  structure  the  corre- 
sponding parts  iu  I  ag  insects,  ex- 
cept that  the  two  inner  terminal 
plates  of  the  labium  are  united  to 
form  a  short  tube.  In  the  higher 
forms  (Macrolepidoptera),  however, 
the  mandibles  (Fig.  241,  mn)  are  quite 
rudimentary  and  the  labium  is  much 
reduced  in  size,  though  its  palps  (Ip) 
are  frequently  large  and  well  de- 
veloped. The  sucking-tube  is  com- 
posed of  the  two  maxill%  (m)  which  are  produced  into  two 
long  tilaments  grooved  on  their  mesal  surfaces,  and  by  their 
apposition  the  tube  is  formed. 

The  metamorphosis  is  in  all  cases  complete,  the  larvro 
being  wormlike  structures  known  as  caterpillars.  Their 
mouth-parts  are  adapted  for  biting,  and  they  live  for  the 
most  part  upon  the  leaves  of  various  plants,  frequently  ac- 
complishing much  destruction.  This  is  especially  the  case 
with  the  Tent-caterpillar  {Clisiocampa),  which  lives  in  colonies 
enclosed  within  a  web  which  is  extended  from  twig  to  twig  as 
the  leaves  are  gradually  eaten ;  various  kinds  of  trees  suffer- 
ing  from  its  ravages.  The  shade-trees  in  cities,  especially 
the  Horse-chestnut,  are  sometimes  greatly  injured  by  the 
caterpillar  of  the  Tussock-moth  (Orgyia),  and  the  larvea  of  the 
common  white  Cabbage-butterfly  (Pieris)  feed  upon  the  leaves 


Fig.  241.— Hrad  op  Moth. 
Sphinx  lif/ustri,  showing 
Mouth- PAUTs   (after  Nkw- 

POIIT). 

a  =  uuttiuuse. 
I  =  lubruiu. 
Ip  =  lubiul  palp  :   that  of 

left  side  removed. 
m  =  uiiixillse. 
mn  =  iii)iii(ii>)le. 
o  =  eye. 


iMB 


TYPE  TRAGHEATA. 


517 


of  the  Cabbage ;  many  other  similar  examples  might  be 
given.  A  few  of  the  Microlepidoptera  possess  aquatic  larva>, 
but  they  form  exceptions.  In  the  typical  caterpillar  there 
are,  in  addition  to  the  three  pairs  of  thoracic  legs,  live  pairs  of 
short  stout  prop-legs  situated  upon  the  third,  fourth,  fifth, 
sixth,  and  tenth  abdominal  segments,  and  the  body  may  be 
covered  with  hairs  of  various  lengths,  as  in  the  larvte  of  many 
moths  (e.g.  the  Woolly  Bear,  Spilosoma),  or  may  possess 
spiny  processes,  as  in  the  larvae  of  the  Mourning-cloak  Butter- 
fly  ( Vanessa)  which  feeds  on  the  "Willow,  or  variously- 
shaped  tubercles,  as  in  the  American  silkworm  (Telea)  and 
the  Cecropia  larva.  In  one  group  of  moths,  the  Geometrida), 
but  two  or  three  pairs  of  prop-legs  occur,  situated  on  the 
more  posterior  segments,  and  in  progression  these  forms 
draw  these  legs  up  close  to  the  thoracic  limbs,  throwing  the 
intervening  portion  of  the  body  into  a  loop,  whence  the  terms 
"measuring-worms"  or  "loopers"  often  applied  to  them. 
In  rare  cases,  as  in  a  few  Microlepidoptera,  the  larva  is 
without  feet  and  maggotlike. 

The  pupa  or  chrysalis  is  of  the  ohtecta  variety,  and  is  fre- 
quently enclosed  within  a  silken  case  termed  the  cocoon, 
spun  by  the  larva  whose  salivary  glands  are  converted  into 
spinning-glands.  A  cocoon  is  more  generally  present  in  the 
Moths  than  in  the  Butterflies,  whose  chrysalids  are  suspended 
by  a  patch  of  silk  to  which  the  hind  end  of  the  pupa  is  at- 
tached or  may  be  in  addition  slung  in  a  silken  loop  passing 
round  the  body  near  the  middle  (Fig.  231). 

14.  Order  Hymenoptera. 

The  Hymenoptera  possess  four  membranous  wings,  with 
comparatively  few  veins  and  not  covered  with  scales  or  hairs 
but  transparent,  the  anterior  pair  being  usually  larger  tlian 
the  posterior.  The  abdomen  is  sometimes  broadly  attached 
to  the  thorax,  as  in  the  Saw-flies  (Tenthredinidje),  but  more 
usually  the  anterior  one  (Bees)  or  two  (Ants)  abdominal  seg- 
ments are  very  narrow,  so  that  the  abdomen  seems  to  be  at- 
tached by  a  stalk.  The  females  possess  ovipositors  which 
may  be  retractile  and  provided  with  a  poison-gland,  forming 


518 


INVERTEBRATE  MORPHOLOGY. 


mxp  - 


efficient  organs  of  offence  and  defence,  as  in  the  Ants,  Bees, 
and  Wasps,  or  else  long  and  slender  and  but  partially  retrac- 
tile and  destitute  of  a  poison-gland,  as  in  the  Saw-flies,  Gall- 
flies, and  IchneumonidsB. 

The  niouth-parts  are  adapted  partly  for  biting  and  partly 

for  licking.  The  mandibles  (Fig.  242, 
mn)  are  well  developed  and  fitted  for 
biting  in  all  forms,  and  in  the  Teu- 
thredinidae  the  maxillge  are  also  like 
those  of  biting  insects,  while  the 
inner  of  the  two  terminal  plates  of 
the  labium  are  united  to  form  a  tube, 
the  outer  plates  remaining  separate. 
In  the  Bees  and  Wasps  the  maxillsB 
{inx)  become  elongated  and  are  no 
longer  adapted  for  biting,  and  the 
inner  terminal  plates  of  the  labium 
are  fused  together  to  form  a  long 
touguelike  structure,  the  glossa  {l)^ 
the  outer  plates  forming  what  are 
termed  the  paraglossa)  {pg).  The 
entire  apparatus  is  adapted  for  biting 
and  also  for  licking  up  the  honey 
contained  in  the  nectaries  of  flowers. 
The  great  majority  of  forms  are 
solitary,  but  a  few  Bees  {^pis,  Bom- 
hiis)  and  Wasps  {Vespa,  Sphex)  and  the  Ants  {Formica, 
Camponotiis)  form  social  aggregations  with  more  or  less 
pronounced  polymorphism,  to  wliicli  reference  has  already 
been  made.  The  Gall-flies  {Cympt<)  lay  their  eggs  upon  the 
leaves  or  stems  of  plants,  at  the  same  time  injecting  a  poison 
which  causes  a  proliferation  of  the  plant-tissues,  forming  a 
gall  in  the  interior  of  which  is  the  larva  of  the  insect ;  while 
many  forms,  such  as  the  Ichneumon-flies,  Proctotrupes,  Ptero- 
malus,  Microgaster,  etc.,  are  parasitic  in  their  larval  stage,  the 
eggs  being  deposited  in  or  upon  the  bodies  of  the  larvsB  of 
other  insects,  a  very  decided  check  being  exerted  upon  the 
larvHB  of  injurious  insects,  such  as  the  Cabbage-butterfly,  by 
these  forms. 


Fig.  242. —Mouth-parts  of 
Bee,     Anlhophora     (after 
Newport  from  Gegbnbaur). 
I  ~  glossa. 
Ip  =  labial  palp. 
mn  =  mandible. 
mx  =  maxilla. 
mxp  =  maxillary  palp. 
pg  =  paraglossa. 


TYPE  TRACBEATA. 


519 


The  larvsB  of  the  Tenthredinidse,  for  example  that  of  the 
Pear-slug  (Selandria),  which  feeds  upon  the  leaves  of  the  pear- 
tree,  resemble  the  caterpillars  in  possessing  prop-legs,  of 
which  there  are  as  many  as  eight  pairs.  In  the  majority  of 
forms,  however,  owing  to  parasitism  or  to  being  in  contact 


Pio.  343. — SpJiex  ichneumonea  (from  Packard). 


Fro  244. — Hypoderma  hovis  ffrom  Packard). 
with  an  abundant  supply  of  nutrition  stored  up  by  the  parents 
(Bees,  Wasps)  or  to  being  fed  and  cared  for  by  the  workers 
among  the  Ants,  the  larvae  are  maggotlike  and  almost  or 
entirely  destitute  of  legs.  The  metamorphosis  is  complete, 
the  pupa  being  a  pupa  libera. 

15.  Order  Diptera. 

In  this  order,  as  the  name  indicates,  but  two  wings  are 
present  (Fig.  244),  which  are  those  of   the  mesothorax,  the 


520 


IN  VERTEBRA  TE  MORPUOLOO  7. 


liietatlioraeic  pair  beiug  usually  represented  by  a  pair  of  club- 
shaped  bodies  ou  the  sides  of  the  segment,  termed  halteres  or 

balancers.  The  wings  are  always  trans- 
parent and  the  veins  by  no  means 
abundant.  In  a  few  forms,  such  as  the 
Sheep-tick  {Mtlophagiis)  and  the  Fleas 
(Pulex),  the  wings  are  entirely  wantiug 
in  harmony  with  the  parasitic  habits 
which  these  forms  possess,  but  they 
form  exceptions  to  the  general  rule. 

The  mouth-parts  are  adapted  for 
sucking  and  also  for  piercing ;  the 
labrum  (Fig.  245,  Ir)  and  labium  {la) 
are  prolonged  into  grooved  processes, 
forming  together  a  tube  within  which 
lie,  in  the  female  Mosquitoes  {Oulex) 
and  Gadflies  {Tabanus),  two  pairs  of 
elongated  needlelike  rods  which  repre- 
sent the  mandibles  {md)  and  maxillae 
{mx),  to  which  a  fifth  unpaired  stylet 
may  be  added  which  arises  as  a  growth 
from  the  lower  wall  of  the  pharynx 
{hy).  In  other  forms  the  maxillge  only 
OP  A  Gnat,  Culex,  the  l^^ve  the  acicular  form,  the  mandibles 
Labrum  tuuned  to  fusing  with  the  labrum,  and  in  all  cases 
ONE  SIDE  (from  hert-  the  maxillary  palps  are  present,  while 

the  labial  palps  are  undeveloped.  In 
the  ordinary  House-fly  {Musca)  the  ex- 
tremity of  the  sucking-tube  is  expanded 
into  a  disklike  structure,  and  in  all 
forms  the  salivary  glands  open  near  the 
extremity  of  the  tube, 
usually  maggotlike  (Fig.  244),  entirely 
destitute  of  feet,  and  in  some  forms  the  head  even  is  indis- 
tinguishable. The  metamorphosis  is  complete,  the  pupa  being 
in  the  Mosquitoes  active,  swimming  about  in  water,  though 
more  usually  it  is  incapable  of  motion,  and  enclosed  within 
the  last  larval  skin,  thus  belonging  to  the  coarctata  variety. 


Fig.  245.— Mouth-parts 


WIG). 

hy  =  hypopharynx,       a 
process  of  labium. 
la  =  labium. 
Ir  ■=  labrum. 
md  =  mandible. 
mx  —  maxillce. 
p  —  maxilltiry  palp. 
The    larvae    are 


TYPE  TUACIIEATA. 


521 


ly 

lis- 


lin 


Development  and  Affinities  of  the  Insecta. — The  early  stages  of  Insect 
development  cannot  be  discussed  here,  belonging  more  properly  to  text- 
books of  Embryology,  but  mention  should  be  made  of  the  remarkable  phe- 
nomenon which  occurs  duriug  the  transformation  from  the  pupal  to  the 
imaginal  conditions  in  those  forms  whose  metamorphosis  is  complete.  In 
describing  the  development  of  the  Acarina  it  was  pointed  out  that  during 
the  transition  from  one  stage  to  the  next  a  iiistolysis  and  subsequent  regen- 
eration »)f  ctu'tain  parts  of  the  body  occurred.  In  the  holometabolic  Insects 
the  same  process  occurs  during  the  pupa  stage,  the  larval  hypodermis, 
tiie  majority  of  the  muscles,  and  the  entire  digestive  tract  and  its  a|)i)en- 
dages  undergoing  degeneration,  and  being  absorbed  and  digested  by  the 
blootl-corpuscles,  tlie  parts  being  formed  anew  from  patches  of  cells  {)resent 
in  the  larva  and  known  as  iinayinal  disks.  The  histolysis  and  regenera- 
tion proceed  pari  passu,  so  that  the  identity  of  the  various  organs  is  pre- 
served throughout  the  process.  The  imaginal  discs  are  to  be  regarded  as 
portions  of  the  original  anlagen  of  the  various  organs  wliich  have  re- 
mained during  larval  life  in  an  embryonic  condition,  springing  into  activity 
and  completing  their  development  during  the  pupal  stage. 

As  regards  the  affinities  of  the  various  orders  of  the  Pterygota,  it  may 
be  pointed  out  that  the  frequent  occurrence  of  Thysanuriform  larvfe  indi- 
cates a  descent  from  Apterygote  ancestors,  and  those  orders  which  present 
larva)  of  a  wormlike  or  maggotliko  form  are  in  all  probability  the  most 
highly  specialized.  It  is  in  these  cases  that  the  complete  metamorphosis 
occurs,  and  it  is  self-evident  that  the  gradual  and  incomplete  metamor- 
phoses are  more  primitive  than  the  complete.  Indeed  all  metamorphosis 
depends  upon  tiie  differences  in  habit  and  structure  of  the  larva  and  imago, 
and  becomes  more  and  more  complete  according  as  the  larva)  and  imagines 
depart  more  and  more  widely  from  the  Thysanuriform  type  of  structure. 
Consequently  it  may  be  concluded  that  tho.se  forms  are  the  most  primitive 
which  retain  most  perfectly  both  in  the  larva  and  imago  the  Thy.sanurid 
characters.  These  are  found  most  perfectly  in  the  Dermaptera,  to  which 
both  in  the  adult  and  larval  stages  the  Corrodentia  (so  far  as  they  have 
not  become  modified  by  parasitism)  and  the  Orthoptera  seem  closely  re- 
lated, and  it  is  interesting  to  note  in  this  connection  that  the  earliest  In- 
sects known  from  the  Paleozoic  rocks  seem  to  have  been  closely  related  to 
tlie  recent  group  of  the  Orthoptera. 

Another  order  which  has  retained  Thysanuran  characters  in  the  larva, 
though  the  imagines  are  more  higlily  specialized  than  are  those  of  the 
Dermaptera,  is  that  of  the  Thysanoptera,  whose  habits  and  mouth-parts 
indicate  affinities  with  the  Rliynchota,  tlie.se  two  orders  together  forming 
a  second  group  traceable  back  to  the  primitive  Pterygota. 

A  third  group  starts  with  tlie  Ephemeridae,  which  lead  up  to  the  Odo- 
nata,  the  larvae  of  the  latter  liaving,  however,  become  greatly  specialized, 
the  resemblances  being  most  marked  in  the  adults,  and  are  indicated  by 
the  character  of  the  wings  and  by  the  mouth  parts.    More  distantly  re- 


622 


INVERTEBRATE  MORPHOLOGY. 


lated  are  the  Neuroptera  with  Thysanuriform  larvap,  probably  to  be  re- 
garded as  a  group  which  has  undergone  a  development  parallel  to  that  of 
the  EphemeridsB  and  Odonata,  the  relationship  being  traceable  back  to  an 
ancestor  common  to  it  and  the  Ephemeridae.  To  this  group  may  also  be 
referred  the  Plecoptera. 

A  fourth  group  includes  those  forms  in  which  the  larvaB  are  provided 
with  prop-legs,  secondary  forms  in  which  all  indications  of  the  Thysanurid 
ancestors  have  disappeared.  Of  such  forms  the  Panorpata  show  relation- 
ships on  the  one  hand  with  the  Ephemerid  group,  and  somewhat  closely 
related  are  the  Trichoptera,  whose  entire  organization  points  to  a  close 
affinity  with  the  Microlepidoptera.  From  the  primitive  Microlepidoptera 
two  lines  of  descent  are  probably  to  be  traced,  one  leading  to  the  Macro- 
lepidoptera  and  the  other  to  the  primitive  Hymenoptera,  the  resemblance 
between  the  larvae  and  the  mouth-parts  of  the  Tenthredinidae,  and  those 
of  the  Microlepidoptera  being  very  striking. 

The  two  remaining  orders,  the  Coleoptera  and  Diptera,  are  very  highly 
specialized,  both  being  holometabolic,  and  the  temptation  is  to  look  for 
their  ancestors  in  forms  with  a  similar  metamorphosis.  This  temptation 
may  bo  justified  in  the  case  of  the  Diptera,  whose  larvae  are  the  most 
modified  of  all,  and  it  is  not  impossible  that  they  have  descended  from 
primitive  Hymenopteran  ancestors,  their  nearest  existing  relatives  being 
the  Tenthredinidae,  whose  sluglike  larvae,  suggest  not  a  little  the  least 
modified  Dipteran  maggots.  With  the  Coleoptera,  however,  the  case  is 
different,  and  it  seems  more  probable  that  their  holometabolisra  has  been 
acquired  quite  independently  of  that  of  the  other  holometabolic  orders. 
The  larvae  of  some  beetles,  notably  those  of  the  Coccinellidae,  are  markedly 
Thysanuriform,  and  prop-legs  do  not  occur  in  the  order.  To  which  of  the 
groups  they  are  to  be  referred  it  is  very  difficult  to  say,  though  the  mouth 
parts  and  the  arrangement  and  structure  of  the  wings  in  the  adults  point 
to  an  affinity  with  the  Orthoptera. 

Granting  a  descent  of  the  Pterygota  from  wingless  ancestors,  it  becomes 
an  interesting  problem  to  discover  the  origin  of  the  wings.  Attempts  have 
been  made  to  show  tluit  they  are  modified  tracheal  branchiae,  a  theory 
which  necessitates  the  derivation  of  the  Pterygota  from  aquatic  ancestors. 
Such  a  derivation,  however,  is  unsupported  by  any  evidence  at  present  at 
our  disposal,  it  being  much  more  probable  that  the  immediate  ancestors  of 
the  Pterygota  were  terrestrial,  just  Jis  Campodea  is  to-day.  The  wings 
arise  in  the  embryo  as  dorsal  outpouchings  of  the  meso-  and  metathorax, 
tracheae  later  pushing  out  into  them,  and  transient  indications  of  out- 
pouchings of  the  prothorax  also  occur  in  some  embryos.  It  has  been  sug- 
gested that  primarily  the  wings  were  platelike  outgrowths  of  the  thoracic 
segments  which  served  to  break  the  fall  and  increased  the  distance  trav- 
ersed by  jumping  Insects,  and  in  support  of  this  view  the  fact  may  be 
mentioned  that  many  Apterygota  are  saltatorial.  The  limitation  of  the 
wings  to  the  meso-  and  metathorax  may  stand  in  some  relation  to  the 
centre  of  gravity  of  the  body. 


at 
of 
gs 

iX, 

t- 

g- 
3ic 
,v- 
be 
he 


TYPE  TRACIIEATA. 


623 


The  Phytogeny  of  the  Tracheata. — It  has  been  the  custom  to  unite 
together  the  Crustacea,  Arachnida,  and  Tracheata  in  a  single  group,  the 
Arthropoda,  characterized  by  tiie  possession  of  a  chitinous  cuticle,  by  tlio 
occurrence  of  jointed  limbs,  and  by  tlie  masticatory  organs  being  modified 
limbs ;  and  furthermore  it  has  oeen  customary  to  consider  the  Arachnida 
and  the  Tracheata  as  closely  related  on  account  of  the  occurrence  in  both 
groups  of  trachea).  The  early  processes  of  development  in  the  tin-eo 
groups  also  show  many  points  of  similarity,  thougii  a  closer  examination 
shows  decided  differences  in  the  details.  How  far  convergent  evolution 
may  have  operated  to  produce  the  similarities  is  the  problem  to  be  settled, 
and  it  can  be  settled  only  by  a  consideration  of  all  the  facts  at  our  disposal 
which  indicate  the  phylogeny  of  the  various  groups,  a  discussion  which 
would  prove  entirely  beyond  the  limits  of  a  text-book. 

It  has  been  pointed  out  that  the  probable  ancestry  of  the  Crustacea  is 
to  be  found  in  the  Annelida,  and  that  the  Arachnida  have  in  all  probability 
descended  from  Eurypterus-like  ancestors,  which  were  certainly  Crustacean 
in  their  affinities.  Are  the  Tracheata  then  also  descended  from  the  Crus- 
tacea and  from  forms  which  possessed  tracheae?  Our  present  knowledge 
of  the  group  negatives  any  such  supposition  ;  it  seems  impossible  that  the 
Tracheata  should,  like  the  spiders,  have  descended  from  Eurypterns-hke 
ancestors  ;  it  must  rather  be  concluded  that  the  similarities  between  them 
and  the  Arachnida  are  due  to  convergent  evolution,  the  embryonic  simi- 
larities to  the  acquisition  of  comparatively  large  amounts  of  food-yolk  in 
the  ova,  distributed  in  a  similar  manner,  and  the  similarities  of  the  adult 
to  the  exigencies  of  a  terrestrial  life.  The  occuiTcnce  of  tracheae  in  both 
groui)s  seems  at  first  an  important  point  of  similarity  to  be  accounted  for 
only  by  a  community  of  descent,  but,  when  it  is  considered  that  in  the 
terrestrial  Isopoda  tracheae  also  occur  in  the  branchial  opercula,  and  that 
their  occurrence  in  these  forms  is  a  purely  secondary  adaptation,  without 
any  phylogenetic  significance,  it  is  evident  that  their  importance  as  indi- 
cations of  affinity  is  much  reduced.  It  may  also  be  pointed  out  that  the 
Malpighian  tubules  of  the  Arachnida  and  Crustacea  are  endoderinal,  whereas 
in  the  Tracheata  they  are  ectodermal,  arising  from  the  ectodermal  hitid- 
gut. 

There  is  little  room  for  doubt  but  that  Peripatus  is  closely  related  to 
the  Annelida,  and  its  relationships  to  the  Myriapoda  are  also  pronounced, 
so  that  the  conc'.jjsion  seems  inevitable  that  the  Tracheata  have  been  de- 
rived from  Annelid-forms,  and  have  therefore  a  phylogeny  practically  inde- 
pendent of  that  of  the  Arachnida.  However,  it  is  possible  that  the  Anne- 
lid ancestors  of  Peripatus  and  those  of  the  Crustacea  were  more  or  less 
closely  related,  and  that  certain  of  the  general  similarities  of  all  the  three 
groups  are  thus  to  be  accounted  for,  though  to  what  extent  we  are  not  at 
present  in  a  position  to  judge.  One  point,  namely,  the  occurrence  of  com- 
pound eyes  of  similar  structure  in  both  groups,  seems  worthy  of  considera- 
tion, since  it  seems  to  be  unexplainable  by  this  hypothesis  and  to  be  a  re- 


624 


INVERTEBRA  TE  MORPIIOLOO  Y. 


markable  instance  of  convergent  evolution.  It  is  to  be  noticed  that  the 
most  primitive  Insects,  the  forms  through  which  affinities  to  the  Crustacea 
if  tliey  exist  must  be  traced,  are  as  a  rule  provided  only  with  simple  eyes, 
a  condition  repeated  in  the  eyes  of  Insect  larvto — a  fact  which  indicates  that 
the  compound  eyes  are  structures  wiiich  were  not  characteristic  of  the 
primitive  Insects,  but  have  developed  within  the  limits  of  the  group  and 
can  therefore  have  no  phyletic  connection  with  the  compound  eyes  of  the 
Crustacea.  Adding  to  this  fact  the  independently-develojMjd  tendency  to 
form  compound  eyes  seen  in  certain  Annelida  and  Pelecypod  Mollusks,  it 
seems  probable  that  notwithstanding  their  remarl<able  structural  similari- 
ties the  compound  eyes  of  Crustacea  and  Insects  have  been  independently 
acquired.  Instead,  therefore,  of  uniting  the  three  groups  together  as  a  type 
Arthropoda  equivalent  to  the  other  types,  it  seems  preferable  to  separate 
them  as  distinct,  just  as  is  done  with  the  Annelida  and  Mollusca,  and  the 
Annelida  and  Prosopygia. 

Starting,  then,  with  the  supposition  that  Peripatus  has  descended  from 
Annelid  ancestors  and  represents  the  ancestors  of  the  Myriapoda,  the  rela- 
tionships of  the  various  orders  of  this  class  and  of  the  Insects  remains  to 
be  traced.  Unfortunately  a  large  gap  exists  between  Peripatus  and  any 
recent  Myriapods,  and  it  is  possible  that  this  class  is  a  heterogeneous  group ; 
indeed  l>y  some  recent  authors  it  has  been  suggested  that  it  should  be 
done  away  with  as  a  class,  the  Chilopods  being  united  with  the  Insecta  to 
form  one  class,  while  the  Diplopods  (perhaps  with  the  Pauropoda  associated 
with  thein)  should  form  a  second.  There  is  no  doubt  but  that  Peripatus 
possesses  many  tracheate  peculiarities,  but  its  affinities  to  the  remaining 
Tracheates  are  much  more  remote  than  those  which  exist  between  the  vari- 
ous groups  of  Myriapoda,  or  between  any  of  these  groups  and  the  Insecta, 
The  character  of  the  various  appendjiges  considered  in  relation  with  the 
nervous  system  seems  to  afford  an  admirable  means  of  indicating  the  relji- 
tionships  of  the  various  groups.  The  brain  of  Peripatus  seems  to  be 
formed  by  the  fusion  of  three  pairs  of  ganglia  ;  the  most  anterior  and  dor- 
sal of  these  gives  rise  to  the  antennal  nerve  and  the  most  posterior  inner- 
vates the  mandibles,  while  upon  the  middle  one,  which  is  closely  related  to 
the  mandibular  ganglion,  the  eye  seems  to  be  placed.  It  may  be  assumed 
that  the  ganglia  with  which  the  eyes  are  associated  represent  the  Annelid 
supraoesophageal  or  cerebral  ganglia  and  may  therefore  be  termed  the  pro- 
tocerebrum,  while  the  antennary  ganglia  form  the  deutocerebrura,  and  the 
mandibular  the  tritocerebrum.  In  the  Myriapods  and  Insects  the  brain  is 
also  composed  of  three  parts  to  which  tlie  same  names  are  applied,  the 
antennae  being  innervated  from  the  deutocerebrum,  while  the  tritocerebrum 
lacks  a  corresponding  appendage,  though  in  certain  Insects  transient  indi- 
cations of  a  tritocerebral  appendage  have  been  seen.  Bearing  these  facts 
in  mind,  the  ganglia  and  appendages  of  the  various  groups  may  thus  be 
tabulated,  and  to  make  the  comparison  complete  the  Orustacea  are  also  in- 
cluded. 


\i^ 


IIJMH 


TYPE  TliACUEATA. 


625 


QanKlion. 

Crufltacea. 

Peripatus. 

Diplopoda. 

Chiiopoda. 

Insecta. 

Deutooere- 

bral. 
Tritocere 

hrtil. 
IstpoHtoral 

4tll       " 
Bth       " 
6th      " 

Antt'niiules 

AlllfllllHJ 

Maiiilibles 

1st  iiiiixillue 

ad 

iHt  til'  racic  limb 

itd 

Antennae  ? 

MandibleM 
Oral  pHpiliBB 
iHt  leiTH 
Sd      '• 
3d     " 
4th    " 
6tb  •' 

Antenna) 

Mandibles 
Maxilla3 

1st  1<'«S 

ad    ■■ 

4lh   " 

Anteiinai 

Mandililt's 
1st  intixiliu3 

ad 

MiixillipedeH 
1st  legs 

Antennas 

MandibleH 
1st  maxillae 
ad 

1st  less 

ad    •• 

8d      " 

It  will  be  seen  from  this  that  in  the  Diplopoda  the  arrangement  is 
intermediate  between  that  found  in  Peripatus  and  that  of  the  Cliilopoda, 
while  these  latter  approach  closely  the  Insecta,  and  this  seems  to  be  the  ac- 
tual relationship,  Scolopeiidrella  forming  an  int(?rmediato  link  between  the 
Chilopods  and  the  Insecta,  approaching  the  Tliysanura  closely  in  the  ar- 
rangement of  the  mouth-parts  and  in  the  number  of  segments  of  which  the 
body  is  composed.  The  Diplopoda,  it  is  true,  pass  through  a  larval  stage  in 
which  but  six  legs  are  present,  and  it  might  at  first  sight  be  supposed  that 
this  indicates  an  affinity  with  the  Insecta,  but  these  legs  do  not  belong  to 
the  same  segments  as  do  those  of  the  Insects,  and  furthermore  the  occur- 
rence of  rudimentary  abdominal  appendages  in  some  Thysanura,  as  well  as 
in  the  embryonic  stages  of  probably  all  Pterygota,  indicates  that  the  In- 
secta have  been  derived  immediately  from  forms  with  many  pairs  of  appen- 
dages, and  these  forms  seem  to  be  represented  most  accurately  by  the  exist- 
ing Scolopendrella. 


SUBKINGDOM  METAZOA. 

TYPE   TRACIIEATA. 

I.  Class  Protracheata.— Annelid-like  forms ;  trunk  not  differentiated 

into  thorax  and  abdomen  ;  with  nephridia.     Peripatus. 
II.  ClasK  Myriapoda.— Elongated  forms;   trunk  not  differentiated  into 
thorax  and  abdomen  ;    posterior  trunk-segments  with  appen- 
diigi^s  in  the  adult. 

1.  Order  Pauropoda. — Small  forms  ;  with  only  one  pair  of  maxilUe  ; 

antennae  ending  in  three  flagella  ;  reproductive  orifices  at  basis 
of  second  pair  of  trunk-appendages.    Pauropns,  Eurypaiiropus. 

2.  Order   Diplofmda. — With   only  one  pair  of   maxillae  ;    antennae 

simple ;  reproductive  orifice  on  second  or  between  second  and 
third  trunk-segments ;  most  of  the  trunk-segments  with  two 
pairs  of  legs.  lulus,  Lysiopetaluin,  Pblydesmns,  Strongylo- 
soma,  Glomeris. 

3.  Order  Chiiopoda.— With  two  pairsof  maxilla  and  with  maxillipeds; 

antenna}  simple;  reproductive  orifice  on  the  antepenultimate 
segment ;  each  trunk-segment  with  a  single  pair  of  legs. 
Geophilus,  iSculopendra,  Lithobius,  iScutiyera. 


526 


IN  VERTEBRA  TE  MORPHOLOO  T. 


4.  Order  Sympftyla.—Vf'\t\\  only  ono  pair  of  maxillffl  and  no  maxilli- 
pods ;  antonnic  simple ;    moHt  of  the  trunk-segments  with  a 
sinjjle  pair  of  lejfs.     Svolopendrella. 
III.  Class  iNSKCTA.— Trunk  difforuutiated  into  thorax  composed  of  throe 
rings  and  an  abdomen  with  typically  ton  segments. 

1.  Subchuss  /l/y^(;/'^//o^a.— Thorax  without  wings;  abdominal  segments 

sometinu'H  with  rudimentary  limbs  in  the  adult. 
1.  Onler  Thysannra. — Abdomen  with  ten  segments,  terminating  in 

three  cerci ;    abdominal  ap])endages  frequently  present.      Le- 

pisma,  Camjmlea. 
3.  Order  Collembola. — Abdomen  with  six  segments  terminating  in 

twospringing-organs  ;  abdominal  appendcages  wanting.  Fbttura, 

Anuiida. 

2.  Subclass  Pterygota. — With  usually  two  pairs  of  wings  situated  on 

the  nu'so-  and  mctathoracic  segments  ;  abdominal  appendages 
wanting  in  adults. 

1.  Order   Denuaptem. — Abdonien  with   forceplike  cerci ;    anterior 

wings  small  and  ehitinous,  posterior  folded  like  a  fan  and  also 
transversely ;  mouth-parts  biting ;  metamorphosis  gradual. 
For/icida,  Labia. 

2.  Older  Orthoptera. — Alwlomen  usually  with  cerci  ;  anterior  wings 

ehitinous,  covering  the  posterior,  which  fold  fanlike  and  some- 
times also  transversely  ;  mouth-parts  biting  ;  metamorphosis 
gradual.  Caloptenns,  Gryllus,  Qryllotalpa,  Periplaneta,  Dia- 
pfieromera. 

3.  Order  Ephemeridce. — Abdomen  with  two  long  cerci ;  wings  mem- 

branous and  richly  veined,  the  anterior  larger ;  not  folded  when 
at  rest ;  mouth-parts  biting,  but  reduced  ;  metamorphosis  in- 
complete.   Ephemera. 

4.  Order  Odonata. — Abdomen  with  two  platelike  cerci ;  wings  mem- 

branous and  richly  veined,  not  f  jided  when  at  rest ;  mouth- 
parts  biting;  metamorphosis  incomplete,  sometimes  approaching 
completeness.     Libelltda,  jEschna,  A(/rion,  Diplax. 

5.  Order  Plecoptera. — Abdomen  usually  with  cerci ;  wings  membra- 

nous, moderately  veined  with  few  cross- veins ;  the  anterior  cov- 
ering the  posterior  when  at  rest ;  mouth-parts  biting ;  meta- 
morphosis incomplete.    Perla. 

6.  Order  Corrodentia. —Abdomen  without  cerci;    wings  sometimes 

wanting  (parasites  and  neuters),  membranous  he  anterior  cov- 
ering the  posterior  when  at  rest;  mouth-parts  bitiiijj  -^^  ta- 
morphosis  incomplete  or  wanting.  Termes  (w '  *h  pc  ymorphism) , 
Atropos,  Liotheum,  Trichodectes. 

7.  Order  Thysanoptera. — Abdomen  without  cc        *vings  someti    .'S 

wanting,  narrow,  poorly  veined,  fringed  wit  lirs  ;  *  e  anterior 
pair  covering  the  posterior  when  attest;  raouih-p  its  piercing 
and  sucking;  metamorphosis  incomplete.     Thrips,  Fhlizothrips. 


TYPK  TliACUEATA. 


627 


8.  Order  Rhynchota. — Abdomen  without  cerci ;    basal  portion    of 

anterior  wings  chitinous.  posterior  winj?s  and  tips  of  anterior 

membranous,  or  else  Imtli  membranous,  the  anterior  the  larger, 

or  both  wanting  ;  moutli-parts  piercing  and  suelcing  ;  metamor- 

phoyis  incomplete. 
Anterior  wings  cliitinous  at  base  (Heniiptera).     Anasa,  Notoneda, 

Belostoma,  Haiuitra,  Hydrometra,  Halobates,  Cimex,  PedkiUus 

(wings  wanting  in  the  hist  two). 
Anterior  wings  both  membranous  (ffoinopfera).     Cicada,  Aspidl- 

ottiSy   Dactylopiiis,  PempUigus,  Aphis  (wings  may  bo  wanting 

in  the  hist  three). 

9.  Order  Coleoptem. — Abdomen  without  cerci ;  anterior  wings  chi- 

tinous. covering  in  the  posterior  wlien  at  rest ;  mouth-parts 
biting ;  metamorphosis  compU^te. 

(a)  Tarsi  of  four  joints,  one  of  them  very  small  {Cryptote- 

tramera).     Coccinella. 
(6)  Tarsi  of  ftve  joints,  one  of  which  is  very  small  (Cryptopen- 
taviera).   CurculionidjB,  Clytus,  Saperdn,  Monohammus, 
Doryphora. 

(c)  Tarsi  of  posterior  legs  four-jointed,  of  two  anterior  pairs 

five-jointed  {Heteromera).     Meloi^,  Lytta. 

(d)  Tarsi  all  five-jointed   and    all   the  joints  of  equal   size 

(Pentamera).  Lampyris,  Elateridip,  Melolontha,  Necro- 
phorus,  Staphylinidae,  Hydrophilns,  Gyrinns,  Jira- 
chinns^  Harpalus,  Carabns,  Calomma,  Cinndela. 

10.  Order  Neuroptera. — Abdomen  without  cerci ;  wings  membranous, 

richly  veined  with  numerous  cross- veins  ;  mouth-parts  biting ; 
metamorphosis  complete.     Corydalis,  Chrysopa,  Myrmeleon. 

11.  Order  Paiiorpata. — Abdomen  sometimes  with  cerci  ;  wings  mem- 

branous with  few  cross-veins ;  mouth-parts  biting,  at  end  of 
cylindrical  rostrum  ;  metamorphosis  complete.     Panorpa. 

12.  Order  I'richoptera. — Abdomen  without  cerci  ;  wings  covered  with 

hairs  or  scales,  posterior  ones  larger  and  folded  taulike  wlieu 
at  rest ;  mouth  -parts  sucking  ;  metamorphosis  complete.  Pliry- 
ganea,  Anaholia. 

13.  Order  Lepidoptera. — Abdomen  witliout  cerci  ;  wings  covered  with 

scales,  not  folded  when  at  rest,  thougli  they  may  overlap;  mouth- 
parts  usually  sucking  ;  nit- ■  ..'•lorphosis  complete. 

Small  forms  (Microlepidoptera).     rj;.'""    Caipucapsa,  Pyralidae. 

Larger  forms  (Macrolepidoptera).  Geom'-tridae,  Clisiocampa^ 
Orgyia,  Telea,  Pieris,  Vanessa,  Papilio. 

14.  Order  Hymenoptera. — Abdomen  without  cerci ;   wings  membra- 

nous, without  scales,  not  folded ;  mouth-parts  biting  and  lap- 
ping ;  metamorphosis  complete. 
Ovipositor  retractile  with  poison-gland  (Aculeata).    Apis,  Bom- 
bus,  Vtspa,  Cumponotus,  Formica. 


628 


INVEHTEBRATE  MOKPHOLOOT. 


Ovii)ositor  non-rotructilo,  without  poison-gland  (Terehrantia). 
Ichneumon,  Froctot rapes ^  Pteromalns,  Microgaster,  Oynips, 
Selandria. 
15.  Order  Diptera. — Abdomen  without  cerci ;  wings  sometimes  want- 
ing, only  the  anterior  pair  mer  present,  posterior  pair  repre- 
sented by  halteres  ;  mouth-parts  piercing  and  sucking  ;  meta- 
morphosis compiote. 

With  wings.     Culex,  Tabaniis,  Mnsca. 

Without  wings.    Pulex,  Melophagus. 

LITERATURE. 

PROTUACHEATA. 

H.  N.  Moseley.  On  the  Structure  and  Development  of  Peripatua  eupensis. 
Philosophical  Transactions  Royal  Society,  London,  cLxiv,  1874. 

F.  M.  Balfour.  T/ie  Anatomy  and  Decelopmeat  of  Penpatu»  capensia.  Quar- 
terly Jouru.  Microscop.  Science,  xxni,  1883, 

E.  Oaffron.  Beitrage  zur  Anatomic  und  Ilistologie  des  Peripaius.  Zoologischc 
Beitrttge,  i,  1885. 

J.  von  Kennel.  Entmcklungsgesrhichte  von  Peripatm  Edwardsii  tend  P.  torqua- 
tua.     Arbeilen  ties  Zool,  Iiislituls  Whrzburg,  vii,  1885;  viii,  188G. 

A.  Sedgwick.  T/w  Development  of  the  Cajte  Species  of  Peripatua.  Quarterly 
Joiirn.  Microscop.  Science,  xxv-xxviii,  18N.5-1888. 

A.  Sedgwick.  A  Monograph  of  the  Sjieciea  and  Dii^trilmtion  of  the  Genua  Peri- 
patua.    Quarterly  Journal  of  Microscop.  Science,  xxviii,  1888. 


MYRIAPODA. 

H.  C.  Wood.     The  Myriapoda  of  North  America.    Trans.  American  Philosoph. 

Soc,  xiii,  1865. 
C.  H.  BoUman.     The  Myriapoda  of  North  America.     Bulletin  U.  S.  National 

Museum,  No.  46,  1898 

B.  Latiel.     Die  Myriapoden  der  daterreichiach-ungarischen  Monarehie.     Wien, 

1880-1884. 
J.  A.  Ryder.     The  Structure,  Afflnitiea,  and  Speciea  of  Scolopendrella.     Pi-oc. 

Academy  Nat.  Sciences,  Philadelphia,  1881. 
J.  Bode.     Polyxenua  lagurua,  de  (leer.     Ein  Jieitrag  zur  Anatomic,  Morphologic 

und  Entwichiungageachichte  der  Chilog nathen .     Zcitschr.  fllr  d.  gcsiimnUe 

Naturwissensch.,  xi.tx,  1877. 
0  vom  Rath.     Beitrage  zur  Kenntnisa  der  Chilognathen.     Bonn,  1886. 

C.  Herbst.     Beitriige  zur  Kenntnias  der   Chilojmlen.     Bibliotheca  Zoologica, 

Heft  i.\,  1891. 

F.  0.  Heathcote.  The  Early  Development  of  lulua  terreatrta.  Quarterly  .Tourn. 
Microsc.  8<!ien«'e,  xxvi,  1886. 

P.  0.  Heathcote.  The  Poatembryonic  Development  of  lulu*  terreatrta.  Philo- 
sophical Trans.  Royal  Soc,  London,  ci.xxix,  1888. 


ki:*.,*...^--./^:^^^.'4UiLi,\ 


TYPE  TJiACIIEATA. 


629 


IN8ECTA. 


CYSTEMATIC. 


London, 


Hiuilbsuuiuu  Iu8t. 


S 


A.  S.  Packard.     A  Guide  to  the  Study  of  limcts.     New  York,  1889. 
Sir  John  Lubbock.     A  Monograph  of  the  Collembola  and  TJiysanura. 

189;i 
H.  Hagen.    Synopsis  of  tlie  Neuroptera  of  North  America. 

Miscellimcous  ColUH^tioii,  iv.  1861. 
C.  H.  Fernald.     Ihe  Orthoptera  of  New  England.     Boston,  1888. 
J.  L.  Leconte.     CluHsiJication  of  the  Coleoptera  of  North  America.     Smithsonian 

Inst.  Miscellaneous  Collection,  in,  1892  ;  xi,  1873. 
J.  L.  Leconte.     Numerous  Papers  on  Colcopteru  in  Proceedings  Acad.  Nat. 

Sciences,  Pliiladelpbia. 
H.  Scudder.     The  Butterflies  of  the  Eastern  United  States  and  Canada,  with 

Special  Reference  to  New  England.     Cambridge,  1889. 
J.  B.  Smith.     Contributions  towards  a   Monograph  of  tlie  Noctuidm  of  North 

America.     Proc.  and  Bulletin  U.  8.  Nat.  Museum. 
A.  S.  Packard.     Monograph  of  the  Oeotnetrid  Moths.    U'jport  U.  S.  Geological 

Survey  of  the  Territories,  x,  1876 
E.  T.  CresBon.     Synopsis  of  the  Families  and  Genera  of  the  Uymenoptera  of 

America  north  of  Xfexico,  together  with  a  Catalogue  ofDescril)ed  Species  and 

Bibliography.     Trans  American  Entomological  Society,  1887. 
H  Loew  and  Baron  Osten  Sacken      Monograph  of  the  Diptera  of  North  Americ't. 

Smithsonian  Institution,  MLscellaneous    Collection,  vi,   1862  ;  vi,  1864 ; 

VIII,  1869 ;  XI,  1873. 
H.  Scudder.     The  Fossil  Insects  of  North  America.    New  York,  1890. 


S, 


See  also  the  publications  of  the  II.  S.  Eiitomological  Commi.ssion  and  the 
Annual  Reports  of  the  U.  S.  Department  of  Agriculture  for  numerous  papers 
by  C.  V.  Riley,  A.  S.  Packard,  and  others. 


STKUCTURAL. 

H.  Orenaeher.     Untersuch^mgen  iiber  das  Sehorgan  der  Arthropoden.    Gottin- 

gen,  1879. 
E.  Burgess      Contributions  to  the  Anatomy  of  the  Milkweed  Butterfly  (Dajiait 

archippus).     Anniversary  Memoirs  Boston  Soc.  Nat.  History,  1880. 
H.  C  HcCook.     The  Honeyants  of  the  Garden  of  tfie  Gods,  etc.     Philadelphia 

1881. 
V.  Oraber.     Die  chordotonal  Sinnesorgane  und  das  Gehorder  Insekten.     Archiv 

fllr  mikroKk.  Aiuit.,  xx,  1883. 
E.  WitlacBil.     i^ur  Anatomic  der  Aphiden.    Arbeiten  a.  d.  zool.  Inst.  Wien, 

IV,  1882. 
Sir  John  Lubbock.     The  Oi'igin  and  Metamorphosis  of  Insects.    London,  1883. 

Ants,  Bees,  and  Wasps.     London  and  New  York,  1888. 

J.  A.  Falmen.     Zur  vergkichenden  Anatomic  'ler  Ausfiirhrnngsgange  der  Sex- 

ualorgane  bet  den  Insekten.    Morpholog  .^ibrbuch,  ix.  1883. 
J.Carriere.    Die   Seliorgane    der    Thiere   mri^leiefiendanatomiach    dargestellt. 

MHnchon.  1885. 


Ml: 


11 


530 


INVERTEBRATE  MORPHOLOGY. 


B.  Orasii.  Anatomia  comparata  dei  Tiaanuri  e  considerazioni  generali  tulP 
oiganizumone  degli  Insetti.  Alti  della  R.  Accad.  Liucei,  Berie  IV  iv, 
1887. 

K.  Heider.  Die  embryonale  Entwicklung  von  HydrophUu8  piceua  L.  Jena, 
1889. 

J.  Van  Bees.  Beitrdge  zur  Kenntnm  der  inneren  Meta?norpIme  wn  Musca 
vomitoria.    Zoolog.  Jahrbucher,  iii,  1888. 

V.  Oraber.  Vergleic/tende  Studien  am  Keimatreif  der  Imekten.  Deukaclir. 
Acad.  Wisseuscb.  Wien,  lviii,  1890. 

H.  T.  Fernald.  The  Relationships  of  Arthropods.  Studies  from  the  Biolog. 
Labor.  Johns  Hopkins  Univ.,  iv,  1890. 

B.  T.  Lowne.  Anatomy,  Physiology,  Morphology,  and  Development  of  the  Blow- 
fly.    London,  1890-91, 

W.H.Wheeler.  A  Contribution  to  Insect  Embryology.  Journ.  of  Morphology, 
VIII,  1893. 

T.  H.  Huxley.  Anatomy  of  Cockroach.  Text-book  of  Anatomy  of  Invertebrate 
Animals.    New  York,  1878. 

W.K.Brooks.  Anatomy  of  Grasshopper.  Hand-book  of  Invertebrate  Zoology. 
Boston,  1890. 


wmmBmm 


TYPE  ECUINODERMA, 


631 


CHAPTER  XVI. 


TYPE    ECHINODERMA. 


The  Echinoderms  are  exclusively  marine  organisms  and 
vary  considerably  in  shape,  some  forms  being  elongated  and 
vermiform,  others  stellate,  and  others  again  almost  spherical. 
Whatever  may  be  the  shape,  however,  a  well-marked  radial 
symmetry  can  be  distinguished,  which  suggested  to  the  older 
zoologists  the  association  of  the  members  of  this  group  with 
the  Ccelentera  in  a  type  Badiata.  The  radii  in  the  Echino- 
derma  are,  however,  almost  invariably  five,  instead  of  four  or 
six  or  some  multiple  of  these  numbers  as  in  the  Ccelentera  ; 
and,  furthermore,  while  in  the  Ccelentera  the  radial  symmetry 
represents  a  primitive  condition  and  any  departure  from  it 
towards  biliterality,  as  in  the  Anthozoa,  is  secondary,  the 
reverse  is  the  case  with  the  Echiuoderma.  The  larval  forms 
of  this  group  are  strictly  bilateral,  and  even  in  the  adults 
certain  organs  or  parts  of  organs  interfere  with  the  regularity 
of  the  pentamerous  arrangement  and  bring  about  a  more  or 
less  pronounced  bilaterality. 

This  may  be  clearly  seen  if  one  of  the  stellate  forms,  such 
for  instance  as  the  ordinary  five-rayed  Starfish  (Fig.  240\ 
be  examined.  This  animal  consists  of  a  central  disk,  at  the 
centre  of  one  surface  of  which,  the  oral  surface,  the  mouth  is 
found,  while  the  anus  occupies  a  somewhat  excentric  position 
on  the  other  surface,  which  may  be  termed  the  aboral  or 
apical  surface.  From  the  edge  of  the  disk  the  five  arms  or 
rays  project  outwards,  and  along  the  median  line  of  the  oral 
surface  of  each  arm  there  extend  outwards  from  rings  around 
the  mouth  a  nerve-cord  and  a  hvdrocoel  canal,  this  latter 
forming  a  part  of  a  peculiar  system  of  vessels  characteristic 
of  tiie  Echinoderms.  In  consequence  of  this  radiation  of 
these  structures  out  along  the  arms,  and  the  arrangement  of 


HI 


n\ 


,(  Jr 


t 


1  ). 


^1 


532 


INVERTEBRATE  MORPHOLOGY. 


the  other  organs  for  the  most  part  iu  couformitj  with  the 
radiation,  the  arms  may  be  regarded  as  representing  the 
radial  axes  of  the  body,  the  interradial  axes  lying  in  the  in- 
tervals between  them.  If  now  the  aboral  surface  of  the  disk 
be  examined,  there  will  be  found  upon  it,  in  one  of  the  inter- 
radii,  a  peculiar  tubercle,  known  as  the  madreporiform  tuber- 


FiG.  246. — Aaterias  arenicola  (after  Aqashiz  from  Vekrill). 


cle,  which  serves  to  place  the  hydrocoel  system  of  canals  in 
communication  with  the  exterior  water.  There  is  but  one 
such  tubercle,  and  but  one  canal  leading  down  from  it  to  the 
hydroctBl  ring  which  surrounds  the  mouth,  and  consequently 
there  can  be  but  one  plane  in  which  the  animal  can  be  di- 
vided into  two  similar  parts.  Therefore  the  Starfish,  though 
superficially  appearing  to  possess  a  radial  sj'inmetry,  is  funda- 
mentally bilateral — a  statement  which  applies  equally  well  to 
any  member  of  the  Echinoderm  type. 

It  does  not  necessarily  follow,  however,  that  the  plane 
which  passes  through  the  madreporiform  tubercle  is  the 
median  plane  of  the  body.  The  larvj©  of  the  Echinoderms 
are  strictly  bilateral  organisms,  no  sign  of  radiality  being 
found  in  them  in  an  early  stage  of  development,  and  it  would 
seem  more  satisfactory  to  take  as  the  median  plane  of  the 


TYPE  ECUINODERMA. 


633 


adult  animal  one  which  corresponds  as  closely  as  possible 
with  the  larval  median  plane.  The  madreporiform  tubercle, 
or  rather  the  pore  which  corresponds  to  it,  and  the  tube 
which  leads  from  it  to  the  rudiment  of  the  hydrocoel  system 
can  readily  be  made  out  in  the  lurv»  of  most  forms,  and  it 
can  be  seen  that  it  lies  to  the  left  of  the  median  plane  of  the 
body.  Indeed  in  the  larvje  of  some  Starfishes  two  pores 
occur  at  an  early  stage  of  development,  one  to  the  left  and 
the  other  to  the  right  of  the  median  plane,  the  latter  subse- 
quently disappearing.  The  madreporiform  tubercle  might 
therefore  be  regarded  as  lying  to  the  left  of  the  median  plane, 
which  will  accordingly  pass  through  the  radius  to  the  right 
of  the  tubercle. 

However,  it  is  impossible  to  tell  how  much  modification 
has  taken  place  during  the  transition  from  the  bilateral  to 
the  radial  condition,  and  it  is  not  impossible  that  the  greater 
portion  of  the  adult  represents  one  of  the  halves  of  the 
embryo,  the  other  half  remaining  more  or  less  undeveloped. 
Furthermore  a  secondary  bilaterality  supervenes  in  certain 
forms  of  Echinoidea  and  Holothuroidea  which  does  not  agree 
with  that  indicated  in  the  preceding  paragraph,  and  is  not 
indeed  the  same  in  the  two  groui)s.  It  seems  therefore  pref- 
erable to  assume  a  perfectly  arbitrary  method  of  indicating 
the  radii  of  the  body,  calling  that  radius  which  lies  ojtposite 
the  madreporiform  tubercle  Ay  that  which  lies  to  the  left  of 
this  when  the  animal  is  held  with  the  oral  surface  upwards 
B,  and  so  on  0,  Z),  E,  following  the  direction  of  the  liauds  of 
a  watch.  The  interradii  may  be  indicated  by  combining  the 
letters  of  adjacent  radii,  the  interradius  between  .1  and  li 
being  denoted  by  A  B. 

The  body-wall  in  the  Echinoderma  is  covered  on  the  out- 
side by  a  usually  delicate,  and  in  some  cases  ciliated,  ecto- 
derm, which  may,  however,  be  indistinguishable  from  the  sub- 
jacent mesodermal  tissues  in  certain  parts  of  the  body. 
Below  this  ectoderm,  when  present,  comes  a  layer  of  meso- 
dermal connective  tissue  consisting  of  relatively  few  cells 
imbedded  in  a  more  or  less  fibrillar  matrix,  and  in  this  con- 
nective tissue  there  are  imbedded  numerous  calcareous  plates, 
in  some  forms,  such  as  the  Holothurians,  somewhat  widely 


634 


INVERTEBRA  TE  MORPHOLOG  T. 


separated  from  one  another  so  that  the  body-wall  has  a  more 
or  less  leathery  consistency,  but  more  frequently  placed 
almost  or  quite  in  contact  with  each  other,  and  uniting  in 
most  of  the  Echinoids  or  Sea-urchins  to  form  a  firm  test  en- 
closing the  principal  vegetative  organs,  a  small  area  or  peri- 
stome around  the  mouth  alone  remaining  but  partially  calci- 
fied and  retaining  a  leathery  consistency.  Spinous  elevations 
are  frequently  developed  upon  these  dermal  plates  (whence 
the  name  of  the  type)  and  may  assume  various  forms,  being 
in  some  cases  quite  long,  movably  articulated  with  the  plates, 
and  supplied  with  muscles  so  that  they  may  aid  in  locomo- 
tion. 

The  arrangement  of  the  calcareous  plates  differs  greatly 

in  the  different  classes  which  com- 
pose the  type,  but  certain  of  them, 
distinguishable  by  their  position 
and  relative  arrangement,  reappear 
in  the  majority  of  the  classes. 
These  plates  are  situated  at  the  oral 
and  aboral  surfaces  of  the  body. 
The  oral  plates  are  not  so  constant 
nor  so  numerous  as  the  aboral  or 
a{)ical,  and  show  a  tendency,  even  in 
those  groups  in  which  they  are  most 
highly  developed,  to  undergo  a 
greater  or  less  amount  of  resorption 
during  development,  being  frequent- 
ly more  pronounced  in  larval  than 
in  adult  life.  Typically  the  oral 
system  consists  of  a  central  oral 
plate,  the  orocentral,  unknown  in 
recent  forms,  but  occurring  in  cer- 
tain fossil  genera,  and  this  is  sur- 
rounded by  a  ring  of  five  plates, 
wliich  may  be  termed  the  oral  plates, 
and  which  have  an  interradial  posi- 
tion. The  apical  system  has  as  a 
central  plate  the  so-called  centro- 
dorsal  (Fig.  247,  CD),  which  in  some  forms  is  replaced  bj'  a 


Fio.  247.  -Disk  and  Aum  of 
ZoroutiUir,  ssiiowincj  thk 
Apical  Systbm  or  Plates 

(after  Sladkn). 
an  =  anus. 
cd  =  centro-dorsal. 
mt  =  madreporite. 

T  =  tcnninul  plate. 

2  =  uuder-basals. 

8  ■=  ba.sals. 

4  ~  radials. 


II*..  ■■y'M.liWMWfW 


TYPE  ECIIINODEliMA. 


535 


inimber  of  small  plates  between  which  the  anal  opening  of 
the  digestive  tract  is  to  be  found.  Forming  a  ring  around 
this  are  frequently  live  plates  possessing  a  radial  jiositiou 
Avhich  are  termed  the  under-basals  (Fig.  247,  2)  anil  are  un- 
represented in  certain  forms  ;  next  to  these  comes  a  second 
circle  of  live  plates,  the  hasals  (Fig.  247,  3),  which  are  inter- 
radial  and  correspond  to  the  oral  plates,  while  next  to  these 
again  is  a  third  cycle,  also  of  five  plates,  the  radials  (Fig.  247, 
4),  whose  name  denotes  their  position.  Numerous  other 
plates  may  intervene  in  the  various  groups  between  the  radials 
and  the  orals,  but  their  number  and  arrangement  is  not  sutti- 
ciently  constant  to  permit  of  homologies  ;  the  oral  and  apical 
systems  are,  however,  represented  more  or  less  j)erfectly  in 
all  but  one  of  the  classes,  and  consequently  deserve  special 
mention. 

A  well-developed  dermal  muscular  system  occurs  in  the 
Holothurians  in  which  the  calcareous  plates  are  scattered 
and  the  body-wall  consequently  capable  of  consi<lerable  con- 
traction and  expansion,  but  in  other  forms  it  is  very  much 
reduced.  In  those  forms  in  which  the  calcareous  plates  are 
simply  in  ajiposition  strands  of  muscular  tissue  pass  from 
plate  to  plate,  a  consideral>le  amount  of  movement  being  pos- 
sible, but  in  the  Sea-urchins,  for  example,  tiie  dermal  muscu- 
lature is  almost  wanting,  being  reduced  to  bands  passing  to 
the  bases  of  the  movable  spines  and  to  the  complicated  mas- 
ticatory apparatus. 

The  c(t'lom  is  somewhat  complicated  in  its  relations,  which 
vary  considerably  in  the  difl'erent  grou])S.  In  all  entoroco'lic 
and  scliizoccelic  portions  are  distinguishable,  tiie  former  in 
the  embryo  arising  as  pouchlike  diverticula  from  the  ])rimi- 
tive  intestine  or  enteron,  and  later  becoming  com])letely  ('(di- 
stricted oft'  from  it.  Much  variation  occurs  in  the  later  his- 
tory of  the  pouches  in  the  various  groups,  but  in  genera!  it 
may  be  stated  that  one  of  them,  the  left,  has  a  portion  con- 
stricted oft'  from  it,  which  forms  the  adult  intior  voscxlar  sys- 
tem or  hydrocwJ,  a  structure  characteristic  of  the  Echinoderms  ; 
and  furthermore  this  same  left  enteroco^l  communicates  with 
the  exterior  by  a  dorsal  ])ore,  situated  in  the  interradius  CD, 
and  represented  in  the  adult  by  one  or  many  pores  opening 


^'SQ 


IN  VEliTEDHA  TE  MOUPUOLOO  Y. 


upon  a  sievelike  calcareous  plate  kuown  as  the  madreporiform 
tubercle  or  madreporite.  The  hydrocoel  in  the  adult  commuui- 
cates  with  the  left  euterocoel  by  a  tube,  termed  the  stone' 
canal  from  the  deposition  of  calcareous  matter  which  occa- 
sionally takes  place  in  its  walls,  and  so  indirectly  opens  to 
the  exterior  through  the  madreporiform  tubercle  (see  Fig. 
2(J5).  The  various  departures  from  this  arrangement  which 
occur  will  be  more  conveniently  considered  in  counection 
with  the  special  descriptions  of  the  various  groups ;  the  cou- 
dition  just  mentioned  may  be  provisionally  accepted  as  rep- 
resenting the  typical  arrangement. 

■  After  the  separation  of  the  hj'drocoel  from  the  left  eutero- 
ccel,  the  latter  and  the  enteroccel  of  the  right  side  increase  iu 
size  aud  finally  apply  themselves  closely  to  the  inner  surface 
of  the  body-wall  and  to  the  outer  surface  of  the  digestive 
tract,  forming  the  peritoneal  lining  of  these  structures.  Where 
the  two  coelomic  sacs  meet  there  are  formed,  of  course,  two 
partitions  extending  from  the  body-wall  to  the  intestine,  and 
suspending  that  structure  between  them.  These  partitions 
are  the  mesenteries,  but  before  the  embryo  reaches  the 
adult  stage  one  of  these  mesenteries  disappears,  the  other 
persisting  in  a  more  or  less  perfect  form.  The  coiling  of  the 
intestine,  which  occurs  frequently  in  the  adult  forms,  brings 
about  complications  of  the  course  of  the  mesentery,  compli- 
cations further  increased  iu  most  cases  by  the  formation  of 
other  partitions  which  may  traverse  a  greater  or  less  portion 
of  the  coelom  either  longitudinally  or  transversely.  One  of 
the  transverse  partitions,  most  frequently  present,  separates 
oft'  more  or  less  completely  from  the  rest  of  the  coelom,  a  por- 
tion of  it  surrounding  the  pharyngeal  region  of  the  digestive 
tract  and  hence  termed  the  peripharyngeal  cavity,  while  iu 
some  cases  a  perianal  cavity  may  similarly  be  formed. 

The  hydrocoel,  whose  origin  has  been  described,  develops 
into  a  tubular  ring  (Fig.  248,  cc)  surrounding  the  {esophagus 
quite  close  to  the  mouth.  Upon  this  ring  in  the  interradii 
one  or  several  saclike  diverticula,  termed  PoUan  vesicles  (p), 
occur,  and  in  one  iuterradius  a  canal,  the  stone-canal  (sc), 
passes  aborally  to  open  into  a  thin-walled  sac  termed  the 
ampulla  of  the  sttme-camd,    which  is  in  reality  a  portion  of 


,i._»Ui»i-t<.««.^- 


TYPE  ECIIINODEliMA. 


nn? 


tho  left  enterocd'l,  partly  r  wholly  separated  oft'  from  tho 
rest  of  that  cavity.  This  ampulla,  as  already  meutioued, 
communicates  with  the  exterior  tlirou^h  the  ma»lr(»porite.  In 
the  radii  tubes  {re)  arise  from  the  v'\n^  which  extend  out  to 
the  aboral  extremity  of  the  body  in  the  elouj^ated  and  s})heri- 
cal  forms,  and  to  the  ends  of  the  rays  in  the  brachiate  forms, 
terminating  frequently  in  tentacular  structures  it)  which  ])ro- 
trude  to  the  exterior,  ]>ushin^  the  ectoderm  before  them. 
Along  the  course  of  these  tubes  lateral  brauches  are  given  ott" 


Fig.  248. 


-Diagram  to  show  tiik  Akuanokmknt  of  the  Hyduoc<el  of 
an  eciiinodkkm. 

a  =  iiinpiillii.  p  —  Poliiiii  vesicle. 

an  =  iixiiil  sinus.  re  =  radial  caiiiil. 

cc  —  circular  ciinal.  »c  —  sloiie-ciiiml. 

M  =  MiadrcporilK.  t  ~  torininal  tentacle. 

tf  =  tube-foot. 


which  terminate  either  in  tentacular  structures,  or  else  in 
tubes  terminating  in  a  sucker,  which,  since  they  play  an  im- 
])ortant  role  in  locomotion,  are  termed  inhc-feet  {tf).  In  many 
forms  a  globular  reservoir  or  ampulla  (a)  is  attached  to  each 
tube-foot,  and  valves  are  found  at  the  junction  the  branch 
l)assing  to  the  foot  with  the  radial  canal,  h.  ...it  the  foot  can 
be  extended  to  a  considerable  distance  by  the  contraction  of 
the  muscles  iu  the  walls  of  the  ampulla  and  the  consequent 
forcing  of  water  into  it.  By  means  of  the  sucker  they  may 
then  adhere  to  foreign  objects,  and  their  contraction  then 
produces  a  movement  of  the  body  towards  the  point  of  tixa- 
tit)n. 


638 


INVERTEBHATE  MOltPUOLOOY. 


In  connection  with  the  stone-canal  a  peculiar  body  is 
developed  in  most  forms.  Its  function  is  u  matter  of  ques- 
tion, it  having  been  at  one  time  taken  for  the  heart  and  at 
another  for  a  gland.  It  is  generally  termed  the  ovoid  gland 
(Fig.  265,  og)  and  consists  of  a  mass  of  cells,  derived  from  the 
peritoneal  lining  of  the  enteroc(rl,  grouped  together  to  form 
a  more  or  less  soli«l  mass.  The  oral  end  of  the  gland  is  pro- 
longed into  a  cordlike  structure  which  seems  to  «uitor  into 
chise  relationships  with  the  oral  lacunar  ring  (see  below),  while 
at  the  other  it  is  continued  out  to  enter  into  close  rehitiouships 
with  the  reproductive  organs  in  a  manner  that  will  be  de- 
scribed when  treating  of  those  organs.  Surrounding  the 
ghind  is  a  sinus — the  axial  sinus  (Fig.  2(55,  as) — separated  oft' 
from  the  enterocad  and,  in  some  forms,  in  communication 
with  the  ampulla  of  the  stone-canal,  and  the  porticm  of  the 
gland  which  passes  oft'  towards  the  reproductive  gland  is  also 
surrounded  by  a  sinus,  or  rather  lies  in  the  wall  of  a  sinus 
which  nniy  or  may  not  communicate  with  the  axial  sinus  but 
has,  like  it,  origin  from  the  general  enteroccel. 

What  has  been  termed  a  blood  system  is  usually  present, 
consisting  of  a  tubular  ring  surrounding  the  cesophagus,  and 
lying  between  the  hydroccel-ring  and  the  nerve-ring.  Five 
branches  may  extend  oft'  from  it  along  the  radii,  preserving 
the  same  relations  to  adjacent  structures  as  does  the  ring. 
These  spaces  seem  to  be  schizoctelic  in  their  character,  and 
may  be  termetl  the  schizocceUc  rimj  ajid  radial  schizocwJic  .sinuses 
in  order  to  avoid  confusion  with  ant^ther  system  of  vessels 
which  S(jmetimes  lie  within  the  sinuses  and  have  also  been 
termed  blood-vessels.  This  latter  system  may  be  termed  the 
lacunar  si/sfem,  and  is  composed  of  a  network  of  vessels  lying 
in  the  walls  of  the  intestine,  and  collecting  usually  into  a 
])eritesoj)hageal  ring  or  plexus  (Fig.  205,  Ir),  with  which  also 
th(!  ovoid  gland  comes  into  connection.  In  the  Echiuoids, 
as  has  just  been  indicated,  ])rolougatiousof  this  pericesophageal 
ring  or  ])lexus  extend  out  in  the  radial  schizocoelicr  sinuses. 

The  fluids  contained  in  the  sinuses,  lacuna*,  hydrocoel,  and 
enterocoel  are  .ill  very  similar,  consisting  of  a  plasma  contain- 
ing amoeboid  cells  sometimes  dee})ly  pigmented.  In  a  few 
forms  hemoglobin  is  present ;  in  the  Ophiuran  Ophiactis  it  is 


TYPK  ECUlNODEliMA. 


d39 


nses 
ids 

leeii 
tho 

liu^' 

|(>  a 
iso 
ds, 
;eal 

iiid 

lin- 

few 

It  is 


contaiued  iu  flat  nou-iiucleatod  dinks,  reseiubling  Mauiiualiau 
reil  blood-corpnscles,  tioatinji;  in  tlu^  plasma  of  tlio  water 
vascular  system  ;  iu  the  Holothuriaus,  Thijune  and  Ci«nNi(in'<i, 
it  is,  however,  coutaiued  iu  auuehoid  corpuscles,  which  are 
most  abuudaut  iu  the  c<elomic  fluid,  thou<{h  occurring  also  iu 
the  water  vascular  tubes. 

The  digestive  tract  is  generally  more  or  less  twisted  into  a 
spiral ;  and  even  when,  as  in  souu'  Holothuriaus,  it  a[>pears  to 
be  straight,  it  is  to  be  regarded  as  a  much-drawn-out  spiral, 
since  the  mesentery  still  retains  a  spiral  arrangement.  In  the 
Holothuriaus,  Echinoids,  and  Starfishes  it  opens  on  the  aboral 
surface  of  the  body,  but  in  the  Crinoids  it  is  bent  upon  itsrlf 
so  that  the  anus  is  on  the  «)ral  surface.  l\i  some  Starfishes 
and  in  all  Ophiurans  no  anus  is  present.  Various  accessory 
structures,  masticatory  apparatus,  ccecal  pouches,  etc.,  are 
found  iu  the  varit)us  groups,  but  their  description  may  be 
deferred  until  later. 

The  nervous  system  may  be  regarded  as  being  composed  of 
three  portions,  one  of  which  has  essentially  the  same  arrange- 
ment as  the  water  vascular  tubes,  consisting  of  a  circumoral 
or  a  periiesophageal  ring  from  which  live  radial  nerves  pass 
oil'  (Fig.  205,  nr  and  rn).  In  the  Starfishes  and  Crinoids  the 
entire  system  is  imbedded  in  the  ectoderm,  but  in  other  forms 
it  sinks  within  the  body-cavity.  From  it  branches  jiass  in- 
wards at  the  mouth  to  suppl}'  the  walls  of  the  <esophagus,  and 
other  branches  form  a  network  coveiiiiL;  the  surface  of  the 
bod}',  supplying  the  sense-organs  which  may  occur  thereon. 
The  radial  nerves,  in  addition  to  sending  branches  to  join  the 
epidermal  plexus,  supply  the  ambulacral  system.  This  por- 
tion oi  the  nervous  system  may  be  termed  the  epidermal 
portion,  and  the  second,  inasmuch  as  it  supplies  the  majority 
of  the  muscles  of  the  body,  may  be  termed  the  muscular 
system.  This  is  not  always  developed,  being  absent  in  Mie 
Crinoids,  but  when  present  accompanies  iu  general  the  epi- 
dermal portion  iu  the  form  of  delicate  nerve-cords  lying  on 
the  inner  surface  of  the  circumoral  ring  and  radial  nerves,  and 
sending  branches  to  the  various  muscles,  including  })()ssibly 
those  of  the  tube-feet.  The  third  or  aboral  ])ortiou  ap])ears 
to  be  entirely  wanting  in  the  Holothuriaus,  but  when  present 


540 


IN  VKUTKliliA  TK  MOHPHOLOO  Y. 


cousists  of  a  riu^  situated  at  the  aboral  surface  of  tbo  body, 
s«'ii(liii<^  oil'  bniiic'hes  to  tbo  reproductive  or^aus  as  well  as,  iu 
some  cases  at  least,  foriuiu^  auustomoses  with  the  epidermal 
system. 

Seiiso-orj^aus  of  various  kiuds  are  developed.  Tactile 
tentacles  occur  at  tbe  extremities  of  the  radii  of  some  forms 
and  round  tbe  moutb  in  others,  while  iu  the  softer-skinned 
Holotburiaus  tactile  papilbe  may  occur.  Eyes  occur  at  the 
extremities  of  tins  ratlial  nerves  of  the  Startishes,  and  have 
also  been  described  as  occurring  iu  some  Echiuoids,  while 
otocysts  occur  in  some  Uob)thuriau8,  sometimes  in  considera- 
ble numbers. 

No  special  excretory  orfjaus  occur  iu  the  Echinodermata, 
the  amceboid  cells  of  the  ctelomic  Huids  perliaps  servinj^  iu 
some  cases  to  remove  the  waste  substances.  They  have  been 
ol)serveil  to  pass  throuj^h  the  body-wall,  in  rej^ions  where  it  is 
thin,  to  the  exterior  and  there  degenerate.  For  the  most 
part,  liowever,  the  waste  products  are  deposited  in  the  tissues, 
or  else  pass  to  the  exterior  by  osmosis.  In  the  Holotburiaus 
8])ecial  branched  appeuda.^es  of  the  terminal  portion  of  the 
intestine  appear  to  take  some  part  iu  excretion,  but  such 
organs  do  not  occur  iu  other  groups. 

The  Echiuoderms  are  almost  invariably  bisexual,  and  the 
reproductive  organs  are  usually  situated  iu  the  iuterradii. 
They  are  enclosed  iu  a  special  coelomic  sinus,  the  genital  .vmws, 
in  whose  wall  may  be  found  the  branches  of  the  aboral  uerves. 
From  each  organ  or  mass  of  reproductive  cells  a  cellular  cord, 
the  genital  rachis,  surrounded  by  the  sinus  may  be  traced, 
except  in  the  Holotburiaus,  to  the  ovoid  glaud,  and  it  a])pears 
probable  that  in  some  ca.ses  at  least  the  reproductive  cells 
originate  iu  a  part  of  the  ovoid  glaud  and  migrate  to  the 
reproductive  organ  along  the  rachis,  becoming  mature  in  their 
final  position.  The  openings  by  which  the  reproductive  ele- 
ments pass  to  the  exterior  vary  both  iu  number  and  position 
iu  the  different  groups,  but  are  usually  situated  on  the  aboral 
surface  of  the  body. 


I 


m 


wmm 


TYPK  KCmyODKUMA. 


541 


I.  Clahs  Ckinoidea. 

The  Criuoula,  or  Sea-lilies  (  Ki^.  '24U),  coiistitute  ii  j^roup  of 
i'urmH  wliicli  in  the  earlier  geolo^icHl  perioiLs  reaciieil  a  liij^li 
j^rade  of  developnitiiit,  but  to-«lay  tlie  class  is  represeiitod  by 
couiparutively  few  Tonus,  for  the  uiost  part  eoutiued  to  deep 


PlO.  249. — Pentaerinus  marlearnnua  (after  Wyville  Thomphon  from  IIertwio). 

water.  One  of  the  most  characteristic  features  of  the  j^roup 
is  the  presence  of  a  more  or  less  ehiugated  cylindrical  stalk, 
one  end  of  which  is  attached  to  stones  or  other  objects  which 
serve  as  supports  for  the  animal,  while  at  the  other  end  is 
the  body  proper,  which  has  a  more  or  less  cuplike  form.  In 
the  peculiar  genus  Ilolopus  the  stalk  is  thick  and  short,  and 
may  be  described  rather  as  the  prolonged  apex  of  the  body 


64. 


INVKHTKliRA  TK  MOliPIlOLOG  Y. 


tliau  as  a  clistinct  stalk,  wliilo  in  other  forms,  such  as  Avte^fon 
and  Actinometra,  the  stalk,  though  present  in  younj^  forms,  is 
entirely  wamting  in  ailult  life,  durinj^  whieh  the  animal  is  free- 
swimming,  tiioiigh  having  the  })ower  of  anchoring  itsolf 
temporarily  to  solid  objects  by  means  of  a  number  of  slender 
processes  terued  cirri  which  project  from  the  apex  of  the  cup 
(Fig.  251,  V). 

The  lower  portion  of  the  cuj),  or  calyx,  is  formed  by  a  num- 
ber of  series  of  calcareous  plates  united  to  each  other  by 
sutures,  while  its  mouth  is  covered  iu  by  a  flat  or  dome- 
sha})ed  disk  in  which  calcareous  plates  may  or  may  not  be 
present.  In  the  centre  of  the  <lisk  is  the  mouth  of  the  animal, 
while  to  one  side  is  the  anus,  lying  in  the  int(!rradius  VI). 
From  the  mouth  live  grooves,  known  as  the  (nnhtddcrdhfronvt's, 
extend  outwards  towards  the  margin  of  the  cuj),  and,  near  the 
margin,  brauch,  being  then  continued  outwards  on  the  oral 
surfaces  of  ten  arms  which  arise  from  the  junctiou  of  the  disk 
and  calyx,  frequently  branching  in  their  course,  and  bearing 
along  their  sides  a  series  of  short  processes  resembling  them 
in  structure,  and  termed  the  pirinKh's,  upon  which  the  anibu- 
lacral  grooves  are  also  continued.  These  arms  are  capable  of 
considerable  movement,  Ix'ing  at  one  time  extended  out  at 
right  angles  to  the  body  or  even  reflexed,  and  at  another 
coiled  up  circinnately  over  the  disk,  the  pinnules  being  at  the 
same  time  bent  inwards  towards  the  median  axis  of  the  arm. 

The  stem  when  ])resent  consists  Of  a  nund)er  of  disklike  or 
cylindrical  calcaitious  plates,  ])laced  one  on  top  of  the  other, 
being  held  togethei-  by  bands  of  conniH^tive  tissue,  an«l  is 
traversed  by  aciMitral  canal  containing  prolongations  of  certain 
of  the  visc(Hal  sti'uctures.  The  t«»rniinal  plate  servers  as  the 
point  of  fixation,  tin;  plates  immediately  above  it  having 
attached  to  them  a  number  of  cirri  which  assist  in  fixation 
and  are,  like  the  stein,  com})os«>d  of  calcareous  ])Iates  contain- 
ing prolongations  of  tln^  central  catial.  In  some  forms,  such 
as  J^entacrinn.s,  whorls  of  cirri  also  occur  at  intervals  all  along 
the  stem,  those  plates  from  which  they  arise  being  ternxMl 
nodal  plates,  a  varying  nund)er  of  plates  destitute  of  cirri 
occurring  between  two  nodes  in  diflercnt  genera.  In  certain 
genera,   however,  such  as  I/iforriiius  and   /i^hiziKrinus,   these 


m:^ 


■PM 


MH'ipw. 


7'  >7'A'  KVIIINODKUMA. 


548 


LV1I1-; 
iitioii 
itaiiN 
such 

ijumI 
cirri 
rtiiiii 

-lioso 


rttem  cirri  iire  entirely  \vuutin«»  except  uear  the  poiut  of  tixu- 
tioii. 

Tiio  uppermost  phite  of  the  stem  is  usually  re<i;ar(leil  as 
forming  tln^  apex  of  tlu>  cal^^x  and  is  termeil  the  vvufrodofsttl. 
Above  this  comes  in  n\ost  recent  forms  a  series  of  usually  live 
(sometimes  three)  interradial  plates,  the  hasals  (^Fig.  2r)(),  />),  hut 
in  one  genus,  ThiiunmfiM-riuns,  there  occurs  between  the  cen- 
troilorsal  and  basal  [dates  a  si^-ies 
of  live  radial  j)lates  which  are 
termed  the  parabasals  or  uiuler- 
basals,  and  which  have  also  betMi 
found  to  occur  in  the  embryo  of 
Aut'doti,  later  on  fusing  with  the 
centrodorsal.  Hu  .'ecMliiig  the  ba- 
8als  are  from  two  *.o  seven  circles 
of  radials  (/•),  each  circle  being 
also  composed  of  Hve  ])lates  t(!rmed 

tlu>  first,  second,  third,  etc.,  radials  Fio.  ar.O.— Thk  aimcai,  sysikm 
a(!cording  to  their  succ»>ssioji 
counting  from  the  centrodorsal. 
In  Antadon  aud  some  of  its  alli«'s 
the  numlxu'  of  cycles  of  radials 
seen  from  the  exterior  is  oiu; 
short  of  the  actual    number  which 

exists,  the  first  radials  IxMug  overlapptid  and  covered  in 
by  the  second;  and  furtlicimoii^  in  the  siinic  forms  tlui 
basals  have  also  been  pushed,  as  it  were,  withii'  Hie  calyx 
and  havji  fustid  to  form  a  single  plate,  tlu»  so-called  ro.sclfc 
/ihtc  (Fig.  'i')!,  lios),  which  nists  upon  the  centrodorsal,  ))ar- 
tiaily  closing  a  cavity  in  that  plate.  '*"'ie  terminal  radi.ils 
usually  present  two  articulating  facets  in  their  dist.il  sur- 
faces and  are  geiKM-ally  known  Jis  tln^  K.rilhirics  (Fig.  'i^O,  a), 
since  the  amis  articulate  with  them.  In  the  genus  Tlnninitf- 
focrimis  between  each  pair  ol"  first  ratUals  an  interiadial  ])late 
occurs,  a  condition  fnMpiently  found  in  fossil  genera  (/),  but 
usually  wanting  in  recent  forms.  Tlu^se  various  plat(>.s  which 
constitute  th*'  apical  sy, cm  are  united  by  suturtis,  the  edges 
of  tin?  various  series  (d'  plates  condng  into  ctnitacit,  so  that  a 
iirm  su|)port  is  afl'orded  for  the  aims. 


ov  MelorrinuH  (kossii,)  (I'miu 

KUOWN). 

(i  ~  Hxilltiry  pliilcs. 
b  Tjz  liiisiil  pliilcs. 
i  —  iiilciTMiii.il  j)l!ites. 
r  —  mdiiil  phiit's. 


rv^ 


\_ 


"^ 


644 


INVEHTEBRATE  MORPIIOLOO Y. 


These  are  in  reality  coutiuuations  of  the  radial  series  of 
])lates  ;  in  fact,  iu  some  forms  certain  of  the  radials  appear  to 
enter  into  the  formation  of  the  arm.  In  most  forms,  however, 
a  series  of  arm-plates  arises  from  eaeli  facet  of  tlie  rive  axillary 
plates,  so  that  the  arms  are  ten  in  number — a  condition  which 
finds  an  exception  in  the  remarkable  genus  TfKotmntocn'ini.s, 
which  ])ossesses  but  rive.  In  some  forms  these  ten  arms 
branch  dichotomously  ;  the  plates  intervening  between  the 
axillaritis  and  the  rir.-t  branching  are  termed  bracMah^  those 
between  the  rirst  and  second  branchings  distichahs,  and  those 
between  the  second  and  third  branchings  pdhnars — terms 
which  are  useful  in  systematic  descriptions.  These  various 
plates  are  united  together  by  ligaments  and  muscles,  or  else 
by  ligaments  alone  (tliis  last  form  of  union  being  known  as  a 
syzff)/!/),  tlie  movements  of  the  arms  noted  above  being  thus 
ren(hM(ul  possible.  The  ])innules  repeat  the  arm  in  their 
structure,  though  usually  on  a  much-reduced  scale.  Thev 
are  situated  on  the  joints  separating  consecutive  })lates  of  the 
arms,  and  are  placed  alternately  on  the  right  and  left  sides 
of  the  arm  which  bears  them.  The}'  appear  at  rirst  to  have 
been  ])r«jduced  by  lattnal  budding  from  the  joints,  but  clos^^" 
exauiination  indicates  that  in  reality  they  represent  a  branch- 
ing, one  of  the  branches  remaining  small,  while  the  other  in- 
creases in  size  and  ])laces  itself  in  the  direction  of  the  axis  of 
the  arm.  The  whole  arrangement  is  comparable  to  that  form 
of  iurioresceuce  termed  by  botauists  a  scorpioid  cyme,  the 
pinnules  representing  the  Hower-jiedicels.  Owing  to  the 
pinnules  being  in  reality  one  of  the  branches  of  a  diclu^tomv, 
it  is  evident  why,  in  those  forms  in  which  the  arms  branch, 
there  is  no  pinnule  at  the  joint  where  the  branching  occurs  ; 
in  addition,  however,  ])innules  are  also  lacking  on  sv/vgial 
joints,  so  that  their  regular  succession  may  be  somewhat  dis- 
turbed. 

As  regards  the  oral  system  of  plates  an  oro-centntl  is  found 
iu  some  fossil  forms,  l)ut  is  unrepresented  in  recent  genera. 
A  circle  of  rive  interradial  f>ral  plates  is  found  in  IIoh^puM, 
Rhiz(}crinus,  //iforriniis,  yininnioforrhiKs,  and  CoJtonocn'fHif,; 
and  in  the  stalked  larva  of  Avfedov,  but  in  the  adults  of  this 
latter  form  and  in  other  genera  than  those  nieution(Hl  these 


g'-iii  j'diWWt.'ir':-^-"!  ';l»>l!!»iiiiP<Miir-'- 


TYPE  ECHINODEKMA. 


r)45 


1.1 
a. 

is 

so 


plates  disappear  duriuj;  growth,  the  disk  being  either  naked, 
or  covered  by  a  number  of  small  i)hites  which  are  tonned  nu- 
ambnhicrals,  certain  of  which  lying  on  either  sid(^  of  th«»  anibu- 
lacral  grooves  receive  the  special  name  t)f  lufatiihuldcrtiis,  or 


covering-plates. 


The  ectoderm  cannot  usuallv  be  distiu'jjnished  over  the 
surface  of  the  calyx  or  on  the  stem,  but  is  j)resent  on  the 
disk  and  on  the  oral  surfaces  of  the  arms  and  pinnules,  b»Mng 
there  non-ciliated  except  along  tlui  ambulacral  gicjoves.  It 
rests  upon  a  connective  tissue  in  which  tlu;  calcareous  pl.ites 
are  developed,  and  from  which  strands,  fio(piently  with  cal- 
caroous  spicules  imbedded  in  them,  usually  traverse  the  body- 
cavity.  The  ligaments  which  unite  the  plates  of  the  ;iriiis 
and  stem  are  fornunl  of  this  conuectiv«i  tissue,  and  contiactile 
fibres  of  a  peculiar  character  aie  s})aringly  developed  in  it, 
stretching  across  the  non-syz^gial  joints  of  the  arms,  ])inMules, 
and  cirri,  and  probably  also  reaching  a  slight  deveh)pment 
in  the  stem. 

The  internal  structure  of  the  Crinoids  is  known  piinci- 
pallj' from  observations  on  Ante(hn,  and  th«^  following  account 
represents  what  occurs  in  that  form.  The  co'lom,  as  already 
stated,  is  traversed  by  numerous  strands  of  connective  tissue, 
and  primarily  consists  of  two  cavities  separated  from  each 
other  by  a  mesentery,  each  cavity  being  continued  out  into 
the  arms,  forming  the  oral  and  aboral  canals  t)f  thest;  struc- 
tures, at  the  extremities  of  which  they  unite.  The  nuisentery 
does  not,  however,  long  persist  in  its  entirety,  but  the  two 
cavities  fuse,  new  membranes,  however,  arising  and  dividing 
them  in  some  species  of  Avialon.  One  of  thes(^  mend)ranes 
(Fig.  251,  vx)  surrounds  the  intestine  and  forms  tlu;  visceral 
sac,  its  presence  rendering  the  evisceration  of  Atilc<h)ii  an 
easily-accomplished  process  and  one  which  is  made  use  of 
by  the  animal  in  unfavorable  conditions,  a  new  visceral  mass 
being  later  regenerated.  The  portion  of  the  ccelom  which 
lies  peripherally  to  this  sac  is  termed  the  circumvisceral 
portion  (cc),  and  that  within  it  the  intervisceral  (/c),  the  latter 
containing  an  axial  cavity  {A.r)  enclosed  by  a  mendtraiu'  sim- 
ilar to  the  visceral  sac  and  continuous  witii  tin-  oral  c(elomic 
cavities  {oc)  of  the  arms,  the  aboral  cavities  («c)  commuuicat- 


I 


046 


IN  VEHTEBliA  TE  MORPHOLOO  Y. 


iug  with  the  circiimvisceral  coelom.  A  portiou  of  one  of  the 
ccelomic  cavities  at  au  early  stajjfe  becomes  cut  oflf  from  the 
rest  of  the  coelom  ami  divided  iuto  live  chambers  whose  walls 
are  formed  of  a  deuse  fibrous  membraue.     This  constitutes 


/Til 


Fio.   351.— Vektical   Skction   Tlinoroii    Antcdon   (combination  of  flKures  by 

I.iDWUi  mill  Mausjiali.i. 


(ic  =  abnral  ciuiiil  nf  aim. 
<in  —  aboral  ncrvi'. 
Aj;  —  axial  sinus. 
Br  =  brachial  plates. 

C  =  cirri. 

cr  =  circiimvisceral  cavity. 
CD  =  cciilrodor.sal  plate. 

CO  =  central  oriran. 
Do  —  dorsal  orj^an. 

gr  —  genital  raclii.s. 
/  =  intestine. 

ie  =  intervisccral  cavity. 


M  —  mouth. 

oe  =  oral  cavity  of  arm. 

on  —  oral  nerve-riug. 

if  —  radial  plates. 

rb  =  radial  lacunar  vessel. 
m  —  radial  epithelial  nerve. 
ros  ■-=  rosetteplale. 
rw  —  radial  liydroca>l-caual. 

se  =  stone-canal. 

T  ■—  oral  tenlacle. 

m  =  visceral  se|)tum. 
tcp  —  water-pore. 


the  elianiborod  (>i'<i;}iii  (co),  which  iu  Anfcdon  lies  iu  a  cavity 
in  the  ceiitrodorsal  plate  and  is  roofed  over  by  the  ro.sette- 
plate,  but  in  other  forms  simply  rests  upon  the  ceiitrodorsal ; 
commuuicatiuj;  with  it  is  the  lower  end  of  a  somewhat  club- 
shaped  structure  termed  the  dorsid  organ  {Do),  which  pro- 
jects orally  jiarallol  to  the  axial  c(elomic  cavity. 

The  ejiitheliuni  of  the  aboral  co'lomic  cavities  of  the  arms 
is  diti'erentiattMl  here  and  there  iuto  peculiar  orgaus  the  cili- 


TYPE  KCIllNODKH.\rA. 


r)47 


atetl  cups,  consistiii}^  of  slight  depressions  lined  l)y  coluinn.ir 
cells  each  of  which  hears  a  long  ciliuni.  These  cups  are 
especially  abundant  in  the  pinnules,  and  serve  to  eremite  a 
circulation  of  the  cceloniic  tiuid,  which,  as  in  other  Echino- 
dernis,  contains  numerous  annehoid  cells  tioating  ticely  in  it. 
The  water  vascular  system,  or  hydroccel,  consists  of  a  ring 
surrounding  the  mouth,  and  sending  outwards  live  radial 
canals  {rw)  which  lie  below  the  ambulacral  grooves  and  are 
continued  along  the  arms  anil  pinnules.  Occasionally  sub- 
ambulacral  calcareous  plates  are  develo})ed  in  tlui  connective 
tissue  below  the  radial  canals,  and  in  some  fossil  forms  these 
plates  assume  a  regular  arrangement  in  two  rows.  At  regu- 
lar intervals  along  the  arms  are  situated  the  and)ulacral  ten- 
tacles, which  are  tingerlike  outpouchings  of  the  radial  canals 
destitute  of  terminal  suckers  and  are  arrangtul  in  groups  of 
three,  the  canals  being  somewhat  enlarged  in  the  region  where 
they  occur,  an  indication  perha|)s  of  the  am})ulho  found  in 
other  groups ;  in  some  forms  the  cavities  of  the  tentacles 
seem  to  be  united  with  those  of  the  canals  only  by  exceed- 
ingly small  oritices,  which  niay  be  closed,  since  the  tentacles 
in  their  greatest  contraction  always  remain  tilled  witii  fluid. 
In  the  neighb()rhood  of  the  mouth  are  a  number  of  oral  ten- 
tacles (Fig.  251,  T)  arising  directly  fioni  tlu^  oral  ring,  and 
differing  from  the  andjulacral  tent.icles  in  not  being  arranged 
in  groups  of  three.  From  the  oral  ring  there  aUo  arise  in 
Anfi'don  a  nund)er  of  ciliated  tubes  (.sf)  which  open  into  tli«3 
ctelomic  cavity,  eairh  one  corresjionding  to  a  stone-canal  of 
the  otlu.'i  Echinoderms.  In  Aiit('<ht)i  tlnu'e  are  as  many  as 
thirty  A  these  canals  in  t'ach  interradius,  and  in  Priiftn  rlmis 
an  even  greater  nundicM*  occurs  ;  but  in  otliei'  forms  they  may 
be  fewer,  lih  '.ocrinuH,  for  exam[)le,  possessing  only  five  in  all, 
on  ^  being  situated  in  t'ach  interradius.  Tii  the  larva  ot"  A)tfv. 
ifon  there  is  at  uu  early  stage  only  one,  communicating  with  a 
])ortion  v>r  one  of  the  primary  co'lomic  cavities,  which  on  its 
part  o])ens  to  the  exterior  by  a  pore,  an  arrangenient  wiiich 
nuiy  be  regarded  as  ty|iic;il  i'or  the  Ei-hiiioih'rms.  LattM*, 
however,  this  portion  of  the  c(elomic  cavity  tlegenei'jitiis,  and 
the  canal  then  opens  directly  intt»  tin;  general  co'loni,  and 
this  communicates  with  the  exterior  by  the  pore.     In  subse- 


Vy- 


V 


"^ 


648 


IN VEHTEDHA TE  MOliPllOLOO  T. 


queut  stages  additioual  stoue-cauuls  develop  from  the  oml 
riij}^',  jiud  at  the  same  time  additioual  jxjres  develop  iu  the 
walls  of  the  body,  forming  what  are  called  the  calyx-pores 
{up).  These  may  reach  a  considerable  uumber,  it  being  esti- 
mated that  iu  Antaloii  there  are  no  less  than  fifteen  hundred 
of  them  scattered  over  the  disk ;  iu  Illiizocrinufi,  Ilyocrinus, 
and  Holopus,  however,  there  are  only  five  pores,  ono  piercing 
each  of  the  oral  jjlates  present  in  these  forms. 

The  schizocu3lic  system  cousists  of  five  radial  siuuses  (Fig. 
251,  rh)  lyiug  between  the  radial  hydroccel  vessels  and  the 
more  superficial  radial  nerve,  together  with,  according  to  some 
authors,  a  circular  sinus  surrounding  the  mouth  into  which 
the  radial  siuuses  open.  A  plexus  of  lacuuie  occurs  iu  the 
walls  of  the  intestine,  and  another  surrounds  the  oesopha- 
gus, this  latter  in  part  aggregating  itself  into  a  structure  re- 
sembling a  lymphatic  gland  and  kuowu  as  the  spongy  body ; 
the  dorsal  organ  likewise  contains  a  dense  network  of  tubes 
lined  with  epithelium.  Along  the  sides  of  the  hydroccel- 
canals,  iu  the  disk,  arms,  and  pinnules,  alternating  in  the  two 
last  with  the  triads  of  tentacles,  in  the  walls  of  the  intestine, 
and  occasionally  elsewhere,  there  are  imbedded  iu  the  con- 
nective tissue  yellowish  si)herical  bodies  known  as  the  .saccvJi. 
The  interior  of  each  sacculus  is  lined  with  cells,  aiul  contains 
a  number  of  pyriform  masses  formed  of  small  highly-refrac- 
tive spherules,  ap])arently  of  an  albuminoid  substance.  The 
fmu'tion  of  these  bodies  is  very  obscure  ;  they  have  been  re- 
garded as  organs  for  secreting  carbonate  of  lime,  as  excretory 
organs,  as  parasites,  as  mucous  glands,  and  lately  as  organs 
of  reserve  iu  which  j)roteid  matter  may  bo  stored  up  for 
future  use.  At  ])resent,  liowevrr,  the  cjuestion  is  an  open 
one,  and  a  function  cannot  with  certainty  be  assigned  to  them. 

The  mouth  iFig.  251,  ni)  is  usually  situated  at  the  centre 
(^f  the  oral  disk,  an<l  opens  into  a  simple  tubular  intestine 
wdiich  coils  once  round  the  ccelomic  cavity  in  the  direction  of 
the  hands  of  a  watch  and  then,  bending  upon  itself,  turns 
orally  to  open  in  the  interradius  CD  U])OU  the  disk.  In  Ac- 
tinoHicfrtt,  a  genus  closely  related  to  Antcthm,  the  intestine 
lies  in  four  coils,  but  there  is  as  a  rule  little  variation  from  the 


^^nJMTlTfcilliniMliri 


7T/W  ECUINODKHMA. 


649 


itaiiis 

ifrac- 

The 


)  [or 
opeu 
hem. 
outre 
'stiiie 
;)ii  of 

iUlllS 

II  Ac- 
stiiie 
n  the 


t'outUtiou  described,  uud  uo  specially-marked  differentia tiou 
of  the  tube  occurs. 

The  liervous  system  of  the  Criuoids  is  characterized  by 
the  remarkable  developmeut  of  the  aboral  i)<)rtioii  and  by 
the  appareutly  eutire  ai)seuce  of  the  muscular  portion.  The 
epithelial  portion  consists  of  a  riu«^  and  live  radial  nerves  [rn) 
which  pass  alonj^  the  ambulacral  jjjrooves  and  out  upon  the 
arras  and  pinnules,  imbedded  throughout  tln'ir  eiitin*  course 
in  the  lower  layers  of  the  ectoderm.     The  aboral  system  is, 


Fkj.  253. — Diagram   of  tiif,  Arranokmknt  of   the  ABouAr,   Nkkvoca 

SYSTIOM   oK   AuUlduu  iaftfi    Mak«hai,i.>. 

a  —  uinis.  cd  —  ceiitrodorsid. 

Br  —  bijicliial  plates.  R  —  radiiils. 

ou  the  othoj-  hand,  much  more  stronj^ly  developed,  and  stands 
ill  intimate  assocnation  with  the  chumbered  or^^'an  in  the  walls 
of  wliich  it  is  imbedded.  Its  central  portion  is  somrwliat 
complex,  as  is  shown  in  Fi<;.  252,  but  may  be  said  to  consist 
of  a  riu<;,  or,  more  properly  speakim:^,  of  a  |>enta^on  from 
which  five  stronf  cords  radiate  out  into  the  arms,  perforatiuj^ 
tlu?  plates  of  which  these  are  composed.  Both  tib">'s  and 
^'an^lion-cells  enter  into  the  compositiui  of  the  (•(.i*is  ;nid 
rinj^,  and  a  complicated  system  of  coEujiissiires  exists.  From 
the  central  porti(»n  bi'auches  are  al!~>  sent  to  the  cii*ri,  ami 
])rol)ably  in  stalked  forms  a  branc  -.  .ms»s  the  central 
cavity  of  the  utalk,  accouapauying  pr  i   u,_-^Lio.iii5  of  thi-  cavities 


^ 


660 


INVERTEBRATE  MORPUOLOOY. 


of  the  chambered  or^au.  The  terminal  branches  of  the  radial 
aboral  nerves  pass  to  the  integument  of  the  oral  surfaces  of 
the  arms  and  to  the  muscles  which  unite  the  various  plates, 
so  that  the  system  governs  and  coordinates  the  movements  of 
the  arms  and  pinnules  as  well  as  of  the  cirri.  The  e})ithelial 
system,  on  the  other  hand,  controls  the  movements  of  the  am- 
bulacral  and  oral  tentacles,  stimulation  of  it  causing  move- 
nnmt  of  these  structures  in  the  immediate  vicinity  of  the 
region  to  which  the  stimulus  is  a})plied. 

Another  system  of  nerv(^-tibres,  consisting  of  a  periocsoph- 
ageal  ring  which  sends  ott'  two  branches  to  each  arnj,  one 
lying  on  each  side  of  each  of  the  ambulacral  grooves,  and 
which  is  connected  with  nerve-fibres  passing  from  the  dorsal 
organ,  has  been  described  as  occurring,  but  its  significance 
has  not  yet  been  satisfactorily  determined.  No  special  sense- 
organs  occur  in  the  Crinoids. 

The  reproductive  organs  are  developed  for  the  most  ])art 
in  the  pinnules,  occasionally  a  slight  development  of  them 
appearing  in  the  arms  or  even  in  the  body  proper  ;  in  Ilolnptis 
alone  tliey  are  confined  to  the  arms.  They  consist  of  tubes 
lined  with  germinal  epithelium  on  their  inner  surfaces  and 
enclosed  within  a  ])rolongation  of  the  C(elom.  They  lie  be- 
tween the  two  c(el(>mi(!  prolongations  of  the  arms  already 
m«3nti()ned,  and  though  tlu;  reproductive  organs  are  developed 
only  in  the  pinnules  as  a  rule,  nevertheless  each  genital  tulu^ 
or  rachis  (Fig.  251,  <//•)  can  be  traied  through  the  arm  to  the 
body,  where  it  terminates  in  connection  with  the  dorsal  organ. 
In  their  development  indeed  they  grow  out  from  this  organ, 
and  it  seems  probable  that  the  (»va  an<l  spermatozoa  mothcM- 
cells  migrate  out  from  it  along  the  lachides  to  I'each  maturity 
in  the  pinnules.  Comparing  this  with  the  condition  in  other 
Echinoderma,  it  seems  clear  that  the  so-call»  d  dorsal  oigan 
of  the  Crinoids  is  homologous  with  the  ovoid  gland  of  the 
other  foi-ms.  The  re))roductive  eh^ments  j)ass  to  the  exterior 
by  one  or  two  ducts  connected  with  each  reproductive  mass; 
the  origin  of  these  ducts  is  unknown. 

The  CriiKtids  socin  to  liiiv*'  bocii  closely  relatod  to  two  jjroups  of  forms 
known  only  us  fossils.  These  were  the  Ci/stoids,  whieli  appt'ur  in  the  Lower 
Silurian  rocks  and  <lie  out  in  the  Carbt»niferou.s,  and  iho  Jilasluidn,  which 


TYPE  ECIIINODEliMA. 


551 


nppear  in  tlio  Upper  !?iluriaii  and  also  disappoar  in  Ihc  Carboniferous. 
For  a  description  of  tliese  forms  referenc*'  nuist  be  made  to  (he  standard 
worivs on  l'aiiuonioloj,'.v.  On  aeconnt  of  tiieir  similarity  to  tlie  Crinoids  they 
have  been  associated  witii  them  in  the  ehiss  /'tliint/oZiKi,  of  which  each  of 
the  groups  formed  an  (tnU'r;  inasiiiiieii,  iiowever,  as  the  present  woriv  is 
concerned  only  with  recent  fornis,  it  has  b»'en  thought  more  convenient  to 
regard  the  Crinoids  as  a  class. 

The  class  Crinoidea  has  been  divided  into  two  orders.  The  Pahrnori- 
iiida,  chiefly  I'ahi'o/.oit-  forms,  characterized  principally  l»y  (he  presence  of 
under-basals  ami  of  a  .series  of  plates  covering  in  the  di.sk  almo.st  com- 
pletely, to  which  may  be  added  (he  usmil  presence  of  interradials  and  tho 
greater  width  of  one  of  the  interradii,  that  in  which  the  aims  occurred 
(see  Fig.  2.")0).  The  Xtitrrinhla,  on  the  other  hand,  included  the  recent 
forms,  the  group  making  its  first  appearance  in  tho  Me.sozoic,  and  is  char- 
acterized by  (h(!  disk  being  only  imperfectly  covered  by  plates,  by  tho 
under-basals  and  interradials  being  absent,  and  the  interradii  all  equal  in 
width.  Transition  forms  between  the  two  groups  occur,  however,  tho 
genera  ////i/<7'/////.v  and  C'dlaiiKwriniis,  for  example,  presenting  certain 
l*ala'o(!rinid  peculiarities  coml)ined  with  Neocrinid  ones,  and  it  seems  more 
satisfactory  to  divide  tho  cla.ss  into  families  only,  leaving  orders  out  of  tho 
question. 

Dere/opiiieHt  of  the  Ciinnhls. — AntedoH  is  tht!  oidy  Crinoi<l  who.se 
development  has  been  studied.  The  eml)ryo  leads 
for  a  time  a  free-swinnning  (i.xistence,  and  possesses 
a  somewhat  ovoidal  form  (Fig.  'ioit)  with  a  tuft  of 
cilia  at  the  smaller  antt^rior  end  and  five  rings  of 
cilia  surrounding  the  body.  Not  far  from  the  autc;- 
rior  end  is  a  sliglit  groove,  and  lower  down  upon  the  ^ 
side  is  a  much  larger  one.     Tliis  larva  .settles  down 


in 


^A^' 


'^ 


002 


IN  VEliTEUUAlK  MOIWUOLOU  Y. 


II.  Class  Asteboidea. 

Tho  Ast»»n)itlpii,  or  StarHslies,  ure  jiU  Huttened  forms,  at 
no  period  of  tlioir  livos  iittufhod  by  a  stulk,  but  crci^piiig 
about  frtM'lv  upon  the  oral  Hurfuce.  In  some  forms  i\\o  body 
is  a  tlatten«Ml  disk  ])euta<^oual  in  outline  (Afiftrinu),  but  more 
usually  (Fij<.  '24(i)  the  five  ladii  are  proU)U«5ed  out  into  tive 
stout  unbranehed  arms,  and  in  some  forms,  such  as  Urisimju, 
the  arms  nniy  be  lon^  and  sleuder  and  more  than  tive  in 
number.  The  mouth  is  situated  in  the  centre  of  the  oral 
surface,  and  the  anus  sli«^htly  excentrically  upon  the  aboral 
surface,  while  the  hvdroco'I  system  of  tubes  is  coutiuetl,  as 
in  the  Crinoids,  to  the  oral  surface  of  tho  body,  except  that 
the  nnidreporiform  tubercle  bv  which  tho  system  commuui- 
cates  with  the  exterior  is  upon  the  aboral  surface  in  the  iu- 
terradius  CI). 

The  ectoderm  is  throuf»liout  ciliated,  and  contains  usually 
numerous  mucous  glands,  while  in  its  lower  buers  j^anj^lion- 
cells  and  uerye-fibrils  form  a  plexus  exteudinj^  over  the  entire 
surface  of  the  body. 

Calcareous  matter  is  deposited  in  the  connective  tissue, 
but  in  the  majority  of  forms  the  ])rimitiye  apical  plates  are 
not  reco«^nizable  in  the  adult;  more  usually  the  aboral  sur- 
face is  covered  by  a  lar}j;e  number  of  small  plates  arranj^ed 
without  any  roj^ularity,  or  else  the  calcareous  matter  forms 
a  reticulum  composed  of  numerous  fused  burs,  short  spiues 
risinjjj  frecpiently  from  the  points  of  union,  lu  embryos, 
however,  and  in  some  adult  forms,  such  as  Zoroasler  (I'Mj^. 
247),  the  apical  system  can  readily  be  made  out,  and  con- 
sists of  a  centrodorsal  plate  (Fig.  247,  C/h,  sometiuies 
<jfrooyed  upon  the  ed^(^  for  the  anus  (an),  surrounde*!  by  tive 
under-basals  (2)  usually  small,  alternating'  with  which  are 
five  basals  (3).  At  the  base  of  each  arm  is  a  radial  (4),  and 
in  end)ry<is  beyond  this  there  is  in  each  radius  another  plate, 
which  as  jj;rowth  takes  place  is  carried  further  and  further 
from  the  radial  and  tinally  forms  the  terminal  plate  (7^  of  the 
arm,  by  which  name  it  is  known. 

Of  the  oral  system  the  orals  are  possibly  represented  by 


■la 


TYl'H  h'(  l/iyoDKli.VA. 


C>r)3 


the  so-culleil  oiloiitoplioic  plutcs,  wliicli  are  ^ouor.'illy  hiuhII 
hikI  iu  nuiiiy  f.ises  rovi^i'd  over  l>y  otiitr  phitfs  (»f  tlir  oiul 
surface,  ami  li(;  in  tlic  iiutiie(liiiti>  iiei<4lilM)rli<»Ml  of  tlio 
iiumtli.  At  tlie  junction  of  tin*  oral  and  al»oral  surfac»'s  of 
the  disii  and  arms  two  series  of  plates  are  frequently  to  he 
found  which  from  their  position  are  ternietl  the  supra  and 
iii/ra-ini(njui<tls,  uud,  iu  aihlition  to  tliese,  s»»ries  of  platrs 
witli  dtilinite  arrangement  are  ileveh)ped  iu  connection  with 
the  water  vascuhir  system.  Thus  aion^  each  side  of  th»'  niid- 
dhi  line  of  each  arm  is  a  series  of  plates,  wliich,  sloping' 
aborully  ami  towards  the  axis  of  the  arm,  nu^et  to  form  the 
tioor  of  an  amhulacral  {groove  which  extends  outwards  from 
the  mouth  to  tin;  extremity  of  the  arm.  Tiiese  are  tim  <ini- 
bnlacral  plates  (Fi^.  2o4,  A),  and  each  series  of  them  is  flanked 
upon  tlie  outer  side  by  a  row  of  adamhnUtrrals  (Fij^.  254,  />) 
whose  uurnber  may  or  may  not  correspontl  with  that  of  the 
and>ulacra]s.  Between  the  adambulacral  series  of  adjacent 
arms  a  series  of  plates  may  be  interposed  upon  the  oral  sur- 
face of  the  disk,  ami  to  these  the  name  of  iuteranihiducrals 
may  be  applied. 

Spines  are  very  freipiently  borne  by  the  j)lates  or  reticu- 
lum of  the  aboial  surface,  but  are  usually  low  and  immova- 
ble, though  iipon  the  marginal  and  adambulacral  plates  they 
are  very  fretpiently  lon<^(>r,  united  to  the  plates  by  a  rudi- 
mentary articular  surfac«i  and  suppliiul  with  muscl('-Hi)res  i)y 
which  they  can  be  moved.  In  adilition  to  these  appendages 
of  the  dermal  skeleton,  others  are  to  bo  found  in  the  Star- 
tislu's,  su(!h,  for  example,  as  tlu*  ciliated  spines  found  in  a 
few  forms,  such  as  Lnidia,  upon  tliv!  marginal  platcis.  These 
spines  arc^  small  and  delicate,  and  grouped  toj^ether,  their 
principal  peculiarity  bein^  that  they  are  cov<ired  by  an 
epithelium  of  hi<rh  columnar  cells  wliich  bear  stron<^'  (rilia.  In 
most  Starrtsh  also  peculiai'  structures  ternnMl  prflirclhtnd'  are 
develoj)ed  iu  connection  with  the  skeh^tou,  but  their  descrip- 
tion may  be  deferred  until  the  Echinoids  are  under  discus- 
sion, in  which  ^roup  they  reach  a  hi^h  ^raile  of  develop 
ment.  Peculiar  to  certain  genera  of  Starftslu.'S,  e.jj;.  I.niilia, 
are  the  /uaxilla',  found  principally  upon  the  aboral  surfac*;  of 
the   body.     They  cousist  of  small  columns  of  carbonate  of 


pp 


^, 


IMAGE  EVALUATION 
TEST  TARGET  (MT-S) 


1.0 


Li  12.8 


lU 

u 
muu 


IM 


IL25  1111.4 


2.5 
2.2 

2.0 


1.6 


Photographic 

Sciences 

Corporation 


<v 


■<( 


•SS 


\ 


<^ 


33  WIST  MAIN  STRUT 

WnSTIR.N.Y.  t4SM 

(716)  t72-4S03 


^ 

^ 


it't  i 


554 


INVEltTEBRATE  MORPHOLOGY. 


lime  imbedded  iu  the  connective  tissue,  and  bear  upon  their 
free  extremity  a  number  of  radiating  spines,  which  vary  in 
the  amount  of  movement  of  which  they  are  capable  in  differ- 
ent species.  The  paxillae  are  frequently  found  in  groups 
around  the  dermal  branchiae,  over  which  the  spines  may  be 
bent  so  as  to  serve  for  protection. 

These  dermal  branchiae  (Fig.  254,  h)  are  pouchlike  evagi- 
nations  of  the  coelomic  cavity  with  thin  walls  composed  of 
ectoderm  and  a  layer  of  ciliated  cells  continuous  with  the 
peritoneal  lining  of  the  coelom.  between  these  two  layers 
there  being  but  a  slight  development  of  connective  tissue  and 


an 


Fig.  254.— Transverse  Section  of  Arm  of  a  Starfish  (inodifled  from 


A  =  ambulacral  plate. 
am  =  ampulla. 
an  =  aboml  uerve. 
B  =  adumbulacral  plate. 
b  =  branchia. 
c  —  digestive'  caecum. 
ec  =  ectoderm. 


LuDWia). 

I  =  schizocoelic  sinus. 
mn  =  muscular  nervous  system. 
JV  =  epithelial  nervous  system. 
o  =  ovum. 

p  =  peritoneal  epithelium. 
pi  =  calcareous  plate. 
rh  =  radial  by drococl- vessel. 


tf  =  tube-foot. 

circular  and  longitudinal  muscle-fibres.  These  pouches  are 
scattered  plentifully  over  the  aboral  surface,  and  in  some 
forms  occur  upon  the  oral  surface  also.  Their  thin  walls  and 
the  extent  of  surface  they  collectively  represent  leave  little 
room  for  doubt  but  that  they  possess  respiratory  functions, 
though  they  may  also  serve  indirectly  in  excretion,  since  it  has 
been  asserted  that  the  amoeboid  cells  of  the  coelomic  hsemo- 


ifm: 


IS! 


TYPE  ECHINODEKMA. 


655 


lymph,  laden  with  excretory  particles,  migrate  to  the  exterior 
through  the  walls  of  the  branchiae. 

The  calcareous  plates  are  imbedded  in  a  connective  tissue 
usually  of  considerable  thickness  and  sometimes  of  a  high 
degree  of  consistency.  Upon  its  inner  side  are  to  be  found 
circular  and  longitudinal  bands  of  muscles,  especially  devel- 
oped in  the  arms,  which  are  capable  of  considerable  move- 
ment. 

The  general  ccelom  is  traversed  by  mesenteries  extending 
from  the  body-wall  to  the  digestive  tract,  which  do  not  require, 
however,  a  detailed  description.  Suffice  it  to  say  that  each 
radial  csecum  of  the  digestive  tract  which  extends  out  into  the 
arm  is  suspended  by  two  longitudinal  mesenteries  (Fig.  254) 
which,  with  the  body-wall  and  caecum,  enclose  a  canal  open- 
ing proximally  into  the  general  coelom.  Extending  vertically 
through  the  caelom  in  the  interradius  CD  is  a  cavity  with 
strong  walls  which  is  in  communication  with  the  exterior  by 
the  madreporiform  tubercle  and  is  the  axial  sinus.  This  is  a 
portion  of  the  coelom  which  is  early  separated  from  the  rest, 
and  in  the  embryo  opens  to  the  exterior  by  the  water-pore, 
the  stone-caual  opening  into  it.  In  the  adult  the  cavity  of  the 
sinus  is  fairly  spacious  and  contains  the  ovoid  gland  sus- 
pended to  its  walls  by  a  mesentery.  Prolongations  of  the 
sinus  accompany  the  genital  rachis  and,  enclosing  the  repro- 
ductive organs,  form  the  genital  sinuses. 

The  schizocoelic  system  consists  of  an  oral  ring  lying 
between  the  nervous  and  water  vascular  rings,  and  of  five 
radial  vessels  which  pass  out  from  this  along  tlie  axes  of  the 
arms.  The  oral  ring  is  divided  by  an  oblique  septum  into  two 
portions,  one  of  which  lies  upon  the  aboral  surface  of  tlie 
other,  and  enters  into  connection  with  the  axial  sinus,  the 
ovoid  gland  abutting  upon  the  septum  in  one  of  the  interrudii. 
The  oral  sinus  also  communicates  with  the  cijelom  by  five 
interradial  orifices.  The  radial  sinuses  are  divided  into  two 
cavities  by  a  median  longitudinal  septum,  and,  like  the  oral 
sinus,  communicate  with  the  ccelom.  A  lacunar  system,  such 
as  occurs  in  the  Crinoids,  is  not  developed  in  the  Asteroidea, 
though  certain  spaces  in  the  wall  of  the  ovoid  gland  and  its 
prolongations  are  perhaps  representatives  of  it.     The  ovoid 


556 


INVERTEBRATE  MORPIIOLOGT. 


I  I      '^l 


gland  is  formed  of  loose  connective-tissue  trabeculfe,  which 
are  covered  by  cells  frequently  found  in  active  division,  and 
are  supposed  to  become  the  amoeboid  corpuscles  of  the  coelom 
and  blood  system. 

The  hydrocoel  consists  as  usual  of  an  oral  ring  and  five 
radial  canals,  the  latter  lying  at  the  bottom  of  the  arabulacral 
grooves  and  therefore  external  to  the  ambulacral  plates  (Fig. 
254,  rh).  Between  each  pair  of  plates  a  branch  passes  upwards 
(i.e.,  aborally),  and  dilates  into  a  globular  sac,  the  ampulla 
(Figs.  254  and  255,  a/w),  which  is  occasionally  double,  and  from 
this  a  cylindrical  tentcclelike  process  passes  outwards  again 
betAveen  two  plates  forming  extensible  processes  (Fig.  254,  th) 
equivalent  to  the  tentacles  of  the  Criuoids.  These  processes 
in  some  of  the  more  primitive  forms,  such  as  Luidia  and 
Asfropecten,  and  the  terminal  ones  at  the  extremity  of  the  arms 
of  all  forms,  are  conical  in  shape,  but  more  usually  the  great 
majority  of  them  are  provided  at  their  extremities  with  suck- 
ing disks,  whereby  they  can  adhere  to  foreign  bodies  and  serve 
thus  as  locomotor  organs.  Hence  they  are  known  as  the 
tube-feet  or  ambulacra.  In  some  forms  they  are  arranged  in 
two  rows,  one  on  each  side  of  the  axis  of  the  arm,  but  in 
others,  as  for  example  the  common  Starfish  Asteria^,  the  suc- 
cessive feet  of  each  row  alternate  with  each  other,  so  that  they 
have  the  appearance  of  being  arranged  in  four  rows.  By 
means  of  the  muscles  of  the  wall  of  the  ampullae  water  can  be 
forced  into  the  tube-feet,  which  may  be  thus  extended,  a 
circular  valve  occurring  in  the  branch  which  passes  from  the 
radial  canal  to  the  ampulla  preventing  the  water  from  passing 
back  into  the  canal.  Contrary  to  what  occurs  in  the  Crinoids, 
there  are  several  appendages  to  the  oral  ring,  in  addition  to 
the  stone-canal.  This  leaves  the  ring  in  the  iuterradius  CD 
and,  passing  aborally,  communicates  with  the  axial  sinus  which, 
as  already  stated,  opens  to  the  exterior  by  the  madreporite. 
This  is  a  complicated  calcareous  sieve-plate  of  some  thickness, 
and  the  union  of  the  canal  and  the  sinus  takes  place  within 
its  substance,  so  that  in  reality  the  canal  seems  to  open  to 
the  exterior.  The  embryonic  history,  and  the  fact  that  injec- 
tions forced  through  the  tubercle  pass  into  both  the  sinus  and 
the  canal,  show  that  what  has  been  described  is  the  true 


\ 


TYPE  ECHINODERMA. 


mi 


relationship.  In  the  walls  of  the  stone-canal  calcareous 
matter  is  deposited,  whence  the  name  applied  to  it,  and  its 
walls  form  projections  extending;  into  the  lumen  of  the  canal, 
the  surface  for  the  ciliated  epithelium  being  thus  increased. 

The  appendages  of  the  oral  ring  are  of  two  kinds,  both 
being  situated  in  the  interradii,  that  containing  the  stone-canal, 
however,  usually  lacking  any  other  appendage.  In  some 
forms  hollow  saclike  structures  open  into  the  ring  by  a 
narrow  neck,  and  are  termed  the  Polian  vesicles ;  their  walls 
consist  of  connective  tissue  in  which  are  situated  muscle- 
fibres,  and  their  interior  is  lined  by  an  epithelium  which 
appears  to  separate  and  give  rise  to  the  amoeboid  cells  of  the 
hydrocoel  fluid.  The  other  kind  of  appendages  occur  generally 
throughout  the  group  and  are  known  as  Tiedeman's  vesicles, 
consisting  of  masses  of  hollow  tubes  arranged  in  pairs  in  one 
or  more  of  the  interradii.  The  epithelium  lining  the  walls  of 
these  structures  also  seems  to  give  rise  to  the  amoeboid  cells, 
both  kinds  of  organs  being  therefore  comparable  to  lymphatic 
glands,  though  the  Polian  vesicles  have  also  been  regarded  as 
reservoirs  for  the  hydrocoel  fluid. 

The  mouth  is  situated  at  the  centre  of  the  oral  surface  of 
the  disk,  and  opens  into  a  short  oesophagus  whicli,  in  some 
forms,  has  connected  with  it  ten  glandular  pouches.  The 
oesophagus  opens  into  a  usually  capacious  cardiac  stomach 
which  is  frequently  lobed  (Fig.  255,  c),  is  eversible  and  pro- 
vided with  special  muscles  for  its  retraction.  Above  this 
comes  the  pyloric  stomach  which  gives  rise  to  five  radial 
pouches,  which  soon  branch  into  a  pair  of  sacculated  pouches 
extending  out  into  the  arms,  and  being  termed  the  radial 
csBca  (t).  From  the  pyloric  stomach  a  short  rectum  passes 
aborally,  interradial  croca  being  sometimes  found  close  to  its 
origin  from  the  pyloric  stomach,  and  opens  upon  the  dorsal 
surface.  In  a  few  forms,  such  as  Luidia,  Astropecten,  and  their 
allies,  the  anus  is  wanting,  but  more  usually  it  is  present  in 
the  region  indicated. 

The  epithelial  nervous  system  consists  of  a  plexus  of 
ganglion-cells  and  fibres  imbedded  in  the  ectoderm  and  cover- 
ing the  surface  of  the  body,  and  of  an  oral  ring  and  five  radial 
nerves  (Fig.  254,  JV)  which,  as  in  the  Crinoids,  are  situated  in 


fi 


658 


IN  VEHTEBRA  TE  MORPHOLOG  Y. 


the  lower  layers  of  the  ectoderm.  Upon  the  aboral  surface 
of  the  oral  riug  aud  the  radial  nerves  sections  show  distinct 
bauds  of  fibres  separated  from  the  riug  and  nerves  by  a 
delicate  layer  of  connective  tissue  ;  these  constitute  the  mus- 
cular system  of  xierves  (Fig.  254,  ?/m),  and  their  branches 
appear  to  be  supplied  to  the  muscles  of  the  body- wall  and  of 
the  ampull*  and  tube-feet.  The  aboral  system  is  but  feebly 
developed  when  compared  with  that  of  the  Crinoids.  A  trans- 
verse section  of  an  arm  shows  lying  between  the  muscles  of 
the  aboral  surface  and  the  peritoneal   mesoderm  a  cord  of 


Pig.  355.— a  Starfish,  Aster acanthion,  with  the  integument  of  the  disk 

AND  RAYS  REMOVED  TO  SHOW  THE  INTERNAL  STRUCTURE. 

a  =  anus.  g  =  reproductive  orgau. 

am  =  ampullae  of  tube-feet.  I  =  liver  caeca. 

ao  =  ambulacral  ossicles.  M  =  madreporite. 

c  =  cardiac  poucb  of  stomach.  A-E  =  the  five  radii. 

nerve-fibres  (Fig.  254,  an),  the  five  cords  converging  towards 
the  centre  of  the  aboral  surface  of  the  body,  the  entire  system 
forming  thus  a  five-rayed  star.  In  position  and  general  rela- 
tions this  system  of  nerve-cords  is  directly  comparable  to  the 
aboral  system  of  the  Crinoids,  and  may  be  regarded  as 
homologous  with  it. 

Special  sense-organs  are  represented  by  the  terminal  ten- 
tacles of  the  radial  hydrocoel  canals,  which,  as  already  stated, 
retain  a  tentacle-like  form  and  do  not  develop  suckers  at  the 
extremity.     Their  walls  are  richly  supplied  with  nerves,  and 


TYPE  ECHINODERMA. 


559 


they  are  surrounded  and  may  be  covered  in  by  the  movable 
spines  of  the  adambulacral  and  marginal  plates.  That  they 
have  a  sensory  function  seems  clear,  but  what  the  exact 
nature  of  the  function  may  be  is  as  yet  uncertain.  At  the 
base  of  the  terminal  tentacle  of  each  arm  is  situated  an  eye, 
consisting  of  a  large  number  of  conical  depressions,  lined  by 
an  epithelium  containing  a  red  pigment,  covered  on  the  out- 
side by  a  cuticle  and  richly  supplied  with  nerve-filrils.  There 
do  not  seem  to  be  present  any  refractive  structures  other  than 
the  cuticle,  and  these  eyes  can  oul}-  convey  to  the  animal  im- 
pressions of  changes  in  the  intensity  of  the  light  falling  upon 
them  ;  they  cannot  form  images  of  external  objects. 

The  reproductive  organs  are  ten  in  number,  two  being 
situated  in  each  arm  (Fig.  255,  g).  Each  consists  of  a  mass 
of  reproductive  cells,  and  is  enclosed  in  a  genital  sinus  (Fig. 
254,  I),  which,  as  already  stated,  communicates  with  the  axial 
sinus.  The  proximal  end  of  each  gland  is  connected  with  a 
cordlike  structure,  the  genital  cord  or  rachis,  the  ten  cords 
uniting  in  a  ring  situated  beneath  the  aboral  surface  of  the 
body,  a  cord  passing  orally  from  this  ring  to  unite  with  the 
tissue  of  the  ovoid  gland.  The  genital  sinuses  accompany  the 
cords,  enclose  the  ring,  and  pass  to  the  axial  sinus  along  with 
the  descending  cord.  A  connection  therefore  exists  between 
the  reproductive  organs  and  the  ovoid  gland,  just  as  in  the 
Crinoids,  and  indeed  the  reproductive  organs  arise  in  the 
embryo  from  an  outgrowth  of  the  ovoid  gland.  In  reality  the 
genital  cords  aro  tubes  containing  in  their  interior  immature 
reproductive  cells  which  seem  to  migrate  from  the  ovoid  gland 
to  the  reproductive  organs  where  they  become  mature.  The 
reproductive  openings  are  usually  placed  upon  the  aboral 
surface  {Asierina  forming  an  exception)  of  the  arms  or  disk  in 
the  interradii  ;  a  single  pore  usually  exists  for  each  gland, 
and  occasionally  there  may  be  several. 

Development  of  the  Asteroidea.—T!he  larval  forms  of  the  Starfislies  are 
known  as  the  Bipinnaria  and  Braehiolaria.  The  former  has  a  somewhat 
triangular  shape,  the  apex  of  the  triangle  being  the  anterior  extremity, 
and  in  the  middle  of  the  ventral  surface  is  a  deep  concavity  in  which  the 
mouth  opens.  The  posterior  border  of  the  concavity  is  formed  by  a  band 
of  cilia  which  is  continued  around  the  lobed  sides  of  the  body  to  the  an- 


N 


660 


IN  VERTEBRA  TE  MOltPIIOLOG  Y. 


% 


terior  extremity,  forruing  a  postoral  ciliated  band,  the  anus  lying  without 
the  area  enclosed  by  it.  In  front  of  the  mouth  is  a  trilobed  region  also 
surrounded  by  a  band  of  cilia,  the  adoral  band.  -In  young  embryos  the 
adoral  and  postoral  bands  are  united  at  the  apex,  separation  only  super- 
vening later.  In  later  stages  two  additional  arms  jire  developed  at  the 
sides  of  the  apical  lobe,  wliich  becomes  like  the  new  arms  destitute  of  cilia, 
and  tipped  with  a  group  of  wartlike  elevations.  This  form  of  the  larva  is 
kno'.vn  as  tlie  Brachiolaria. 

A  peculiar  process,  amounting  almost  to  a  metamorphosis,  occurs  during 
the  transformation  of  the  larva  into  the  Starfish.  Calcareous  plates  of  tlie 
aboral  system  make  their  appearance  on  the  dorsp.I  surface  of  the  stomach 


Fig.  256.— Bipinnarta  op  AsteraeantJiion  (after  agassiz). 
an  =  anus.  fiy  =  liydrocoel.  m  =  mouth. 

near  the  posterior  end  of  the  body,  and  oral  plates  on  the  ventrjil  surface 
of  the  same  organ.  These  two  systems,  at  first  rather  widely  separated, 
gradually  approach  each  other,  and  at  the  same  time  the  internal  organs 
assume  the  adult  form.  Finally  the  two  series  of  plates  unite,  enclosing 
between  them  the  hydrocoel,  a  portion  of  the  digestive  tract  and  of  tlie  coe- 
lom.  The  original  mouth  and  anus  are  obliterated,  and  indeed  the  anter- 
ior half  of  the  larva  takes  no  part  in  the  formation  of  the  adult  animal, 
but  is  gradually  absorbed. 

A  highly-developed  faculty  for  regeneration  occurs  in  the  Asteroidca, 
the  disk  being  able  to  regenerate  lost  arms ;  and  indeed  an  arm,  with  which 
a  small  fragment  of  the  disk  is  in  connection,  has  the  power  of  regenerat- 
ing all  the  missing  parts.  Specimens  of  tlie  common  Starfish  Asterias  are 
in  consequence  frequently  found  with  one  or  more  of  the  arms  bifid  at  the 
tip,  or  even  with  an  abnormal  number  of  arms. 


1^ 


TYPE  ECIIINODERMA. 


501 


III.  Glass  Ophiuboioea. 

The  Ophiuroidea,  or  Brittle-stars,  resemble  the  starfishes 
closely  in  their  general  form,  consisting  of  a  central  disk  from 
which  five  arms  radiate  (Fig.  257).  The  arms,  however,  are 
in  all  cases  slender  and  distinctly  marked  off  from  the  disk, 
and  in  Astrophyfon  branch  dichotomously.  Closer  examina- 
tion reveals,  however,  considerable  differences  from  the  Star- 
fishes ;  there  is  no  anus,  the  madreporite  is  on  the  oral  sur- 


FiG.  257. — OpMoglypha  aculeata  from  the  ABORAii  Suufack  to  show  thk 
Peksistent  Apical  System  of  Plates. 

(The  arms  are  cut  off  close  to  the  disk). 
1  =  ceutrodorsal  plate.  3  =  basals. 

3  =  under  basals.  4  =  radials. 

face,  there  are  no  visible  ambulacral  grooves  on  the  arms, 
which  are  more  or  less  circular  in  section  and  do  not  con- 
tain caBcal  processes  of  the  digestive  tract.  Furtliermore,  on 
each  side  of  each  of  the  five  radii  there  is  upon  the  edge  of 
the  oral  surface  of  the  disk  a  slitlike  opening,  divided  into 
two  parts  in  Ophioderma,  and  leading  into  a  thin-walled  cili- 
ated sac,  which  is  to  be  regarded  as  an  invagination  of  the 
wall  of  the  body.  There  are  thus  ten  of  these  genital  bursoi 
as  they  are  termed,  two  being  situated  in  each  interradius. 
They  seem  to  have  a  respiratory  function,  and  serve  also  for 
the  exit  of  the  reproductive  elements,  in  some  forms,  e.g.  Am- 


662 


INVEHTEBHATE  MORPHOLOGY. 


phium  sqitamata,  even  serving  as  brood-poucLes  iu  which  the 
youn^  develop. 

The  ectoderm  is  indistinguishable  over  the  greater  portion 
of  the  body  iu  the  adults,  becoming,  as  in  the  Criuoids,  con- 
founded Avith  the  mesoderm.  Calcareous  plates  are  largely 
developed  in  this  tissue  (except  in  Ophiomyxa  and  its  allies), 
giving  to  the  disk  and  arms  a  brittleness  which  has  suggested 
the  popular  name  for  the  group.  The  extent  to  which  the 
apical  system  of  plates  is  distinguishable  in  the  adults  varies 
considerably  even  in  members  of  the  same  group,  and  while 
in  some  forms  (Fig.  257)  all  the  plates  represented  in  the 
IStariish  Zoroader  can  be  distinguished,  in  others  only  the 
radials  or  the  basals  or  both  are  visible.  At  the  tip  of  each 
arm  is  a  plate  comparable  to  the  terminal  of  the  Aste- 
roidea,  and  in  addition  there  are  frequently  present  series  of 
interradials  or  interbrachials,  the  most  aboral  plates  of 
which  separate  the  radials  from  each  other  and  extend  round 
to  the  oral  surface,  abutting  on  five  large  plates  known  as  the 
buccal  shields  and  corresponding  to  the  orals  of  other  forms. 
On  the  aboral  surface  of  the  disk  above  the  origin  of  each  arm 
there  is  a  pair  of  plates  termed  the  radial  shields,  which 
must  not,  however,  be  confused  with  the  radial  plates  extend- 
ing along  the  aboral  surfaces  of  the  arms. 

These  latter  form  a  complete  series  extending  from  the 
disk  to  the  terminal  plates,  and  form  the  aboral  wall  of  the 
arms,  their  lateral  walls  being  formed  by  another  series  of 
plates,  the  adambulacrals  (Fig.  258,  Ad),  while  still  another 
series,  the  superamhidacrals,  form  their  oral  walls.  Between 
each  adambulacral  plate  and  its  successor  is  a  pore  (usually 
bounded  by  a  number  of  small  plates)  through  which  the 
tube-feet  are  protruded,  the  radial  water-vascular  canals  being 
situated  in  the  interior  of  the  arm.  The  cavity  of  the  arms 
is  occupied  almost  entirely  by  a  linear  series  of  calcareous 
masses  termed  the  vertebral  or  ambidacral  ossicles  (Figs.  258 
and  260,  A),  each  of  which  consists  of  two  halves,  usually 
firmly  united  by  suture.  The  ossicles  are  united  by  well-de- 
veloped articular  surfaces,  and  have  attached  to  them  muscles, 
whereby  a  considerable  amount  of  motion  is  possible  for  the 
arms  as  a  whole,  the  motion  being  almost  entirely  in  a  hori- 


TYPE  ECIIINODEltMA. 


r)03 


zoutul  plane,  except  in  Astrophijtnn  and  its  allies,  in  which 
the  arms  may  be  coiled  up  over  the  oral  surface,  in  a  manner 
similar  to  what  is  found  in  the  Crinoids.  These  ambulacral 
ossicles  seem  to  correspond  with  the  similarly-named  plates 
of  the  Asteroidea. 

In  the  neighborhood  of  the  mouth  certain  modiiieations 
in  the  arrangement  of  some  of  these  plates  occur.  The  two 
halves  of  each  first  ambulacral  ossicle  (Fig.  258,  A^  are  widely 
separated,  and  come  into  close  relatitm  with  the  similarly- 
separated  ossicles  of  adjacent  radii,  forming  a  buccal  shield. 
The  plate  so  formed  rests  upon  the  aboral  surface  of  the  first 
adambulacrals  {Ad^),  which  unite  in  pairs  in  a  similar  manner. 


Ad, 

fA.^! 

A. 

A.I4 

M, 

>. 

ky^ 

Ad, 

Fig.  258.— Diagram  to  snow  the   atirangement  op  the  ciKCUMonAL 

PLATES  OF  AN   OPHIUHAN  (after  LuDWiG). 

A  =  ambulacral  plates.  p  =  pala  angularis. 

Ad  =  adambulacrals.  T  =  torus. 

/  =  intenadial.  t  =  oral  tentacles. 

101'  =  radial  hydrocoel-vessel. 

forming  a  triangular  plate,  termed  an  oral  angle-piece,  lying  in 
an  interradius,  and  partly  covered  on  its  oral  surface  by  a 
buccal  shield.  At  the  sides  of  the  buccal  shield  are  the  so- 
called  lateral  buccal  shields  (Ad,),  which  are  in  reality  the 
second  adambulacrals  of  adjacent  arms,  and  cover  in  the 
second  arabulacrals  (A^),  which  serve  as  supports  for  the 
oral  angle-piece.  Along  the  margins  of  the  oral  surface  of 
this  are  a  series  of  spines,  the  bitecal  papillce,  while,  at  the  apex 
of  the  triangle,  are  the  dental  papillce.  The  vertical  edge  of 
the  piece  is  furnished  with  a  number  of  stout  projections,  the 


,J 


564 


IN VEltTKDHA  TE  MOIWIIOLOG  Y. 


I 


I! 


pnl(e  angulares  (Fij^.  260,  p\  whose  buses  geuerally  fuse  to 
form  a  supportiuj^  plate,  the  torm  aiujulariH  {T). 

Spiues  deveh)ped  iu  couuectioii  with  the  dermal  skeletou, 
leaving  out  of  coiisideiatiou  the  oral  augle-pieces,  may  be  en- 
tirely wanting,  but  iu  many  forms  they  are  borne  in  vertical 
rows  upou  the  adambulacrals,  and  are  usually  movable.  Iu 
a  few  forms,  especially  those  inhabiting  rocky  bottoms,  pe- 
culiar hooked  spiues  are  situated  on  the  oral  surface  of  the 
arms  towards  their  extremities,  and  seem  to  serve  au  adhe- 
sive function.  Pedicellariie  are  absent,  except  iu  Astrophyton 
and  allied  genera. 

The  coelom  (Fig.  260,  c)  is  of  comparatively  slight  extent, 
the  cavity  of  the  disk  beiug  largely  occupied  by  the  digestive 
tract,  and  that  of  the  arms  by  the  ambulacral  ossicles.  In  the 
disk  the  cavity  is  traversed  by  uumerous  bands  which  ex- 
tend from  the  body-wall  to  the  wall  of  the  dig-^stive  sac, 
and  from  the  wall  of  the  oesophagus  a  membrane  extends 
outwards  and  orally  to  be  attached  to  the  peribuccal  plates, 
forming  a  septum  (Fig.  260,  s),  enclosing  a  cavity  surround- 
ing the  cBsophagus,  the  peripharyngeal  space  (p.§),  which  is 
completely  separated  from  the  rest  of  the  coehim.  In  Ophio- 
thrix  and  some  other  forms  a  second  septum  occurs  parallel 
to  the  one  just  mentioned,  so  that  the  peripharyngeal  space 
is  double.  The  coelom  of  the  arms  consists  of  two  portions, 
one  lying  on  the  aboral  and  the  other  on  the  oral  side  of  the 
series  of  ambulacral  ossicles.  The  aboral  cavity  is  expanded 
laterally  so  as  to  partially  surround  the  ossicles,  but  this  lat- 
eral portion  is  traversed  opposite  each  ossicle  by  a  calcareous 
lamella,  and  is  thus  separated  into  series  of  chambers 
which  open  into  the  undivided  aboral  portion,  termed  the 
aboral  or  dorsal  canal.  An  axial  sinus,  standing  iu  close  re- 
lationship to  the  ovoid  gland,  exists,  but  presents  some  fea- 
tures not  found  in  the  Asteroidea.  It  consists  iu  Amphiura 
squamata  (Fig.  259)  of  three  distinct  portions  completely  sep- 
arated from  one  another  ;  one  of  these  is  the  so-called  am- 
pulla (am)  of  the  stone-canal ;  the  second  (s)  lies  in  close 
relation  to  the  ovoid  gland,  which  is  developed  on  its  axial 
wall ;  while  the  third  is  comparatively  small,  and  is  associated 
with  the  genital  cords  (fjrr),  and  the  mass  of  cells  in  the  ovoid 


TYPtS  ECIIINODEUMA. 


665 


11 


{{luud  from  which  these  arise.  Tliese  two  last  cavities  are 
said  to  be  portious  of  the  general  coilom  which  beconie  sepa- 
rated oil'  duriug  developiueut,  aud  are  not  simple  exteusions 
of  that  portion  of  the  cwlom  into  which  the  stone-canal  opens 
in  the  embryo,  and  which  persists  as  the  ampulla. 


1 


Kti^  359.       DiAGUAM   SHOWING    THE   RELATIONSHIPS    OF    THE    StoNE-CANAL, 

Axial  Sinus,  etc.,  in  Aniphiura  tiquamtda  uuifv  MacBridk). 

am  =  ampulla  of  stone-cant/i.  If  =  riiig-nerve. 

(J  --  geuital  bursa.  o  =  ovoid  gluud. 

gr  =  origin  of  genital  rachis.  pe  =  pore-canal. 

M   -  mouth.  ps  =  peripburyngeal  space. 

mp  —  madreporite.  «  =  sinus. 

mu  -"■  nmsclf.  ae  --  sloueoaiial. 

The.se  differences  from  what  occurs  in  the  Asteroidea  do  not  imply, 
however,  a  want  of  homology  between  tlie  axial  sinu.sos  of  the  two  groups. 
The  entire  sinus  is  after  all  a  separated  portion  of  the  cu'lom.  and  it 
makes  little  difference  whether  it  be  all  separated  off  at  an  early  stage 
in  the  development,  as  in  the  Asteroidea,  or  only  a  part  of  it,  the  njst  de- 
veloping later  from  the  general  coelomas  in  theOpliinrnidea  and  Criiioidea 
the  dorsal  organ  of  the  latter  being  homologous  with  the  axial  sinus 
minus  the  ampulla  of  the  stone-canal  of  the  Ophiuroidea  and  Asteroidea. 

Lyinj;  on  the  aboral  surface  of  each  radial  nerve-cord  is 
a  radial  schizoccelic  sinus  (Fig.  260,  hr),  which  communicates 
with  an  oral  sinus  surrounding  the  mouth.  The  relations  of 
this  system  are  similar  to  those  of  the  schizoccelic  system  of 
the  Asteroidea,  aud  num  rous  communications  between  it 
and  the  coelomic  cavities  occur.  It  contains,  however,  a 
system  of  canals,  which  correspond  to  the  lacuuse  occurring 


666 


INVERTEBRATE  MORPHOLOGY. 


ill  the  walls  of  the  CBSophagus  in  the  Crinoids.  They  have 
been  termed  blood-vessels  in  the  Oi3hiuroidea,  the  sinuses 
which  surround  them  being  termed  the  perihiLMual  canals; 
they  follow  the  course  of  these  latter,  a  process  of  the  ovoid 
gland  coming  into  connection  with  the  oral  lacunar  ring. 
This  gland  (Fig.  259,  o)  is,  as  in  other  groups,  partly  asso- 
ciated with  the  lacunar  system  and  partly  with  the  genital 
a[)paratus.  It  lies  in  the  wall  of  the  axial  sinus  and  projects 
into  it  so  as  almost  to  fill  it.  At  one  extremity,  as  stated,  it 
comes  into  connection  with  the  oral  lacunar  ring,  and  at  one 
point  in  its  wall  it  contains  a  mass  of  cells  from  which  the 
genital  cords  pass  out  to  the  reproductive  organs,  accom- 
panied by  strands  of  the  lacunar  tissue. 

The  hydroccel  has  the  usual  arrangement,  and  is  confined 
t)  the  oral  surface  of  the  disk  and  arms.  The  radial  canals 
(Fig.  260,  tor)  lie  on  the  oral  surface  of  the  ambulacral  ossi- 
cles, extending  to  the  terminal  plate,  and  ending,  at  least  in 
those  forms  which  have  simple  arms,  in  a  terminal  tentacle. 
At  regular  intervals,  corresponding  in  number  to  the  ambu- 
lacral ossicles,  the  radial  canals  give  off  transverse  branches, 
which  pass  outwards  in  the  substance  of  the  ossicles  (Fig.  258), 
and  make  tlieir  exit  through  the  ambulacral  pores  between 
successive  adanibulacral  plates  to  terminate  as  tube-feet.  No 
anipulhe  occur  on  these  transverse  branches,  though  a  circu- 
lar valve  occurs  just  Avliere  each  branch  becomes  continuous 
with  the  tube-foot.  The  feet  are  simple  conical  structures 
destitute  of  a  terminal  sucker,  and  do  not  therefore  serve 
for  locomotion.  Their  walls  are  richly  supplied  with  nerves, 
and  in  some  forms  are  provided  with  numerous  papilho  ap- 
l)areiitly  sensory  in  function.  Surrounding  the  mouth  are 
ten  buccal  tentacles  (Fig.  260,  ht)y  which  correspond  to  the 
first  two  pairs  of  tube-feet  of  each  radius  of  the  Asteroids, 
but  arise  by  fine  branches,  Avliich  later  divide,  and  are 
directly  connected  with  the  oral  ring- canal  (Fig.  258,  t). 
These  seem  to  be  undoubtedly  sensory  and  perhaps  olfactory 
in  function.  The  oral  ring-canal  usually  has  attached  to  it 
in  each  interradius,  except  that  in  which  the  stone-canal  lies, 
a  single  Poliau  vesicle  (Fig.  260,  PV),  though  in  Ophiactis 
two,  three,  or  even  four  vesicles  may  occur  in  each  interra- 


-iW^^ifc  ■*-*'»■ 


TTPt:  ECIIINODERMA. 


567 


-e 


ip. 
Ire 
Ihe 

lis, 
Lre 

0. 

it 


lis 
ra- 


dius. The  stone-caual,  as  has  beeu  uotetl,  opens  into  a  spe- 
cial portion  of  the  cceloni,  the  ampulhi  of  the  stone-canal, 
and  this  again  coiumunicates  with  the  exterior  bv  a  tube 
opening  by  a  pore  placed  in  the  adults  on  the  oral  surface 
of  the  botly  in  one  of  the  buccal  shields  ;  primitively  the 
opening  is  situated  on  the  aboral  surface,  only  later  migrat- 
ing to  its  tinal  position.  In  Astrophyton,  in  which  the  buccal 
shields  are  wanting,  the  madreporiform  tubercle  occurs  on 
the  oral  surface  of  the  disk  in  one  of  the  interradii,  and  in 
some  species  there  may  be  five  tubercles,  one  in  each  iuterra- 
dius,  a  multiplication  of  the  pores  and  stone-canals  occurring 
also  in  other  genera,  such  as  Amphiura  and  Ophiolepis. 

In  consequence  of  the  position  of  the  pore  or  tubercle  the  position  of 
the  stone-caual  in  the  Ophiuroidea  is  very  ditl'eront  from  that  which  it 
possesses  in  other  groups.  Wheresis  in  these  it  passes  aborally  from  the 
water  vascular  ring,  in  the  Brittle-stars  it  hangs  down  from  the  ring 
towards  the  oral  surface  of  the  body.  All  those  structures  too,  such  as 
the  axial  sinus  and  the  ovoid  gland,  which  are  usually  associated  with  the 
camd,  undergo  a  similar  transformation  of  position,  whicli  is  possil)!)'  on 
account  of  the  distance  from  the  oral  surface  at  which  the  water  vascular 
ring  is  situated  (sec  Fig  2r»S)). 

The  digestive  tract  is  very  simple.  The  mouth  guariled 
by  the  oral  angle-pieces  opens  into  a  short  ojsophagus  (i'ig. 
2(50,  ()),  which  communicates  with  a  capacious  saclike  stomach, 
slightly  ])ouched  out  in  each  radius,  but  not  extending  into 
the  cavities  of  the  arms.  There  is  no  anus  in  any  mend)er 
of  the  group. 

The  epithelial  nervous  system  of  the  Ophiuroidea  is  asso- 
ciated with  the  general  ectoderm  in  very  young  specimens, 
but  later  sinks  into  the  cavity  of  tlie  body  by  a  process 
which  may  be  compared  to  an  invagination.  Consequently 
a  tube  is  formed  lying  within  the  body-wall  on  the  oral  sur- 
face of  the  body,  the  radial  nerves  (Fig.  260,  nr)  being 
situated  in  its  aboral  wall.  This  tube  forms  the  epinviwai 
sinus,  and  the  cavity  it  encloses  seems  to  be  in  reality-  a  por- 
tion of  the  exterior,  though  it  may  be  schizocnelic.  The 
oral  ring  of  the  nervous  system  is  not  enclosed  in  an  epi- 
neural  canal,  but  remains  in  connection  with  the  ectoderm 
at  the  lower  extremity  of  the  cBsophagus,  being  pushed  thus 


11 


568 


IN  VERTEBRA  TE  MORPHOLOQ  Y. 


i 


*; 


aborally  by  the  development  of  the  oral  angle-pieces.  The 
radial  nerves  are,  however,  contained  in  the  wall  of  the  sinus, 
coming  to  the  surface  of  the  body  at  the  tips  of  the  arms, 
where  they  terminate  by  fusing  with  the  general  ectoderm. 
The  muscular  nervous  system  is,  as  in  the  Asteroidea,  closely 
associated  with  the  oral  ring  and  radial  nerves,  lying  on  their 
aboral  surface  and  separated  from  them  only  by  a  thin  layer 
of  connective  -Issue.  The  aboral  system  consists  of  a  ring 
situated  beneath  the  aboral  surface  of  the  body,  from  which 
branches  pass  off  towards  the  reproductive  organs.  Indeed 
the  entire  system  is  intimately  associated   with  the  genital 


Fig.   260.— Section   through   an    Ophiuuan  showing  Structuke  (after 

LUDWIG). 

A  —  umbulacral  ossicles.  0  =  moiitb. 

br  —  schizocoelic  siuus.  p  =  pala  angularis. 

bt  -  buccal  teutacles.  pa  =  peripbaryngeal  space. 

0  ~  ctKioin.  PV  =  Polian  vesicle. 
M  —  muscle.  S  =  peripbaryngeal  septum 

nr  —  radial  nerve.  T  =  torus  augularis. 

wr  —  bydrocoel- vessel. 

cords,  and  its  course  can  be  understood  from  a  description 
of  these  structures.  No  special  sense-organs  other  than  the 
terminal  and  buccal  tentacles  and  the  tube-feet,  already  de- 
scribed, occur  in  the  Ophiuroidea. 

As  already  stated,  the  genital  cord  arises  from  a  group  of 
cells  in  the  wall  of  the  ovoid  gland  (Fig.  259,  gr)  and  passes 
in  an  iuterradius  towards  the  aboral  surface  of  the  body, 
carrying  with  it  a  portion  of  the  axial  sinus.  Arrived  at  this 
point  the  sinus  and  cord  form  rings,  the  aboral  nerve-ring 
lying  in  the  wall  of  the  sinus,  Avhile  the  genital  cord  lies  in  its 
interior,  attached  to  its  wall  by  a  lamella  of  connective  tissue. 
From  the  genital-cord  ring  ten  short  branches  are  given  off 


TYPE  ECUINODERMA. 


569 


which  enlarge  into  ten  saclike  reproductive  organs,  lying 
in  close  contact  with  the  walls  of  the  genital  biirsje.  The 
ova  and  spermatozoa  migrate  from  their  point  of  origin  in 
the  ovoid  gland  along  the  genital  cords,  which  also  contain 
prolongations  of  the  lacunar  tissue  of  the  gland,  and  mature 
in  the  reproductive  pouches.  During  the  spawning-time  the 
lobes  of  the  reproductive  organs  protrude  into  the  bursje, 
pushing  before  them  the  thin  walls  of  these  pouches,  and  the 
reproductive  elements  when  mature  burst  through  into  the 
cavities  of  the  bursse,  whence  they  make  their  way  to  the 
exterior,  or  else,  as  in  Amphiura  squamata,  undergo  their  de- 
velopment in  the  pouches. 

From  what  has  been  said  it  may  be  seen  that  the  genus 
Astrophyton  and  its  allies  diifer  in  many  respects  from  the 


Fig,  261.— Pluteus  Larva  of  Echinarachnius  parma  (after  Fewkbs). 
a  =  oBSopliiigus.  m  =  raoutb. 

e  =  rudimeut  of  adult,  «  =  calcareous  skeleton. 

at  =  stomach. 

other  Ophiuroids,  having  arms  sometimes  branched  and 
capable  of  being  curled  in  over  the  oral  surface,  possessing 
pedicellarisB  and  lacking  buccal  shields,  not  to  mention  other 
peculiarities.  Consequently  the  class  Ophiuroidea  may  be 
regarded  as  consisting  of  two  orders,  the  EuiiYALiDA,  includ- 
ing Astrophyton,  commonly  known  as  the  Basket-star,  Tri- 
chaMer,  in  which  the  arms  do  not  branch,  and  other  similar 
forms,  and  the  Ophiurida,  which  includes  the  other  genera, 
such  as  Ophiothrix,  Ophioderma,  Ophiolepis,  Amphiura,  etc. 


i! 


670 


IN  VERTEBRA  TE  MORPHOLOQ  T. 


Development  of  the  Ophiuroidea. — Except  in  a  few  cases,  such  as  Am- 
phiura  squamata,  whose  habits  have  already  been  referred  to,  the  devel- 
opment of  the  young  Ophiuran  takes  place  outside  the  body  of  the  parent 
in  the  surrounding  water  and  a  typical  larval  form  occurs.  This  is  known 
as  the  Pluteus  (Fig.  261)  and  in  its  general  form  resembles  the  Bipinnaria 
of  the  Starfish.  The  ciliated  band,  however,  does  not  divide  into  an 
adoral  and  a  postoral  portion,  but  remains  continuous,  and  the  lateral 
lobes  become  long  armlike  processes  supported  by  a  special  skeleton, 
formed  of  calcareous  rods  developed  in  their  interior.  The  mode  of  devel- 
opment of  the  young  Ophiuran  from  this  larva  resembles  closely  that  de- 
scribed for  the  Asteroidea. 

Glass  IY.  Echinoidea. 

The  Echinoidea  present  greater  differences  in  shape  than 
are  found  in  any  of  the  other  groups  of  Ecliinoderms,  being 
more  or  less  spherical,  oval,  discoid,  pentagonal,  or  heart- 
shaped,  but  they  are  all  characterized  by  the  absence  of  arms,^ 
by  the  calcareous  plates  being  immovably  united  (except  in  a 
few  forms,  such  as  Asthenosoma,  where  their  edges  overlap 
and  they  are  consequently  movable)  to  form  a  firm  test,  and 
by  a  great  development  of  movable  spines  upon  the  plates, 
Avheuce  the  popular  name  of  Sea-urchins  usually  applied  to 
members  of  the  group.  The  test  is  covered  by  ciliated  ecto- 
derm, below  which  is  a  plexus  of  nerve-fibres  and  ganglion- 
cells  which  coordinate  the  movements  of  the  spines,  to  whose 
bases  muscle-fibres  are  attached. 

The  test  presents  certain  variations  in  the  different  forms, 
but  there  are  also  certain  features  which  are  to  be  considered 
typical  for  the  group.  The  apical  system  of  plates  is  usually 
well  developed.  A  centrodorsal  is  present  in  the  genus  )Sa- 
lenia,  but  in  all  other  recent  forms  it  is  replaced  by  a  series  of 
small  plates  which  constitute  the  periproct  in  the  simpler 
forms,  since  in  these  they  surround  the  anus.  These  plates* 
are  surrounded  by  a  circle  of  five  basals  (Fig.  262,  g),  usually 
termed  genitals  o  i  account  of  the  reproductive  ducts  opening 
by  a  pore  upon  them.  The  five  radials  (o)  are  also  repre- 
sented, alternating  with  the  basals ;  and  since  the  terminal 
tentacle  of  the  radial  hydrocoel-canals  protrudes  through  a 
pore  situated  upon  them,  and  more  especially  since  a  pigment- 
spot,  supposed  to  be  an  eye,  lies  frequently  at  the  base  of 


TYPE  ECUINODERMA. 


571 


this  tentacle,  these  plates  are  usually  kuowu  in  this  group 
as  the  oculars.  Starting  from  each  ocular  and  each  genital, 
rows  of  plates  pass  outwards  and  downwards  towards  the 
mouth,  the  test  being  formed  of  twenty  such  rows  extending 
in  meridional  lines  from  the  aboral  to  the  vicinity  of  the  oral 
pole.  The  rows  are  grouped  together  in  pairs,  five  of  the  pairs 
starting  from  the  geuital  plates  and  the  other  five  from  the 
oculars ;  and  since  a  number  of  the  plates  in  these  latter  rows 
are  perforated  for  the  emission  of  tube-feet,  they  are  generally 
known  as  the  ambulacral  plates  (Fig.  262,  A),  while  those  of 


Pig.  262— Figure  showing  thk  Abrakgement  of  the  Apical  System  of 

Plates  of  Sirongylocentrotus. 
A  =  ambulucml  ureas.  /  =  iDterambulacral  areas. 

an  =  anus.  m  =  inadreporite. 

g  =  genital  plates.  o  =  ocular  plates. 

the  intervening  pairs  are  termed  the  interambulacrals  (/).  The 
pores  for  the  tube-feet  are  almost  always  double,  and  are 
situated  usually  near  that  side  of  a  plate  which  abuts  upon 
the  adjacent  interumbulacral ;  in  some  forms  but  a  single 
pair  of  pores  occurs  on  each  plate,  but  more  usually  two  or 
more  pairs  are  found  (Fig.  262) — an  arrangement  which  indi- 
cates that  such  plates  are  formed  by  a  fusion  of  several 
smaller  ones,  each  of  which  is  represented  by  one  of  the  pairs 
of  pores. 

In  the  more  primitive  forms  one  of  the  genital  plates  is 
transformed  into  the  madreporiform  tubercle  (Fig.  262,  m), 
but  in  others  the  limits  of  the  tubercle  may  extend  so  as  to 


i^l^ 


lil 


1« 


672 


INVERTEBRATE  MORPHOLOGY. 


include  all  the  plates  of  the  apical  system,  and  at  the  same 
time  the  anal  opening  may  leave  its  position  near  the  centre 
of  the  apical  system  and  become  situated  in  the  interradius  AB, 
either  at  the  margin  of  the  flattened  disklike  test,  or  even  on 
its  oral  surface.  A  marked  bilaterality  of  form  is  thus  de- 
veloped, which  may  become  still  more  pronounced  by  a  mi- 
gration of  the  mouth  away  from  the  centre  of  the  oral  surface 
along  the  line  of  the  radius  U,  which  at  the  same  time  be- 
comes more  or  less  altered  in  size  and  form,  and  consequently 
dissimilar  to  the  other  radii  (Fig.  263).     In  these  cases  it  is 

possible  to  recognize  in  addi- 
tion to  oral  and  aboral  surfaces 
anterior  and  posterior  poles 
and  a  right  and  left  side,  the 
median  line  of  the  body  pass- 
ing in  front  through  the  radius 
D  and  posteriorly  through  the 
interradius  AB.  Three  of  the 
radii,  6',  D,  and  J?,  thus  lie  in 
the  anterior  half  of  the  body, 
and  for  descriptive  purposes 
these  have  been  termed  the 
trivium,  while  the  two  posterior 
ones,  A  and  B,  constitute  the 
bivium. 

The  mouth,  which  is  usual- 
ly situated  in  the  centre  of  the 
aboral  surface,  is  surrounded  by 
ah  area,  the  peristome,  which 
has  imbedded  in  it  only  a  few  scattered  calcareous  plates  and 
consequently  possesses  a  somewhat  leathery  consistency. 
An  oral  system  of  plates  cannot  be  distinguished  in  adult 
Echinoids. 

The  marked  bilateral  symmetry  referred  to  above  as  occurring  in  cer- 
tain Echinoids  is  undoubtedly  a  secondary  condition,  those  forms  in  which 
the  mouth  is  central  and  the  anus  approximately  so,  and  whose  bilaterality 
is  indicated  only  by  the  madreponform  tubercle,  being,  there  is  every 
reason  to  believe,  the  most  primitive.  The  bilaterality  cannot  be  regarded 
as  a  reversion  to  the  more  primitive  symmetry  of  the  larva,  since  in  the 


Fig.  263. — A  PETALosTfCHous  Echi- 
NOiD,     Brissopsis    lyrifera,    pnoM 
THE  Aboual  Surface  with  the 
Spines  removed  (after  A.  Aqassiz). 
D  =  modified  ambulacrum. 
/  =  fasciole. 


TYPE  ECUINODEliMA. 


673 


latter  the  hydrocoel-pore  lies  to  the  left  of  the  median  lino,  while  in  the 
bilateral  Echiiioids  it  is  situated  in  the  right  anterior  interradius.  Nor 
can  it  bo  regarded  as  indicating  a  primitive  adult  symmetry,  which,  though 
usually  disguised,  exists  in  all  the  Echinoderras,  since,  as  stated,  the  forms 
in  which  it  is  most  pronounced  are  the  most  highly  differentiated  meniljers 
of  their  group.  It  should  be  mentioned  that  in  the  less  differentiated 
forms  the  radiality  is  disturbed  by  the  arrangement  of  the  plates  bordering 
upon  the  peristome,  as  well  as  by  the  unpaired  madreporiform  tubercle. 
For  if  the  rows  of  plates  of  each  urnbulacral  region  be  indicated  alternately 
a  and  ft,  proceeding  contrary  to  the  direction  of  the  hands  of  a  watch,  so 
that  the  posterior  iuterambulacral  region  is  bordered  by  the  plates  Ba  and 
Ah,  then  it  will  be  found  tluit  the  plates  bordering  tlie  peristome  in  the 
rows  Ah,  Ba,  Ca,  Db,  and  Ea  are  large  and  usually  pierced  by  a  double 
pore,  while  those  of  the  rows  Aa,  Bb,  Cb,  Da,  and  Eh  are  smaller  and 
pierced  usually  by  only  one  pore.  This  arrangement  does  not,  however, 
necessarily  point  to  any  special  plane  of  bilaterality,  but  is  interesting  on 
account  of  its  constant  occurrence  in  both  the  bilateral  and  the  more  radial 
forms,  whence  it  furnishes  a  means  of  identifying  the  plane  of  bilaterality 
in  the  latter. 


Projecting  inwards  from  the  inner  surface  of  the  test  in 
the  neighborhood  of  the  peristome  are  frequently  to  be  found 
calcareous  plate-  or  pillar-like  processes  termed  ouricula'  iFig. 
265,  au),  which  may  either  be  confined  to  the  iuterambulacral 
plates  or  occur  also  on  the  ambulacrals,  uniting  in  some  forms, 
such  as  Strongylocentrotus,  in  pairs,  so  as  to  form  arches 
through  which  the  radial  hydroccel-cauals  and  nerve-cords 
pass.  In  the  flattened  disklike  forms,  such  as  Echinarachnius, 
these  pillars  are  much  more  numerous,  extending  from  the 
oral  to  the  aboral  surfaces  of  the  test.  Attached  to  the  outer 
surface  of  the  test  are  numerous  spines,  each  of  which  is 
hollowed  out  at  its  base,  the  holloAV  fitting  over  the  convexity 
of  a  tubercle  upon  the  test.  This  ball-and-socket  articulation 
allows  of  a  free  movement  of  the  spines  in  any  direction,  a 
movement  which  is  effected  by  muscles  extending  from  the 
test  to  the  base  of  each  spine,  and  forming  a  sheath  around 
its  base.  The  spines  thus  serve  as  efficient  organs  of  locomo- 
tion, usurping  this  function  entirely  in  some  forms,  while  in 
others  they  are  aided  by  the  tube-feet.  They  also  in  some 
forms  serve  as  defensive  structures,  as  in  Diadema,  where  they 
are  long  and  slender  and  readily  penetrate  the  skin  of  less- 
protected   animals,  or  in   Asthenosoma,  in  which  the  larger 


I 

!  •  i 


074 


IN  VKHTKHHA  TK  MOHVllOLOU  Y. 


Hpiues  are  uoinewhut  oultir^ed  towartla  the  tip,  the  eularge- 
iiient  contiiiuiii^  ii  (loiHon-ghind  whoHu  socrotioii  is  iujeoted 
into  tho  woiiihI  {)I'(>(1iu'<h1  by  the  spine.     Pedicel  hiring  wliich 
luive  alreiuly  hetui  noted  as  oeenrrinjj;  in  the  Asteioidea  and 
the  Euryaliil  Ophinroideu,  are  rielily  deveh)ped  in  tho  Echi- 
iioids,  more  especially  iu  the  neighborhood  of  the  mouth  and 
auns.     Tliev  assume  varying  forms,  in  the  typical  one  (Fig. 
liS^)  being  composed  of  a  stalk  surmounted  by  three  calca- 
reous   pieces   or   teeth,    hinged    iipou    the 
stalk,  and  capable  of  being  divaricated  and 
approximaited  by  means  of  muscles.     Kach 
tooth   bears   cushionlike    elevations    which 
are    tactile  in  function,  so    that   the    three 
teeth    ar«^    vigon>usly   approximated    when 
touciied    by  any    ft)reigu    body.      In    s»)me 
})tHlicellaria>    the   teeth  are  very  much    re- 
ducetl    iu    size,    but   in    their    place    three 
mucous   glands  are   developed,    structures 
sometimes   found   also  in    association  with 
well-developed  teeth.     The  functions  of  the 
Pio.  364.— PRmcRi.-    pedicellaria>    may    be    various ;    they    may 
i.AuiA  KuoM  I>ovo     serve    for   the  prehe*nsion    of    prey    or  for 
.•/,/,.m      mnlUita    p,.^,t^>ction,   and  thev  have  also  been  seen 
1H  -  miiscle-iibres.     to  remove  excreta  from  the  surface  of  the 
test  in  tho  neighborhood  of  the  anus.     Iu 
the  bilateral   Echini>ids  a   third   form  of  spine  is  found,  of 
small  si/0  and  covered  by  a  richly-ciliated  epidermis.     These 
davuhx\  as  they  are   termed,  are  usually  associated  together 
in  groups  of  consiilerable  extent  termed  semiles  or  /(tscioles, 
iH'curriug  especially  in  the  neighliorhood  of  the  plates  per- 
forated for  the  emission  of  tube-feet  (Fig.  2(>3),  and  iu  the 
vicinity    of  the    anus.     The   davuh©   have   a  rich  supply  of 
uerve-tibres,  and  are  on  this  account   supposed    to   be    sen- 
sory in  fuuctiou,  though  they  may  also  assist   in   renewing 
the  water  iu  the   vicinity   of  the  tube-feet,  which   probably 
assist  to  a  greater  or  less  extent  in   respiration.      A   fourth 
variety  of  appendage  to  the   test   is   formed    by    the  sphre- 
ridiiif  which    consist    of    a    stalk    surmounted    by    an    oval 
mass  of  carbonate  of  lime  traversed  in  all  directions  by  deli- 


TYPE  BCHINODBRMA. 


676 


cate  eaualH.  These  orgauH,  which  are  uHuallj  quite  small, 
are  Hituated  iu  the  vicinity  of  the  ambulacral  poreH  or  neur 
the  mouth,  ami  are  supposed  to  have  a  seusorj,  perhaps 
olfactory,  function. 

Owing  to  the  presence  of  the  firm  test  the  muscular  system 


Pig 

Al 
amp 
an 
aa 
au 

Co 

O 

Od 

Op 

Or 

hr 

1 

Ir 

M 


.  265. — DlAOnXM   8HOWINO 

—  Aristotle's  lauteru. 

=  ampulla. 

=  aboral  nerve-iiug. 

=  axial  siuus. 

=  auricula. 

=  c'xterual  brancbia. 

=  coelom. 

=  reproductive  organ. 

=  genital  duct. 

=  genital  jwre. 

=  genital  racbis. 

=  bydroccel-riiig. 

=  (Bsopbagus. 

=  lacunar  ring. 

=  niadreporite. 


TIIK  SruUCTUUE  OP  AN   EciTINOID. 

nr  =  epitbelitil  nerve-ilng. 

oc  =  ocular  plate. 
og  =  ovoid  gland. 
pa  =  perianal  space. 
pp  =  periproctal  space. 
pph  =  peripbaryugcal  space. 
pv  =  Polian  vesicle. 

R  =  rectum. 

rh  =  radial  bydrocoel-vessel. 
rn  =  radial  nerve. 

«c  =  stone  canal. 

si  =  sipbon. 
8p  =  spine. 

if  —  tube-foot, 

tt  =  terminal  tentacje. 


is  but  feebly  developed,  being  represented  by  the  muscles 
attached  to  the  spines,  pedicellariaB,  etc.,  and  by  those  which 
move  the  parts  of  the  masticatory  apparatus  when  it  is 
present. 

The  coelom  (Fig.  265,  Co)  is  comparatively  spacious,  though 


676 


IN  VEHTEBHA  TE  MOHPUOLOG  T. 


traversed  iu  soiue  forms  by  the  calcareous  pillars  already 
meiitioued,  as  well  as  by  a  perforated  uieseutery  extending 
from  the  iuuer  surface  of  the  test  to  the  iiitestiue  aud  follow- 
ing the  convolutions  of  the  latter.  In  the  bilateral  Echiuoids, 
which  as  a  rule  swallow  large  quantities  of  sand,  the  mesen- 
tery is  much  stronger  than  iu  other  forms,  and  additional 
mesenterial  bands  are  added  to  assist  in  the  support  of  the 
intestine.  As  in  the  Ophiuroids,  a  circular  partition  extends 
from  the  cesophagus  outwards  to  be  inserted  into  the  test  iu 
the  neighborhood  of  the  auricuhe,  enclosing  a  peripharyngeal 
space  (Fig.  265, />/>//),  which  has  no  communication  with  the 
rest  of  the  coelom,  and  ctintaius  the  organs  of  mastication  when 
these  are  present.  Iu  many  forms  the  partition  is  pouched 
out  into  five  radial  diverticula  which  project  into  the  general 
ccehim  and  are  known  as  the  organs  of  IStewart  or  as  internal 
branchiae.  In  those  radial  Echiuoids  in  which  these  structures 
are  absent,  ten  lobed  diverticula  of  the  floor  of  the  peripharyn- 
geal space  project  upon  the  outside  of  the  body  at  the  margin 
of  the  peristome,  a  pair  being  situated  iu  each  iuterambula- 
cral  region  ;  these  are  termed  external  brauchi}©  (br).  At  the 
aboral  surface  of  the  body  iu  the  radial  forms  a  partition 
similar  to  that  enclosing  the  peripharyngeal  space  is  found, 
surrounding  the  terminal  portion  of  the  intestine  and  enclos- 
ing a  subperiproctal  cavity  {pp),  while  within  this  occurs  a 
second  partition  shuiting  off  a  perianal  space  {pa).  Muscular 
fibres  occur  in  these  partitions,  and  it  has  been  suggested  that 
by  contracting  and  thus  compressing  the  fluid  contained  iu 
the  spaces  they  serve  to  close  the  lumen  of  the  rectum  and 
the  anus. 

As  regards  the  axial  sinus  (as)  the  Echiuoids  resemble  the 
Asteroids  aud  Ophiuroids  in  that  the  portion  of  the  coelom 
into  which  the  larval  stone-canal  opens  persists  iu  the  adult 
and  forms  a  pouch  extending  downwards  towards  the  oral 
surface  parallel  to  the  stone-canal,  the  ovoid  gland  {og)  devel- 
oping in  its  walls.  Into  the  upper  portion  of  it  the  adult 
stone-canal  {sc)  opens,  and  it  communicates  with  the  exterior 
through  the  madreporiform  tubercle  {M). 

The  so-called  blood-vessels  are,  as  in  the  Ophiuroidea, 
portions  of  the  lacunar   system,  and  are  contained  in  peri- 


TYPE  ECUINODKHMA. 


577 


hsBiual  canals,  as  the  radial  sclii/uciflic  Hiuiises  have  been 
termed  iu  this  group,  as  iu  the  Ophiurids.  These  canals  con- 
sist of  live  tubes  lying  between  the  radial  liydroccel-canals 
and  the  radial  nerve-cords,  and  terminating  blindly  at  their 
oral  extremities  by  coming  into  contact  with  the  peripharyn- 
geal partition  ;  the}-  are  not  continued  within  the  partition  and 
there  is  no  circumoral  sinus.  The  hicuuar  system  consists  of 
live  radial  lacunje,  lying  iu  the  perihsemal  canals,  penetrating 
into  the  peripharyngeal  space,  where  they  unite  into  a  circular 
circumoral  lacuna  (/r),  from  which  branches  j)ass  to  the  walls 
of  the  digestive  tract  and  which  is  in  connection  with  the 
lacuuiu  of  the  ovoid  gland.  This  structure,  as  stated,  lies  iu 
the  wall  of  the  axial  sinus,  and,  as  iu  other  forms,  stands  in 
close  relationship  to  the  reproductive  organs,  its  lacuna)  being 
continued  into  the  walls  of  the  genital  cords. 

The  hydrocoel  has  the  usual  arrangement  of  a  perieso- 
phageal circular  canal  {hr)  from  which  five  radial  canals  pass 
off  {rh),  each  terminating  in  a  tentacle  {tt)  perforating  an 
ocular  plate.  From  the  periojsophageal  ring  the  stone-canal 
(sc)  passes  aborally  to  open  into  the  axial  sinus  close  to  the 
madreporiform  tubercle,  and  in  addition  in  the  radial  Echi- 
noids  the  ring  has  attached  to  it  in  each  interradius  a  spongy 
structure  which  is  usually  termed  a  Polian  vesicle  {pv\ 
though  these  structures  iu  other  groups  are  saclike.  The 
tube-feet  ((/*)  which  perforate  the  ambulacral  plates  are  in  the 
majority  of  forms,  and  especially  in  the  radial  ones,  very 
extensible  and  provided  at  the  tip  with  a  sucking-disk,  and  so 
assist  the  spines  in  locomotion.  Two  pores  as  a  rule  exist 
for  each  foot ;  through  one  of  these  the  branch  issuing  from 
the  radial  canal  passes,  and  through  the  other  a  branch  passes 
back  from  the  foot  into  the  interior  of  the  body  to  terminate 
in  a  saclike  ampulla.  The  feet,  however,  near  the  aboral 
surface  are  frequently  branched  and  lack  a  sucker,  serving  a 
respiratory  function  rather  than  a  locomotor,  and  in  the 
bilateral  Echinoids,  in  which  frequently  the  tube-feet  occur 
only  on  the  aboral  surface  of  the  test,  nearly  all  the  feet  may 
assume  a  tentaclelike  or  pinnate  form  and  become  respiratory. 

The  digestive  tract  in  all  those  forms  in  which  the  mouth 
occupies  the  centre  of  the  oral   surface  is  provided  with  a 


678 


IN  VEUTEUHA  TE  MOHPIIOLOO  J. 


pbaryux  surrouiuled  by  u  complicuted  culcareouH  luuHticatory 
apparatus  usually  termed  Aristotle' h  hmtern  (Fij;.  265,  Al^  and 
Fig.  260).  When  most  highly  developed  it  has  the  form  of  a 
peutagoual  pyramid,  whose  apex  is  directed  towards  the 
mouth  and  consists  of  five  similar  portions  united  together. 
Each  portion  contains  an  elongated  ribbonlike  tooth  (Fig. 
266,  /)  lying  in  an  interradius  and  projecting  slightly  beyond 
the  lips  of  the  mouth,  though  for  the  greater  portion  of  its 
^.  length    imbedded   in    a   calcareous 

socket  or  alveolus  (a)  composed  of 
a  right  and  a  left  half  united  above 
by  epiphyses  (e).  Between  each 
pair  of  alveoli,  at  their  basal  ends 
is  another  calcareous  piece  termed 
a  radius,  and  below  each  of  these, 
i.e.  on  its  oral  surfaces,  lies  another 
piece,  the  radula  (r).  Muscles  pass 
to  this  complicated  apparatus  from 
the  auriculsB  and  from  one  piece  to 
the  other,  producing  approximation 
and  divarication  of  the  projecting 
tips  of  the  teeth.  The  presence 
of  this  apparatus  brings  it  about 
that  the  circumoral  hydrocoel  and 
lacunar  rings  are  forced  back  some  distance  from  the  mouth, 
surrounding  the  oesophagus  just  where  it  leaves  the  lantern. 
It  seems  well  accordingly  to  speak  of  these  rings  as  being 
perioesophageal  rather  than  circumoral. 

On  leaving  the  lantern  the  digestive  tract,  starting  in  the 
interradius  I)E,  passes  around  the  ccelomic  3avity  in  the  direc- 
tion of  the  hands  of  a  watch,  until  it  reaches  the  interradius 
CDy  when  it  bends  abruptly  on  itself  and,  on  another  plane, 
nearer  the  aboral  surface,  retraces  its  course  almost  to  its 
point  of  starting,  whence  it  passes  to  the  anus.  The  portion 
of  the  intestine  immediately  succeeding  the  pharynx  is  termed 
the  oesophagus  and  is  succeeded  by  a  slightly  wider  intestine, 
the  junction  of  the  two  parts  being  in  some  forms  further 
indicated  by  the  occurrence  at  that  point  of  a  large  coecum. 
As  a  rule,  however,  appendages  to  the  digestive  tract  are  rare, 


Fig.  266  —Aristotle's Lan- 
tern FROM  Arbacia. 
a  =  alveolus. 
e  =  epiphysis. 
}■  =  radula. 
t  =  tooth. 


TYPK  ECIUNODEHMA. 


:m 


the  ouly  one  occurring  with  auy  niuiked  degree  of  coustuucy 
being  the  aiphon  (Fig.  205,  «/),  a  tube  which  uriHes  from  the 
d'Hophugus  und  ruiiH,  ch)sely  applied  to  tlie  iutestiue,  to  open 
ugiiin  into  it  at  the  extremity  of  tiie  oral  coil.  The  function  of 
this  structure  appears  to  bt>  respiratory,  but  it  is  U)  be  noteil 
that  it  is  wanting  in  all  the  members  of  (me  of  the  orders  (the 
Clypeufitrouifn)  into  which  the  group  may  be  diviiled. 

The  epithelial  nervous  system  has  the  usual  arrangement 
consisting  of  a  peri(eso])hageal  ring  (Fig.  2(55,  nr)  and  live 
radial  cords  (rn).  As  in  the  Ophiuroidea,  these  latter  struc- 
tures have  withdrawn  themselvt  s  from  the  ectoderm  and  sunk 
within  the  body-cavity,  and  accordingly  there  is  to  be  found 
an  epineural  sinus  lying  below  the  nerve-cords.  Below  the 
nerve-ring,  however,  no  sinus  is  to  be  found,  and  it  seems 
possible  that  it  may  have  fused  with  the  peripharyngeal  space. 
The  exiliernity  of  each  radial  cord  fuses  with  the  ectodernj  in 
passing  through  the  pore  in  the  ocular  plate,  and  is  distrib- 
uted to  the  walls  of  the  terminal  tentacle.  A  muscular 
nervous  system  is  present,  consisting  of  five  masses  lying  on 
the  aboral  surface  of  the  radial  nerve-cords  just  where  they 
join  the  ring,  and  apparently  having  no  direct  connection 
with  each  other  ;  they  send  fibres  to  the  muscles  of  the  mas- 
ticatory apparatus,  and  are  said  to  be  wanting  in  those  forms 
which  lack  this  organ.  The  visceral  system  consists  of  a 
ring  (an)  lying  near  the  margin  of  the  subperiproctal  cavity 
and  imbedded  in  its  wall.  From  the  ring  five  branches  arise 
which  pass  to  the  walls  of  the  ducts  of  the  reproductive 
organs. 

Sense-organs  of  various  kinds  have  already  been  referred 
to,  such  as  the  terminal  tentacles  of  the  hydrocoel  canals,  the 
fascioles,  and  the  splneridia.  In  addition  to  these,  pigment- 
spots  occurring  on  the  ocular  plates  have  been  regarded  as 
eyes,  and  somewhat  complicated  structures  of  a  bright  blue 
color  which  occur  abundantly  over  the  surface  of  the  test  in 
a  species  of  Diadema  have  also  been  regarded  as  light-percip- 
ient organs. 

As  in  other  forms,  the  reproductive  system  consists  of  the 
genital  cords  and  the  reproductive  organs.  The  former  have 
their  origin  from  a  single  cord,  which  is  a  hollow  tube  lined 


,  '1 ; 


580 


IN  VJCliTEBIiA  TE  MOIWIIOLOO  T. 


iutcrujilly  by  immature  germ-cells  aud  is  coiiueeted  tit  its  oral 
extroiiiiU  with  the  ovoid  j^laiul.  It  passes  thence  to  the 
aboral  surface  of  the  body,  whore  it  ft)rnis  a  riiifj;  (Fij^.  ii()(),  j/r) 
from  which  iu  each  interradius  a  branch  passes  outwards  to 
expaud  iuto  a  hij^hly  rai*»Mnoso  sac,  the  reproductive  orj^au 
((r).  In  some  forms  the  iiund)er  of  the  organs  may  be  re- 
duced to  four  or  even  to  two,  thongli  tive  is  to  be  regarded  as 
tiie  typical  number.  Each  organ  opens  to  the  exterior  by  a 
special  duct  {Gd),  usually  opening  on  a  genital  plate,  but 
sometimes  iu  au  interr.idius  outside  the  genital  plates. 

As  already  noted,  there  is  considerable  variety  iu  the  rela- 
tive positions  o^'cupied  by  the  mouth  aud  sinus,  and  many 
differences  of  structure  are  associated  with  these  variations. 
It  is  i)ossible,  in  fact,  to  divide  the  Echinoidea  iuto  three 
orders,  which  are  marked  out  by  the  positions  of  the  openings 
of  the  digestive  tract. 


) 


1.  Order  Desmostioha. 

In  these  forms  the  mouth  occupies  the  centre  of  the  oral 
surface,  and  the  anus  approximately  that  of  the  aboral  sur- 
face, tiie  radial  symmetry  usual  among  Echinoderms  being 
well  marked.  Tiie  body  is  usually  more  or  less  spherical  in 
form,  though  occasionally  somewhat  tiattened  ;  all  the  ambii- 
lacral  plates  are  perforated  for  the  emission  of  tube-feet,  and 
all  tive  and)ulacral  areas  are  equally  developed  (Fig.  202).  In 
the  members  of  this  order,  consequently,  the  bilaterality  is 
marked  externally  only  by  the  position  of  the  madreporiform 
tubercle. 

The  primary  ambulacral  plates  frequently  fuse  to  form 
secondary  plates  each  of  which  is  perforated  l)y  several  pairs 
of  pores,  as  many  as  six  occurring  on  some  plates  in  Sfrongy- 
hwenfrofm.  The  spines  are  sometimes  exceedingly  long,  as 
in  Dicuiema,  and  are  usually  well  developed,  bt^ing  in  Arhncia 
equal  in  length  to  about  half  the  diameter  of  the  body.  The 
auricuho  are  the  only  representatives  of  the  calcareous  })late3 
ov  bars  which  extend  from  the  oral  to  the  aboral  surface,  and 
an  Aristoth^'s  lantern  is  always  well  develo})ed,  its  alveoli 
being  much  longer  than  broad.    In  this  order  external  branohisB 


TYPK  ECIflNODKliirA. 


nsi 


are  gonorjilly  prosent,  tho  }j;ouus  Cithwls  uiul  its  iilli«>s  alono 
Ijickin^  tlu»m  jiiul  possossiu^  Stewart's  organs.  It  is  custom- 
jir}',  therefore,  tt)  ilivi*le  the  orihu'  into  two  sub-groups:  tlio 
Entohuanchiata,  inelutliu";  th(>  Cidaridtv,  aiul  the  KtvroimANcm- 
ATA,  iucludiug  all  other  tonus,  such  us  SfroKji/loceutrotuii  imd 
Arbncia. 


lu 


as 


'Z.  Order  Clypeastroidea. 

In  this  order,  nanu^d  from  one  of  the  jj;euora  contained  iu 
it,  tho  mouth  still  occupies  the  centre  of  the  oral  pole,  but  tho 
anus  is  situated  in  the  interradias  Ali,  either  at  the  marj4;in 
of  tho  flattened  test,  as  in  hchlnarachuius  (l*^ig.  'Mil),  or  on  its 
oral  surface,  as  in  MeUita.  In 
Clypeaster  the  body  is  but  slightly 
flattened,  but  in  tho  two  other 
genera  already  mentioned  the 
flattening  is  carried  to  such  an 
extent  that  the  test  has  a  more 
or  less  disklike  sha])e,  whouce 
the  term  Hand-dollars  applied  to 
certain  forms. 

In  accordance  with  the  shift- 
ing of  the  anus  from  the  centre  „       .„^     -■■' ^.ktsm 

P  ^.  .     .  ^    .    Pio.  liOT— ACi,Yi'KAHTm»ii>  E(  in- 

Of      tho      apical      system      certain      ^,„„      KclUnamrhmuH     parma, 

changes  take  place  iu  it,  the  kijom  tiik  Ahouai.  Sukkack 
most  marked  being  an  extension  with  tuk  Spinkh  kok  tuk  most 
ot    ilie    madreporiform    tul)ercle 

until  it  includes  all  the  genital  and  ocular  plates,  the  genital 
pores  being  forced  outwards  so  as  to  lie  on  one  of  the  inter- 
ambulacral  plates,  and  in  some  cases  the  ])ore  <»f  the  posterior 
interradius  AH  xi\\([  the  corr<\sponding  reproductive  organ  is 
\\M\i\\\^{Hvhimtravhnim).  The  perforatiMl  ainbulacral  plates 
ar(^  as  a  ruh^  conlined  to  the  apical  ])ortion  of  tlu*  aboral 
surface,  the  ]dates  n(\-ir  the  margin  of  th«'!  t(>st  and  thosi»  upon 
the  oral  surface  biMug  imperforate.  The  t)erforat(Ml  plates 
are  of  course  very  narrow  at  the  apical  end  of  the  series,  and 
gradually  'Milarge  as  they  [)ass  towards  tho  edge  of  the  tost, 
jdving  thus  the  appearance  of  live  flower-petals,  ami  houco 


682 


INVERTEBRATE  MORPHOLOGY. 


the  ambulacra  or  areas  occupied  by  perforated  plates  are 
termed  petaloid.  In  Echinarachnitis,  for  instance,  the  plates 
after  having  reached  their  greatest  width  retain  it  to  their 
abrupt  termination  (Fig.  267),  the  petals  being  then  termed 
open,  but  in  other  forms,  e.g.  Mellita,  they  contract  again 
peripherally,  in  which  case  the  ambulacra  are  said  to  be 
closed. 

The  pores  belonging  to  each  pair  are  generally  united  by 
a  groove,  and  are  termed  yoked  pores,  and  in  Mellita,  for 
example,  in  addition  to  the  pair  of  yoked  pores  on  each  plate 
there  is  a  third  one  situated  near  the  middle  line  of  the  am- 
bulacrum. The  iube-feet  which  project  from  the  yoked  pores 
are  frequently  pinnate  in  form,  while  those  emitted  through 
the  single  pores  are  simple  and  tentaclelike.  The  spines  are 
generally  very  small,  though  those  of  the  oral  surface  serve 
for  locomotion. 

In  Mellita,  towards  the  periphery  of  the  test,  the  imper- 
foriate  ambulacral  plates  of  the  radii  A,  B,  C,  and  ^do  not 
meet,  leaving  elongated  holes  passing  through  the  test,  and 
the  same  thing  also  occurs  with  the  plates  in  the  interradius 
AB,  so  that  altogether  five  such  holes  exist.  In  other  forms, 
instead  of  holes,  notches  occur  at  the  margin  of  the  test,  and 
other  interambulacra  than  that  in  which  the  hole  occurs  in 
3Iellita  may  be  affected.  Calcareous  columns  extend  from  the 
oral  to  the  aboral  surfaces  of  the  test,  being  especially  abun- 
dant towards  the  periphery,  and  calcareous  plates  uniting  the 
two  surfaces  occur  on  either  side  of  each  ambulacrum.  An 
Aristotle's  lantern  is  present,  but  the  alveoli  are  usually 
broader  than  long. 


3.  Order  Fetalosticha. 

In  this  order,  as  its  name  indicates,  the  ambulacra  are 
usually  petaloid,  and  the  bilaterality  indicated  in  the  Clypeas- 
troids  is  more  pr<iuouuced,  since  neither  the  mouth  nor  the 
anus  retains  its  original  position  at  the  centre  of  the  oral  or 
apical  surface.  The  anus  lies  in  the  posterior  interradius 
A  B,  while  the  moutli  has  moved  forwards  to  a  greater  or  less 
extent  along  the  radius  D.     The  test  is  oval  or,  frequently', 


TYPE  ECHINODERMA. 


583 


somewhat  beart-sbaped,  owing  to  tbe  anterior  radius  D  being 
more  or  less  depressed  so  as  to  form  a  groove  (Fig.  263). 

Tbe  madreporiform  tubercle  extends  usually  tbrougb  tbe 
centre  of  tbe  apical  system  into  tbe  posterior  interiadius, 
tbus  dividing  tbe  apical  system.  Tbe  posterior  genital  pore 
is  obliterated  in  all  members  of  tbe  order,  and  tbe  reproduc- 
tive gland  corresponding  to  it  disappears,  so  tbat  but  four 
reproductive  organs  and  pores  are  present  {Spatangufi).  In 
some  forms,  bowever,  tbe  reduction  of  tbe  reproductive  organs 
and  pores  is  carried  still  furtber  by  tbeir  disappearance  in  tbe 
rigbt  anterior  iuterradius  CD,  and  tbose  of  tbe  left  anterior 
iuterradius  DEvn&y  also  disappear,  tbe  genital  plate  becom- 
ing part  of  tbe  madreporiform  tubercle,  so  tbat  tbe  number  of 
reproductive  organs  and  genital  pores  may  be  reduced  to  two 
{Moira).  Tbe  ambulacra  are  usually  dissimilar,  especially  in 
beart-sbaped  forms,  in  wbicb  tbe  anterior  a,mbulacrum  be- 
comes mucb  modified.  Fascioles  are  generally  present,  ar- 
ranged in  different  manners  in  different  species,  in  some  sur- 
rounding tbe  ambulacra,  in  otbers  forming  a  ring  in  tbe  vicinity 
of  tbe  anus,  and  in  otbers  arranged  in  numerous  patcbes  or 
lines.  Spines  are  abundantly  present  and  are  usually  of  a 
moderate  lengtb.  Tbe  moutb  is  bounded  bebiud  by  a  well- 
marked  lip  or  labrum,  produced  by  tbe  extension  forwards 
below  tbe  moutb-opening  of  tbe  plates  of  tbe  posterior  inter- 
ambulacrum  AB.  Tbere  is  no  Aristotle's  lantern  in  tbe 
Petalosticba. 


Bevelupment  of  the  Echinoidea. — The  development  is  in  its  general 
features  very  similar  to  that  of  the  Ophiiiroids,  the  free-swimming  larva? 
having  a  closely-similar  form  and  being  known  by  the  same  name.  The 
Eehinoid  Pluteus  (Fig.  261)  may,  however,  in  some  cases  be  distinguished 
by  the  occurrence  of  two  (Arbaeki)  or  three  (Spatangus)  processes  ui)on 
the  posterior  portion  of  the  body,  which  are  wanting  in  the  Ophiurid  larvae; 
and  furthermore  in  some  Echinoid  Plutei  (Arbacia)  two  earlike  lobes  fringed 
with  cilia  occur  upon  the  sides  of  the  posterior  portion  of  the  body  and 
are  known  as  ciliated  epaulettes.  The  young  Sea-urchin  develops  in  tlie 
posterior  portion  of  the  body  of  the  larva  as  in  other  forms,  the  armlike 
processes  and  the  praeoral  lobe  of  the  larval  body  being  gradually  resorbed. 

The  relationships  of  the  various  groups  have  already  been  referred  to 
and  need  not  be  again  discussed  here,  exce|)t  to  repeat  the  statement  that 
the  evidence  at  our  disposal  indicates  that  tbe  Debiuosticba  are  the  most 


584 


INVERTEBRATE  MORPHOLOGY. 


primitive,  while  the  Clypeastroidea  and  Petalostieha  are  secondarily-de- 
rived forms.  Tlie  bilaterality  of  these  latter  forms  is  not  to  be  regarded, 
therefore,  as  having  any  phylogenetic  significance. 


" 


Class  Y.  Holothvroidea. 

The  Holotlmrians  (Fig.  268)  are  characterized  so  far  as 
their  form  is  concerned  by  being  elongated  in  the  oral-aboral 
axis,  having  thus  a  somewhat  wormlike  form,  the  mouth  be- 
ing at  or  near  one  extremity  and  the  anus  at  the  other,  except 
in  the  genus  Hhopalodina,  in  which  the  two  openings  are  ap- 
proximated.    As  a  rule  the  body  is  cylindrical,  but  in  some 

forms,  such  as  Psolm  and  the  Elasipoda, 
there  is  a  well-marked  flattened  ventral 
siirface.  Three  of  the  radial  hydrocoel- 
cauals  lie  upon  this  ventral  surface,  the 
other  two  being  dorsal,  and  it  is  usual  to 
apply  the  term  trivium  to  the  ventral  radii 
and  bivium  to  the  dorsal.  It  must  be 
recognized,  however,  that  this  use  of  the 
terms  does  not  imply  a  homology  with 
the  radii  similarly  named  in  the  Echiu- 
oidea,  since  in  the  latter  the  radii  C,  Dy 
and  K  constitute  the  trivium,  whereas  in 
the  Holothuriaus  it  is  the  radii  yl.  By 
and  E. 

The  mouth  is  surrounded  by  a  circle 
of  tentacles  varying  in  number  from  ten 

^     «««     «  ^       to  thirty.     There  are  at  first  five  primary 

Fig.  268.— Pentocto/ron-        ,     /      .    ,  ,.  ,    .  m-  i  •  T 

doaa,  A  HoLoTHDRiAN.  teutacles,  interradial  m  position,  which 

are  formed  in  connection  with  five  ca3cal 
outgrowths  of  the  hydrocoel-ring,  and  the  tentacles  subse- 
quently formed  receive  branches  from  the  five  primary  cseca. 
In  shape  the  tentacles  vary  considerably,  being  cylindrical 
in  some  forms,  arborescent  or  pinnate  in  others  (Fig.  268), 
and  in  others  peltate,  and  in  some  forms  they  are  retractile. 

The  exterior  of  the  body  is  usually  covered  by  an  epithe- 
lium over  which  a  cuticle  may  be  developed,  but  in  some 
forms  the  ectodermal  cells  sink  into  and  become  fused  with 
the  subjacent  connective  tissue.     The  calcareous  skeleton  is 


TYPb:  ECUINODEUMA. 


585 


lU 

cle 
teu 
arv 
licli 
3cal 
3se- 
eca. 
ical 
68), 
e. 

he- 

)me 

ith 

is 


but   feebly  developed   iu    the   Holothurians,  the  body- walls 
having  as  a  rule  a  somewhat  leathery  consistency,  though  in 
Elpidia  interlocking  spicules,  and  in  Deima  and  Psolm  closely- 
approximated  or  overlapping  plates,  give  the  skin  considerable 
rigidity.     In  the  majority  of  forms,  however,  the  calcareous 
skeleton  is  represented  by  scattered  plates  of  various  shapes, 
perforated,  knobbed,  sometimes  wheel-shaped  as  in  Ohirodofa, 
or   associated  with  an  anchorlike  spicules  in  Synapta,  and 
are  not  sufficiently  numerous  to  give  a  rigidity  to  the  integu- 
ment, which  in  Synapta  may  even  be  thin  and  translucent. 
There  is  no  indication  of  an  apical  system  of  plates,  though  iu 
MnUeria  a  circle  of  five  plates  surrounds  the  anus,  but  the 
oral  system  is   represented  in    a  species   of  Psolm  by  five 
plates  which  may  be  closed  over  the  mouth  and  tentacles. 
In  other  parts  of  the  body  than  the  integument  calcareous 
matter  is  also  frequently  deposited,  especially  in  the  connec- 
tive tissue  of  the  wall  of  the  peripharyngeal  cavity,  the  pharjn- 
geal  ring  (Fig.  269,  b)  so  formed  consisting  typically  of  five 
radial  ossicles,  grooved  or  perforated  by  the  radial  nerves 
and  hydroccel-canals,  and  of  five  interradial  ossicles  alternat- 
ing with  them,  though  in  those  forms  in  which  the  number  of 
tentacles  is  greater  than  ten  the  number  of  the  interradial 
ossicles  may  be  increased.     Spicules  are  also  found  in  the 
mesentery,  and  in  some  forms  plates  or  a  calcareous  network 
develops   in   the   wall   of  the   pharynx.     Spines   are   rarely 
present,  though  the  plates  of  Echinocucumis,  Elpidia,  and  a  few 
other  forms  bear  them,  and  pedicellarise  are  entirely  wanting. 
The  coelom  is  traversed  by  several  mesenteries  uniting  the 
digestive  tract  to  the  body-wall,  the  most  constant  being  the 
so-called  dorsal  mesentery  (Fig.  269,  m\  which  lies  iu  the 
anterior  portion  of  the  interradius  CD.     The  portion  of  the 
coelom  which  surrounds  the  cesophagus  is  separated  from  the 
rest,  as  in  the  Echinoidea,  and  forms  the  peripharyngeal  space, 
and  similarly  in  some  forms  {Oucumaria,  Holothuria)  a  perianal 
space  surrounds  the  terminal  portion  of  the  digestive  tract. 
In  Synapta  and  its  allies  there  are  attached  by  slender  pedun- 
cles to  the  body-wall  along  the  line  of  the  mesenteries,  and 
hanging  freely  in  the  body-cavity,  numerous  ciliated  urnlike 
bodies,  which  probably  function  similarly  to  the  ciliated  cups 


f 


686 


INVERTEBRATE  MORPHOLOOT. 


It 


of  the  Oriuoids  ia  maiutaiuiug  a  circulation  of  the  ccelomic 
fluid. 

So  far  as  is  known,  the  portion  of  the  coelom  which  in  the 
embryo  opens  to  the  exterior  by  the  water-pore  and  with  which 
the  stone-canal  communicates  in  the  Asteroids  and  Echiuoids 
does  not  persist  in  the  adult  Holothurian,  and  consequently 
there  is  no  axial  sinus,  and  it  is  doubtful  if  a  structure  com- 
parable to  the  ovoid  j^laud  of  other  forms  exists.  Schizocoelic 
sinuses  corresponding  to  the  perihaemal  canals  of  the  Echinoids 
occur  in  their  usual  posiLion  between  the  nervous  system  and 
the  hydrocoel-canals,  and  consist  of  a  ring  accompanying  the 
nerve-ring  and  five  radial  canals  which  abut  against  the  ring 
at  their  oral  ends  but  seem  to  be  completely  separated  from 
it  by  septa.  A  lacunar  system  is  well  developed,  consisting 
of  a  plexus  in  the  walls  of  the  intestine,  the  various  branches 
uniting  to  form  a  dorsal  and  a  ventral  intestinal  vessel,  which, 
passing  forwards,  unite  with  a  lacunar  ring  surrounding  the 
oesophagus  at  about  the  level  of  the  hydrocoel-ring.  From 
this  ring  five  radial  lacunaB  extend  backwards,  lying  in  the 
connective  tissue  between  the  radial  perihsemal  sinus  and  the 
hydrocoel-canals,  and  giving  branches  to  the  tentacles  and  the 
tube-feet.  A  lacuna  also  extends  from  the  perioesophageal 
lacunar  ring  to  the  reproductive  organs  arising  from  a  thick- 
ened portion  of  the  ring,  and  this  thickening  has  been  re- 
garded as  the  rudiment  of  the  ovoid  gland. 

The  hydrocoel  has  the  usual  arrangement,  consisting  of  a 
ring  (Fig.  269,  c)  surrounding  the  oesophagus  behind  the  ring 
of  peripharyngeal  ossicles,  and  having  arising  from  it  a  stone- 
canal  which  in  the  majority  of  forms  hangs  freely  in  the 
coeloraic  cavity,  where  it  terminates  in  a  madreporiform  plate. 
In  the  embryo  it  as  usual  opens  upon  the  surface  of  the  body, 
and  this  condition  is  retained  in  many  Elasipoda,  in  which  the 
canal  opens  upon  the  dorsal  surface  of  the  body,  probably 
indirectly  through  the  intervention  of  an  ampulla,  as  in  other 
forms.  In  the  majority  of  forms,  however,  the  connection  with 
the  exterior  becomes  lost,  the  ampulla  which  is  present  in  the 
embryo  disappearing,  and  occasionally  a  number  of  secondary 
canals  develop.  A  single  Poliau  vesicle  (e)  is  usually  attached 
to  the  ring,  but  in  some  cases  the  number  of  these  structures 


TYPE  ECUINODERMA. 


587 


may  be  considerably  increased.  Five  interradial  canals  arise 
from  the  ring  and  pass  forwards  to  the  tentacles,  branching  if 
these  structures  are  more  than  live,  and  in  some  forms  (Holo- 
thuria,  Chirodota)  tliese  tentacular  canals  are  provided  with 
ampullee.  Five  radial  canals  also  pass  backwards  from  the 
ring  in  all  forms  except  the  Synaptidio,  corresponding  to  tlie 
radial  hydroccel-canals  of  other  Echinoderms  and  bearing 
tube-feet. 

The  distribution  of  these  latter  structures  is  peculiar  in 
many  forms.  In  MoJpadia  although  the  canals  are  present 
the  tube-feet  are  entirely  wanting,  as  they  are  also  from  the 
dorsal  canals  of  Psolus  and  from  the  median  canal  of  the 
trivium  of  the  Elasipoda ;  when  present  they  may  be  arranged 
along  the  lines  of  the  radial  canals  {Cucumaria,  Penfacta,  Fig. 
268)  or  may  be  scattered  irregularly  over  the  surface  of  tlie 
body  ( jT/i/yowe).  In  form  they  also  vary  considerably,  being 
either  simple  fingerlike  processes  or  else  tipped  with  a 
sucker.  Frequently  the  tube-feet  are  not  retractile,  and  in 
the  Elasipoda  they  take  the  form  of  strong  well-developed 
conical  processes  arranged  in  pairs. 

Owing  to  the  absence  of  a  firm  test  in  the  Holothurians 
there  is  a  much  more  extensive  development  of  the  muscular 
system  than  in  other  Echinoderms.  The  inner  surface  of  the 
body- wall  is  formed  by  a  layer  of  circular  muscle-fibres,  and 
on  each  side  of  each  radial  hydrocosl-canal  is  a  longitudinal 
muscle-bundle  (Fig.  269,  p)  from  which  in  some  forms  special 
bundles  pass  to  the  peripharyngeal  ossicles  and  serve  as 
retractors  of  the  tentacles  and  mouth-disk. 

As  stated  the  mouth  is  usually  at  the  anterior  end  of  the 
body  at  the  centre  of  a  disk  surrounded  by  the  tentacles,  but 
in  the  Elasipoda  it  has  a  somewhat  ventral  position.  The 
digestive  tract  is  a  simple  tube,  which  occasionally  has  a  per- 
fectly straight  course  from  mouth  to  anus,  but  more  frequently 
it  is  bent  twice  upon  itself,  so  that  there  is  an  anterior  de- 
scending limb  (Fig.  269,/),  an  ascending  (g),  and  a  posterior 
descending  one  (h).  The  terminal  portion  of  the  posterior  de- 
scending limb  is  dilated,  forming  a  cloaca  (/)  from  whose  wall 
muscle-bands  (q)  radiate  to  the  walls  of  the  body.  This  cloaca 
is  rhythmically  contractile,  and  has  opening  into  it  except  in 


m 


! 


i 


! 


m 


688 


INVERTEBRATE  MORPHOLOGY. 


the  Synaptidte  two  much-brauclied  structures  termed  the  re- 
spiratory trees  (k).  As  their  name  indicates,  these  structures 
are  supposed  to  have  a  respiratory  function,  but  it  is  possi- 
ble that  they  may  also  aid  in  excretion,  the  waste  products 
of  metabolism  collecting  in  the  cells  lining  the  interior  of  the 


Pio.  269.— Diagram  representing  the  Internal  Anatomy  op  a  Holo- 
THURIAN  (after  LuDWio  from  Leunis). 


a  =  tentacles. 

b  =  calcareous  pharyngeal  ring, 
c  =  hydrocoel  ring. 
d  =:  stone-canals. 
e  =  Pol  Ian  vesicle. 
/,  g,  h  —  intestine. 
}  =  cloaca. 


i'  =  cloacal  opening. 
k  =  respiiatory  trees. 
I  =  Cuvierian  organ. 
m  =  dorsal  mesentery. 
n  =  duct  of  reproductive  organ. 
0  =  reproductive  organ. 
p  =  longitudinal  muscles. 


g  =  radiating  muscles  of  the  cloaca. 

tubular  branches  and  being  carried  to  the  exterior  by  a 
desquamation  of  the  cells.  In  addition  in  a  small  number  of 
forms  {Holothuria)  there  occur  upon  one  side  of  the  cloaca  a 
large  number  of  slender  tubes  (l),  which,  at  the  will  of  the 
animal,  can  be  evaginated  so  as  to  project  through  the  anal 
opening.  These  constitute  the  organ  of  Cuvier,  the  function 
of  which  is  not  as  yet  satisfactorily  explained. 


TYPE  ECUINOVEHMA, 


589 


J  re- 
ares 
jssi- 
ucts 
the 


lOLO- 


in. 


by  a 
)er  of 
aca  a 
f  the 
aual 
ictiou 


The  epidermal  nervous  system  consists  of  a  pericjosopha- 
geal  ring  and  live  radial  nerves  as  in  other  forms,  and  in  addi- 
tion live  iuterradial  nerves  pass  from  the  rinjj;  tu  the  tentacles. 
The  system  is  almost  completely  separated  from  the  ectoderm, 
with  which  in  the  embryo  it  is  connected,  and  lies  within  the 
tissues  of  the  body,  the  radial  nerves  only  at  their  posterior 
extremities  passing  through  the  tist-viesof  the  body-wall  to  fuse 
with  the  ectoderm.  In  accordance  \vith  this  arrangement  an 
epineural  sinus  accompanies  each  radial  nerve,  though  absent 
from  the  pericesophageal  ring.  The  muscular  nervous  sys- 
tem has  an  arrangement  similar  to  that  of  the  sinuses,  being 
well  defined  in  connection  with  the  radial  nerves,  though 
wanting  in  the  pericesophageal  ring.  No  trace  of  the  aboral 
nervous  system  has  been  discovered  in  the  Holothurians. 

The  tentacles  serve  as  tactile  organs,  and  may  have  also 
an  olfactory  function  in  addition  to  being  respiratory,  and  in 
the  Synaptids  numerous  ciliated  depressions  Avith  a  strong 
nerve-supply  are  found  upon  them.  No  visual  organs  occur 
in  the  Holothurians,  but  in  the  Synaptids  and  Elasipoda 
otocysts  occur.  In  Synapta  there  are  ten  of  these  organs,  im- 
bedded in  the  connective  tissue  in  the  neighborhood  of  the 
pericesophageal  nerve-ring,  one  lying  on  each  side  of  each 
radial  nerve  close  to  its  origin  from  the  ring.  Each  otoc  st 
contains  a  number  of  otoliths,  and  is  lined  interiorly  by  cells 
which  are  provided  with  cilia.  In  Elpidia  the  number  of 
otocysts  is  fourteen  and  in  other  Elasipoda  the  number  may 
be  much  greater,  amounting  to  about  thirty-six  in  a  species  of 
Kolga. 

The  Synaptids  and  Molpadids  are  hermaphrodite,  but  all 
other  Holothurians  are  bisexual.  The  reproductive  organs 
(Fig.  269,  o)  consist  of  one,  or  in  some  cases  two  bundles  of 
csQcal  tubes  which  are  attached  to  the  left  side  (or  on  both 
sides)  of  the  dorsal  mesentery,  and  open  by  usually  a  single 
duct  {n)  upon  the  dorsal  median  line,  sometimes  within  the 
circle  of  tentacles,  but  usually  behind  it.  The  caeca  are  lined 
interiorly  with  an  epithelium  from  which  the  ova  and  sperma- 
tozoa develop. 

The  arraugement  of  the  reproductive  organs  in  the  Holothurians  is  de- 
cidedly different  from  what  occurs  in  other  groups  of  Echinoderms,  and  the 


iW^ 


li 


i,i 


590 


IN VKHTEBIiA  TB  MORPHOLOQ  Y. 


diflferences  are  associated  witli  the  absence  or  reduction  of  the  ovoid  gland 
and  of  an  aboral  nervous  system.  The  number  of  the  organs  is  very  much 
reduced,  and  no  genital  cords  have  as  yet  been  discovered.  It  is  interest- 
ing to  note,  however,  the  existence  of  a  genital  lacuna  mentioned  above, 
in  association  with  which  the  reproductive  organs  seem  to  develop,  and  it 
may  be,  as  stated,  that  the  lacunar  thickening  from  which  it  arises  is  to  bo 
regarded  as  representing  tiie  ovoid  gland,  which,  as  has  been  seen,  is 
intimately  connected  with  the  lacunar  system  in  other  forms.  It  seems 
probable  that  in  harmony  with  the  shortening  of  the  stone-canal  and  its 
separation  from  the  body-wall,  and  with  the  abortion  of  the  axial  entero- 
coel,  there  has  been  a  shortening  of  the  genital  cords  so  that  the  aboral 
ring  no  longer  exists,  and  tlie  reproductive  organs,  reduced  in  number, 
develop  directly  upon  the  wall  of  the  genital  lacuna.  It  must  be  remarked 
that  in  some  forms  there  is  no  distinct  genital  lacuna,  but  the  reproductive 
organs  are  associated  with  the  intestinal  lacunae,  a  condition  which  may 
be  secondary. 

Development  of  the  Holothuroidea. — The  typical  larva  of  the  Holothu- 
rians  is  known  as  the  Auricularia  (Fig.  270),  and  is  distinguished  from 

that  of  the  Asteroids,  Ophiuroids,  and 
Echinoids  by  being  destitute  of  armlike 
processes.    In  later  stages  the  ciliated 
bands  fuse  in  such  a  manner  as  to  form 
.si  a  series  of  circular  bands  surroiuuling 
the  barrel-shaped  larva  and  recalling  the 
-dp  larva  of  the  Crinoids.     By  the  gradual 
elongation  of  this  larva  and  the  disap- 
pearance of  the  ciliated  bands  the  adult 
form  is  acquired,  there  being  no  absorp- 
tion of  any  extensive  portion  of  the  larval 
body  as  in  the  Brachiolaria  and  Pluteus. 
The  Phylogeny  of  the  Echfnoderma. 
— The  Echinoderms  form  a  well-defined 
group  showing  little  indication  of  afflni- 
FiG.  270— Auricularia  Larva  OF  ties  with  other  forms,  and  the  establish- 


Synapta  (after  Semon). 
dp  =  dorsal  pore. 
//  =  hydrocoel. 
pt  =  prinuiry  tontticlcs. 
at  =  secondary  tcutucles. 


ment  of  a  plausible  phylogeny  is  an 
unusually  difficult  task.  One  thing, 
however,  seems  certain  from  their  de- 
velopmental history,  and  that  is  that  they 
have  been  derived  from  primitive  bilat- 
'  eral  forms,  and  that  the  radiality  charac- 

teristic of  the  adults  has  been  secondarily  acquired.  The  larva?  are  strictly 
bilateral,  there  being  indications  that  originally  two  water-pores,  situated 
symmetrically  upon  the  dorsal  surface,  existed.  The  first  question  to  be 
decided  then  is  the  cause  of  the  radial  symmetry  seen  in  the  adult. 

Bilaterality  in  the  animal  kingdom  is  usually  associated  with  an  antero- 
posterior differentiation,  and  this  with  a  definite  axis  of  progression.    Thus 


TYPE  EVUINODEHMA. 


691 


mtero- 
Tbus 


in  an  Annelid  the  most  anterior  metamero  is  that  which  is  apt  to  flrst  come 
into  contact  with  new  conditions  of  environment,  and  consequently  it  has 
become  specially  provided  with  sense-organs  for  the  perception  of  these 
new  conditions  whether  favorable  or  unfavorable;  similarly  the  conditions 
affecting  the  dorsal  and  ventral  surfaces  are  different  and  conseiiuently  a 
dorso-ventral  differentiation  exists;  while  on  the  other  hand  the  conditions 
affecting  the  two  sides  of  tlie  body  are  apt  to  be  alike,  and  consequently 
the  differentiations  which  occur  on  eaejj  side  of  the  median  line  are  usually 
alilve.     Bilaterality  is  then  associated  witli  a  free- mode  of  life. 

Conversely,  radiality  is  usually  associated  willi  indefinite  axes  of  pro- 
gression or  with  a  fixed  mode  of  life,  and  the  idea  suggests  itself  that  the 
radiality  of  the  Eciuuodernis  may  be  the  result  of  a  fixation  of  the  bilateral 
larva.  The  majority  of  recent  Echinoderms  are,  it  is  true,  free  forms,  the 
Crinoids  alone  being  sessile,  but  it  will  be  found  that  geologically  the  free 
forms  are  the  latest  to  appear,  and  that  the  Pelmatozoa  are  especially  char- 
acteristic of  the  Palooozoic  roclis.  This  would  imply  that  the  Crinoids  are 
to  be  regarded  as  the  most  perfect  representatives  among  recent  foi-ms  of 
the  ancestral  types,  an  idea  which  is  borne  out  by  certain  points  in  Echiiio- 
derm  development.  Thus  the  calyx  of  the  Crinoid  is  developed  in  the 
posterior  portion  of  the  oval  larva,  the  anterior  portion  being  occupied  by 
the  stalk;  in  the  Brachiolaria  and  Pluteus  larvae  the  adult  body  is  developed 
in  the  posterior  portion  of  the  larva,  the  anterior  portion  undergoing 
absorption,  an  arrangement  which  may  be  explained  by  supposing  tliat 
originally  the  adult  forms  which  possess  these  larvae  were  stalked,  but 
secondarily  acquired  a  free  mode  of  life,  and  that  as  a  result  the  stalk  dis- 
appeared, the  body,  corresponding  to  the  calyx  of  the  Crinoid,  still  develop- 
ing in  its  accustomed  position  in  the  posterior  portion  of  the  larva.  In 
the  Holothurians  there  is  not  apparently  that  distinction  into  anterior  and 
posterior  regions  in  the  larva  which  obtains  in  other  forms,  but  it  is  to  bo 
noted  that  in  the  young  Cuciimaria,  for  example,  there  is  a  well-marked 
praeoral  lobe  which  later  on  disappears  and  may  represent  the  partially- 
aborted  stalk  region  of  other  forms. 

Attempts  have  been  made  to  trace  out  a  phylogeny  taking  the  Holo- 
thurians as  a  primitive  type  and  tracing  them  back  to  sucli  forms  as  the 
Gephyrean  worms,  the  presence  of  respiratory  trees  in  both  forms  suggest- 
ing a  possible  affinity.  Or  again  the  later  barrel-shaped  larva  with  five 
tentacles  has  been  considered  to  represent  a  common  ancestor  for  all  the 
various  groups  of  Echinoderms,  the  term  Pentactaa  being  given  to  the 
hypothetical  ancestor.  Neither  of  these  views,  however,  affords  any  clue 
to  the  origin  of  the  radiality,  and  the  structure  of  the  Holothurians  departs 
so  widely  from  that  of  the  other  groups  that  it  seems  preferable  to  consider 
them  as  most  remote  from  the  common  ancestor.  Furthermore  as  regards 
the  Pentactoea  it  is  difficult  to  understand  how  there  should  be  so  much 
similarity  in  structure  of  the  different  forms  if  they  all  differentiated  in- 
dependently from  this  common  ancestor. 

From  the  evidence  at  our  disposal  it  seems  far  more  logical  to  accept  a 


>  1 1 


692 


IN  VEIt  TKBIiA  TE  MOIil'IIOLOG  Y. 


>  11 


1 1 


11 


stalked  form  for  an  ancestor,  and  to  consider  the  Crinoids  as  approaching 
it  more  nearly  than  any  other  recent  forms.  It  has  been  suggested,  with 
iio  little  reason,  that  tlie  Cystoidea  were  the  ancestors  of  the  Crinoids  and 
perhaps  of  most  of  the  other  groups  as  well.  A  full  consideration  of  this 
point,  as  well  as  of  the  details  of  the  conversion  of  the  bilateral  remote 
ancestor  into  the  radial  form,  would  carry  us  beyond  the  scope  of  this  work, 
and  reference  must  be  had  to  special  works  treating  of  these  questions 
(P.  and  F.  Sarasin,  O.  BUtsehli). 

As  stated,  the  relationships  of  the  Echinoderras  to  other  groups  is  a 
question  which  has  not  yet  been  satisfactorily  settled.  Attention  may  bo 
called,  however,  to  the  remarkable  similarity  of  the  yo)7*a/«a-larva  of 
Bahinoglossus  (p.  606)  to  the  Eciiinoderm  larva,  a  similarity  so  great  as  to 
suggest  aflSnity.  This  suggestion  may,  however,  be  postponed  until  the 
Tornaria  has  been  described. 

SUBKINGDOM  METAZOA. 

TYPE  ECIIINODERMA. 

I.  Class  Crinoidea. — Usually  stalked  ;  with  ten  (or  five  )  arms,  provided 
with  lateral  pinnules,  arising  from  the  margin  of  the  cup-shaped 
body.    Dermal  skeleton  well  developed. 
In  adult  life  free-swimming.     Antedon,  Actinometra. 
Fixed  throughout  life,  stalk  with  numerous  whorls  of  cirri. 

Pentacrinus. 
Fixed  throughout  life,  slender  stalk  with  cirri  either  wanting 
or  only  on  distal  joints.     Rhizoorinus,  Calainocrinus,  Hyo- 
crinus,  Thaumatocrinus. 
Fixed  throughout  life,  stalk  short  and  stout.    Holopus. 
II.  Class  AsTEROiDEA. — Free  forms  ;  stellate  or  pentagonal  in  shape ;  arms 
containiug  ciecal  processes  of  digestive  tract ;  ambulacra  limited 
to  oral  surfaces. 
Dermal  skeleton  reticulate  ;  no  paxillaB  ;  anus  present.    Aste- 

rias,  Bnsinga,  Asterina,  Zoroaster. 
Dermal  skeleton  of  separate  platos  ;  paxillae  present ;  no  anus. 
Astropecten,  Liiidia. 

III.  Class  Ophiuroidea. — Free  forms  ;  steDato  in  shape  ;  arms  not  contain- 

ing caecal  processes  of  the  digcsti  ve  tract ;  ambulacra  limited  to 
oral  surface  ;  ambulacral  ossicles  contained  within  the  arms. 

1.  Order  OpJmtrida.j—Avms  unbranched ;  madreporiform  tubercle 

on  mouth-shield.     Ophiura,  Ophiotepis,  Amphiura,  Ophiactis, 
OpMothrix,  OpJiioderma,  Ophiomyxa. 

2.  Order  Euryalida. — Arms  usually  branched  ;   mouth-shields  not 

well  developed  ;  usually  several  madreporiform  tubercles.    -4*- 
trophyton,  Trichaater. 

IV.  Class  EcHiNoiDEA.— Free  forms;    without  arms;   test   composed  of 


TYPK  KVniNODKHMA. 


593 


twenty  rows  of  plates ;  aiiibiiliuTa  oxtendltig  from  oral  to 
aboral  siirfauos  or  else  liinitt'd  to  aboral  surface. 

1.  Order  Desmostieha. — Ambulacra  all  similar,  extending  from  oral 

to  aboral  surfaces;  mouth  and  anusiu  centre  of  the  oral  and 
aboral  surfaces  respectively  ;  nuisticatory  apparatus  present. 

With  internal  branchiiu,     Cidaris. 

With  external  branchiie.  Asthenosoma^  Saknia,  Diadftna, 
Arbacia,  StroiKjylocentrotus. 

2.  Order  Clypeastroidea. — Ambulacra  all  similar,  limited  to    the 

aboral  surface  ;  mouth  in  centre  of  oral  surface,  anus  exoentric  ; 
masticatory  apparatus  present.  Clypeaster^  Evhinamclinius, 
Mdlita. 

3.  Order  Petalosticha. — Ambulacra  more  or  less  dissimilar,  limited 

to  the  aboral  surface ;  mouth  and  anus  both  excenlric;  masti- 
catory apparatus  wiinting.     Moira,   Spatangns,  liriasopsis. 
V.  Class  IIoLOTHURoiDEA. — Free  forms;  body  mostly  elongate  and  ver- 
miform ;  mouth  surrounded  by  circle  of  10-30  tentacles  ;  skele- 
ton usually  of  small  scattered  plates. 

1.  Order  Elasipoda. — Bilateral  symmetry  well  marked  ;  stone-canal 

frequently  opens  to  exterior ;  no  respiratory  trees.  Deimu,  El- 
pidia. 

2.  Order  Pedata. — Bilateral  symmetry  as  a  rule  not  particularly 

well  marked ;  respiratory  trees  present ;  stone-canal  not  open- 
ing to  exterior  ;  tube-feet  well  developed. 

Tentacles  branched  dendritically.  Cucmnaria,  Psolus,  Thy- 
one. 

Tentacles  pinnate.    Rhopalodina. 

Tentacles  peltate.     Hohtfmrfa,  Mulleria. 
8.  Order  i4porto.— Bilateral  symmetry  not  well  marked  ;  stone-canal 
not  opening  to  exterior  ;  tube-feet  wanting. 

Respiratory  trees  present.     Moipadia 

Respiratory  trees  wanting.    Synapta,  Chirodota. 


i 


LITERATURE. 

GRNERAL. 

Johannes  HttUer.  Ueber  die  Larven  und  die  Metaphorphoaen  der  Echinodermen. 
Abbandl.  KK.  Acad.  Wissensch.  Berlin,  1848-5& 

P.  H.  Carpenter.  The  Oral  and  Apical  Systems  of  Echinoderms.  Quarterly- 
Journal  Microscop.  Science,  xviii,  1878  ;  xix,  1879. 

W.  P.  Sladen.  Tli£  Homologies  of  the  Primary  Larval  Plates  in  the  Test  of  the 
Braehiate  Echinoderms.  Quarterly  Journal  Microscop.  Science,  xxiv, 
1884. 

0.  Hamann.  Die  toandemden  Urkeimzellen  und  ihre  Beifungsstdtten  bei  den 
Echinodermen.    Zeitschr.  fUr  wissuusch.  Zoologie,  xlvi,  1887. 

P.  ft  F.  Sarasin.     Ueber  die  Anatomie  der  Echinothuriden  und  die  Phylogenie 


694 


INVEHTEBHATE  MORPHOLOGY. 


der  Echiaodermen,    Ergebuisse  uulurwiss.  Forsfluingcu  auf  Ceylou,  i, 

1888. 
H.  Bury.     Studies  in  the  Embryology  of  t/ie  Echinoderms.     Quarterly  Journ. 

Microscop.  fcscieiice,  xxix,  laSW. 
M.  Neamayr.     Die  Sttiinme  dcs  Thierreicfis,  etc.     Wieu  u.  Prug,  1889. 
L.  Cnenot.    Etudes  morphoLoyiques  sur  lea  Echinodermes.    Archiveii  de  Biolu- 

gie,  XI,  ISUl. 
0.  Btitschli.     Versuch  der  Ableitung  des  Echinoderms  am  einer  bilateralen  XJr- 

Jorm.    Zoitschr.  flir  wisseiiscb.  Zuulogiv,  LUi,  Siippl.,  1891iJ. 
H.  Ludwig.    Echiaodermen.    Btouus  Klassen  uud  Orduungeu  des  Tbiurreicbs, 

III.    (In  course  of  publication.) 


\\9 


CRINOIDEA. 

W.  Carpenter.  Memoir  on  the  Structure,  Physiology,  and  Development  of  Ante- 
don  rosaceus.  Pliilosopb,  Transactions  Royal  Society,  Loudon.  CLVi, 
1866. 

H.  Ludwig.  Bntriige  zur  Anatomic  der  Crinoiden.  Zeitscbr.  filr  wissenscb. 
Zoologie,  XXVIII,  1877. 

A.  H.  Marshall.  On  the  Nervous  Syxtem  of  Aniedon  rosaceus.  Quarterly 
Journ.  Microscop.  Science,  xxiv.  1884. 

£.  Perrier.  Memoire  sur  l' organisation  et  la  dereloppement  de  la  Comutule  de  la 
Mediterranee.  Nouvelles  Aroliivcs  du  Museum  d'Hisl.  Nut.  de  Paris,  S""* 
ser.,  IX,  1886  ;  S"""  ser.,  i,  1889  ;  ii,  1890. 

H.  Bury.  The  Early  Stages  in  the  Development  of  Antedon  rosacea.  Pbiloso- 
pbical  Transactions  of  tbe  Royal  Society,  London,  CLXXix.  1888. 

0.  Hamann.  Anatomic  der  Ophiuren  und  Crinoiden,  Jcuaiscbe  Zeitscbr., 
XXIII,  1889. 

A8TER0IDEN. 

A.  Agassis.  77ie  Embryology  of  the  Starfish.  Memoirs  from  tbe  Museum  of 
Com  p.  Zool.,  V,  1877. 

H.  Ludwig.  Beitrage  zur  Anatomic  der  Asteriden.  Zeitscbr.  flir  wissenscb. 
Zoologie,  XXX,  1878. 

0.  Hamann.     Die  Asteriden.    Jcmu.  1885. 

L.  Cuenot.  Contributions  d  I'etude  anaiomique  dcs  Asterides.  Archives  de  Zool. 
ex  per.  et.  gen.,  2""*  ser.,  v,  1887. 

G.  W.  Field,  llie  Larva  of  Asterias  vulgaris.  Quarterly  Journal  of  Micro- 
scop. Science,  xxxiv,  1893. 


OFHIUKOIOEA. 

Th.  Lyman.  Ophiurida  and  Astrophytida.  Illustr.  Catalogue  Museum  Comp. 
Zool.,  1865. 

H.  Ludwig.  Beitrage  mr  Anatomic  dtr  Ophiuren  Zeitscbr.  filr  wisseuscb. 
Zool.,  xxxi.  1878. 

H.  Ludwig.  Das  Mundskclett  der  Asterien  und  Ophiuren.  Zeitscbr.  filr  wis- 
senscb. Zool.,  XXXII,  1879. 

H.  Ludwig.  Neue  Beitrage  zur  Anatotnie  der  Ophiuren.  Zeitscbr.  fUr  wis- 
senscb. Zool.,  XXXIV,  1880. 


TYPE  ECIIJNODEHMA 


595 


P.  H.  Carpenter.  On  the  Apical  System  of  the  Ophiurids.  Qiiuiturly  Joura. 
Microscop.  Scieuce.  xxi,  1881. 

L.  Cuenot,  Etudes  anatomiques  et  morphologiquea  sur  lea  Ophiures.  Ar- 
chives de  Zool.  exper.  et  gen.,  2'"«  ser.,  vi,  1888. 

0.  Hamann.  Anatomie  dei'  Ophiuren  u/ui  Crtnoiden.  JeuaiscLe  Zeitsclir., 
XXIII.  1889. 

E.  W  MacBride.  T7ie  Development  of  the  OeniUil  Organs,  Ovoid  Gland,  Axial 
and  Aboral  Simises  in  Amphiura,  etc.  Quarterly  Jouru.  Microscop.  Sci- 
ence, XXXIV,  1893. 

ECIIINOIDEA. 

A.  Agassiz.     Revision  of  the  Echini.    Illustr.  Catalogue  of  the  Museum  of 

Comp.  Zool.,  No.  7,  1872-74. 
R.  Roenler.     Recherchea  sur  les  Echinides  des  cutes  de  Provence.     Aunales  du 

Musee  d'Hist.  Nat.  de  Marseille,  i,  1883. 
S.   Loven.     Etudes  sur   les   Echinoidees.      Kougl.   Sveusk.   Vetensk.    Akad, 

Hand!.,  ii,  1884. 
H.  Ayers.     On  the  Structure  and  Fmiction  of  the  Sphceridia  of  the  Echinoidea. 

Quarterly  Jouru.  of  Microscop.  Science,  xxvi,  1886. 
0.  Hamann.     Anatomie  und  Ilistologie  der  Echiniden  und  Spaiangiden.     Jena- 

ischeZeitschr.,  XXI.  1887. 


UOLOTnUROIDBA. 

E.  Selenka.     Beilriige  zur  Anatomie  und  Systematik  der  Holothurien.     Zeitschr. 

flir  wisseusch.  Zool.,  xvii,  1867  ;  xviir.  1868. 
H.  Theel.     Report  on  tlte  Uolothuroidea.     Scientific  Results  of  the  Voyage  of 

H.M.S.  Chulleuger.     Zoology,  iv,  1882. 
0.  Hamann.     Die  Holothurien.    Jena,  1884. 
R.  Semon.     Die  Entmcklung  der  Synapta  digitata  und  ihre  Bedetitung  fi'tr  die 

Phylogenie  der  Echinvdermen.     Jeuaische  Zeitschr.,  xxii,  1888. 
H.  Ludwig  und  Ph.  Barthels.     Beitriige  zur  Anatomie  der  Uolot/iurien.     Zeitschr. 

filr  wisseusch.  Zoologie,  Liv.  1893. 
Th.   Mortensen.      Zur  Anatomie  und  Entwicklung  des  Cucumaria  glacialis 

(l^ungman).    Zeitsclir.  llir  wissenBch.  Zoologie,  lvu,  1894. 


596 


INVERTEBRATE  MORPHOLOGY, 


CHAPTER   XVII. 


TYPE  PROTOCHORDAT '. 


il^* 


The  type  Protochordata  contains  a  number  of  forms  which 
present  certain  features  of  similarity  to  the  Chordata  (Verte- 
brata),  one  member  of  the  type,  Amphioxus  being  frequently 
considered  as  belonging  to  that  group,  which  is  to  be  regarded 
as  the  most  highly  differentiated  of  all  the  types  composing 
the  Animal  Kingdom. 

The  various  groups  of  -the  Protochordata  differ  greatly  in 
general  appearance,  but  certain  structural  features  of  great 
morphological  importance  are  common  to  all  of  them.  These 
may  be  briefly  stated  as  (1)  a  notochord,  consisting  of  a  more 
or  less  well-developed  rod,  arising  from  the  mid-dorsal  line 
of  the  digestive  tract  and  either  extending  the  entire  length  of 
the  body,  or  else  limited  to  its  anterior  or  its  posterior  part, 
or  even  present  only  during  larval  life,  as  in  the  majority  of 
the  Tunicata ;  (2)  branchial  slits  which  place  the  cavity  of  the 
pharynx  in  communication  with  the  exterior  and  serve  as 
respiratory  organs  ;  (3)  a  central  nervous  system,  situated  in  the 
mid-dorsal  line  of  the  body,  and  arising  in  some  forms  as  an 
ectodermal  invagination. 

Metamerism  is  but  feebly  indicated  in  the  majority  of 
cases,  some  forms  possessing  only  three  mesodermal  somites, 
while  others,  such  as  some  of  the  Tunicata,  show  traces  of 
it  only  in  the  posterior  region  of  the  body,  Amphioxus  being 
the  only  form  in  which  it  is  at  all  well  marked.  Limbs 
do  not  occur  in  any  members  of  the  group,  nor  are  there  any 
special  jaws  or  organs  of  mastication.  All  the  members  of 
the  group  are  marine,  and  the  various  classes  possess  a  wide 
geographical  distribution. 

I.  Glass  Hemichobda. 

The  members  of  this  class  are  characterized  by  the  noto- 
chord being  a  comparatively  small  diverticulum  of  the  an- 


TYPE  PROTOCHORDATA. 


697 


terior  portion  of  the  digestive  tract  aud  coutaining  a  more  or 
less  perfect  cavity  which  communicates  with  the  cesophagus. 
The  body  is  divisible  into  three  regions :  an  anterior  praioral 
epistome  or  proboscis,  a  median  collar  region,  and  a  posterior 
visceral  sac  or  trunk  region.  The  ccelom  is  divided  into 
corresponding  regions,  a  pair  of  pores  placing  the  collar  com- 
partment  in  communication  with  the  exterior,  while  one  or 
two  additional  pores  perform  a  similar  office  for  the  epistome 
or  proboscis  cavity.  The  nervous  system  remains  in  connec 
tion  with  the  ectoderm,  and  its  principal  portion  is  situated 
in  the  collar  region. 

1.  Order  Pterobranchia. 

This  order  contains  but  two  genera,  RhabdopUura  and 
Cephalodiscus,  the  former  occurring  in  deep  water  oil'  the  coast 
of  Norway,  while  the  latter  was  obtained  by  the  "  Challenger  " 
Expedition  in  the  Straits  of  Magellan. 

Bhahdophura  (Fig.  271)  is  a  colonial  form,  consisting  of  a 


Pig.  271.— Colony  of  Rhabdopleura  (after  Lankbsteb). 

stolonlike  system  of  tubes  ramifying  over  the  surface  of 
stones,  etc.,  and  giving  off  shorter  lateral  tubes  each  one  of 
which  contains  an  individual.  The  tubes  are  composed  of 
ohitinlike  material  and  form  a  "house"  for  the  colony,  and 


w 


698 


INVERTEBRATE  MORPHOLOGY. 


are  traversed,  except  towards  the  extremities  of  the  lateral 
tubes  where  the  individuals  occur,  by  a  chitinous  rod  which 
results  from  the  chitiuization  of  what  was  once  the  stem  of  the 
various  polyps.  Each  of  these  is  stalked  (Fij?.  272),  the  stalk 
(C)  becoming  continuous  below  with  the  chitinous  rod,  and 


iir 


Fig.  272. — Individual  of  Rhabdopleura  (slightly  modified  after  Lankkstbr). 

(N.B. — The  tentacles  of  one  side  of  one  arm  only  are  represented.) 

B  =  aual  papilla.  O  =  tentacle. 

C  =  stalk.  Oa  =  arm. 

D  =  epistome.  /  =  intestine. 

E  =  trunk  regiob.  K  =  sensory  papilla. 

F  =  collar  region.  N  =  uotochord. 

each  consists  of  three  well-marked  regions.  What  may  be 
termed  the  anterior  portion  of  the  body  is  formed  by  a  large 
disklike  epistome  (/)),  beneath  which  on  the  ventral  surface  is 


TYPE  PROTOCIIORDATA. 


599 


the  mouth.  Behind  these  is  what  is  termed  the  collar  region 
{F)j  which  bears  upon  its  dorsal  surface  two  long  arnilike 
processes  {Ga\  each  carrying  a  double  row  of  ciliated  tentacles 
( 6r)  arranged  pinnately.  The  third  region  is  the  visceral  sac 
{E),  from  the  posterior  and  ventral  portion  of  which  the  stalk 
arises,  while  dorsally  and  anteriorly  it  carries  a  stout  papilla 
{B)y  at  the  extremity  of  which  the  anus  opens. 

The  digestive  tract  consists  of  a  straight  a^sophagus  trav- 
ersing the  collar,  and  having  connected  with  it  on  the  dorsal 
surface  a  short  blind  process  {N)  whose  cavity  communicates 
with  that  of  the  cesophagus.  This  is  the  rudimentary  noto- 
chord.  The  cesophagus  opens  into  a  large  saclike  stomach, 
from  the  lower  end  of  which  the  intestine  (/)  arises,  and,  bend- 
ing upon  itself,  runs  forwards  to  open  on  the  anal  papilla. 

The  nervous  system  consists  of  a  thickening  of  the  ecto- 
derm on  the  dorsal  surface  of  the  collar  region,  where  is  also 
found  a  small  ciliated  elevation  {K)  supposed  to  be  sensory ; 
no  other  special  sense-organs  occur.  On  each  side  of  the 
collar  a  pore  occurs  which,  by  a  short  ciliated  canal  which 
perforates  the  wall  of  the  body,  places  the  coelom  of  the  collar 
in  communication  with  the  exterior,  and  may  be  regarded  as 
representing  an  excretory  organ.  No  epistome-pore  or  branchial 
slits  have  yet  been  observed. 

Cephaloducus  resembles  Rliahdopleura  in  being  colonial,  but 
the  houso  is  gelatinous,  and  the  various  buds  formed  from  the 
short  stalk  do  not  remain  in  connection  with  each  other,  but 
early  separate  from  the  parent.  Each  polyp  (Fig.  273)  con- 
sists of  three  regions — an  anterior  pra3oral  portion  which  forms 
a  large  epistome,  a  middle  collar  region,  and  a  posterior  visceral 
sac  ;  the  body-cavity  being  divided  into  three  corresponding 
regions.  Two  epistome-pores  occur,  the  canal  leading  from 
the  ca3lom  to  them  passing  through  the  anterior  part  of  the 
nervous  system  (n).  The  dorsal  ectoderm  of  the  collar  region 
is  thickened  to  form  the  central  nervous  system,  and  on  each 
side  of  this  is  a  cluster  of  six  tentacles  (^),  each  ending  in  a 
knoblike  dilatation  and  bearing  numerous  lateral  pinnules 
arranged  in  two  rows.  At  the  sides  the  collar  is  continued 
backwards  as  a  pair  of  lateral  folds  which  slightly  overlap  the 
anterior  portion  of  the  visceral  sac  and  form  the  operculum, 


T 


600 


INVERT J£BHATE  MORPHOLOGY. 


S0 


upon  the  inner  surface  of  which  there  is  on  each  side  a  collar- 
pore. 

The  mouth  (m)  opens  beueath  the  epistome  into  an  oesoph- 
agus, which  in  the  collar  region  bears  a  dorsal  diverticulum, 
the  notochord  {x),  projecting  forwards  into  the  epistome.  In 
this  same  region  there  is  on  each  side  a  branchial  slit  («/?), 
structures  which  are  apparently  wanting  in  lihabdopleura. 
Behind  the  oesophagus   opens  into  a  saclike   stomach  from 


Fig.  273. — Diagrammatic  Longitudinal  Section  thuough  Ceplialodiscus 

(after  Ehlers  from  Korschelt  aud  Heider). 

a  =  anus.  n  —  nervous  system. 

ex  =  excretory  organ.  sp  =  branchial  slit. 

g  =  ovary.  t  =  tentacles. 

m  =  mouth.  x  =  notochord. 

which  the  intestine,  bending  upon  itself,  passes  forwards  to 
open  (a)  upon  the  dorsal  surface  of  the  visceral  sac,  a  short 
distance  behind  the  collar. 

The  collar-pores  probably  serve  as  excretory  organs,  and 
it  has  been  stated  that  the  epistome-pores  open  into  well- 
developed  tubes  (ex)  terminating  in  the  epistome  cavity  in  a 
dilatation ;  they  also  have  been  regarded  as  excretory.  The 
reproductive  organs  (g)  are  paired  sacs,  which  open  on  the 
dorsal  surface  just  in  front  of  the  anus.    No  circulatory  system 


TYPE  PliOTOCUORDATA. 


601 


occurs  in  the  Pteiobraucliia,  and  nothing  is  yet  known  con- 
cerning the  embrj'ouic  development  of  either  of  the  genera 
contained  in  the  order. 


Ids  to 
short 

and 
I  well- 
in  a 
The 
the 
rstem 


2.  Order  Enteropneusta. 

The  order  Enteropneusta  contains  a  small  number  of 
closely-related  forms  which,  until  recently,  have  been  grouped 
together  in  a  single  genus,  Hahmoglossus,  and  which,  notwith- 
standing that  they  present  in  some  respects  a  decided  advance 
in  structure  over  the  Pterobranchia,  yet  may  ])r()])erly  be  as- 
sociated in  a  class  with  that  order. 

All  the  species  live  buried  in  sand,  and  the  body  is  elon- 
gated and  vermiform,  the  digestive  tract  being  })ractically 
straight  and  the  anus  terminal.  The  anterior  portion  of  the 
body  (Fig.  274,  pr)  has  the  form  of  a  cylindrical  })roboscis, 
corresponding  to  the  epistome  of  the  Pterobranchia  and 
united  to  the  second  region  by  a  narrow  neck,  upon  the  dor- 
sal surface  of  which  is  a  pore  (occasionally  two)  which  places 
the  proboscis-coelom  in  communication  with  the  exterior, 
while  upon  its  ventral  surface,  just  where  it  joins  the  second 
region,  is  the  mouth. 

The  second  region  is  comparatively  short  and  forms  the 
collar  (c),  its  posterior  border  being  prolonged  backwards  for 
a  short  distance  in  the  form  of  a  fold  on  each  side,  as  in 
Cephdlodiscus.  These  folds  enclose  between  their  inner  walls 
and  the  sides  of  the  anterior  part  of  the  third  portion  of  the 
body  a  space  which  communicates  behind  freely  with  the 
exterior  and  is  known  as  the  atrium.  The  posterior  or  trunk 
region  of  the  body  is  much  longer  than  either  the  proboscis 
or  collar  regions,  and  contains  the  greater  portion  of  the 
digestive  tract  and  the  reproductive  organs.  Anteriorly  it  is 
somewhat  flattened  and  presents  on  the  dorsal  surface  a 
ridge,  on  either  side  of  which  are  to  be  found  a  varying  num- 
ber of  U-shaped  pores  (br)  with  ciliated  margins.  These 
pores  lead  into  short  tubes  opening  internally  into  the  cavity 
of  the  digestive  tract  and  are  the  branchial  slits.  The}'  seem 
to  increase  in  number  during  the  life  of  the  animal,  the  pos- 
terior ones  being  merely  circular  openings,  as  the  anterior 


602 


INVERTEBRA  TE  MORPHOLOO  Y. 


\P 


ones  are  in  the  youuger  stages  of  development,  a  tonguelike 
valve  later  growing  down  from  the  dorsal  border  of  the  pore 
and  giving  it  its  U-shaped  form.  Water  flows  in  at  the  mouth 
and  passes  out  through  the  branchial  slits,  which  thus  pos- 
sess a  respiratory  function.     A  few  of  the  anterior  slits  open 


Fig.  374. — Balanoglossus  Kowalewskii  (after  Minot  from  Spenobl). 
br  =  branchial  slit.  c  =  collar.  pr  =  proboscis. 

externally  into  the  atrium,  being  covered  over  by  the  back- 
wardly-projecting  atrial  folds  of  the  collar,  but  the  majority 
are  quite  uncovered  and  are  plainly  visible  from  the  exterior. 

The  ectoderm  contains  numerous  mucous  glands  and  is 
ciliated  throughout,  no  external  cuticle  or  "  house,"  such  as 
occurs  in  the  Pterobranchia,  being  developed.  Below  it  rests 
upon  a  thin  basement-membrane. 

The  coelom  is  clearly  marked  out,  and  consists  of  three 
portions  completely  separated  from  one  another  and  corre- 
sponding to  the  three  body  regions.  The  proboscis-ccelom 
(Fig.  275,  A,  pc)  is,  in  its  anterior  portion,  a  simple  unpaired 
cavity  lined  with  delicate  cells  and  traversed  by  circular  and 
longitudinal  (tm)  muscle-fibres.  Posteriorly  it  is  prolonged 
into  two  horns  between  which  lies  a  mass  of  tissue  consisting 


TYPE  PROTOCUOliDATA. 


603 


of  several  diflferent  organs.  The  centre  of  the  mass  is  occu- 
pied by  the  uotochord  («c),  which  is,  as  in  the  Pterobranchia, 
a  forwardly-directed  diverticulum  of  the  dorsal  wall  of  the 
anterior  portion  of  the  digestive  tract  and  contains  a  more  or 
lebd  distinct  cavity  communicating  with  that  of  the  cysophagus. 
Dorsal  to  the  noLochord  lies  a  contractile  heart  (//),  and 
dorsal  to  this  again,  and  surrounding  it,  a  sac  {ps)  containing 
a  few,  or  in  some  cases  many,  cellular  elements.  This  pro- 
boscis-vesicle, as  it  has  been  termed,  may  possibly  represent  a 
portion  of  the  proboscis-ccelom,  which  would  then  consist  of 
two  separate  cavities,  originally  right  and  left,  one  of  which 
becomes  very  large  and  fills  the  gr^ate^  portion  of  the  pro- 
boscis, while  the  other  remains  quite  small.  Surrounding 
these  structures  are  a  number  of  folds  of  the  splanchnic 
layers  of  the  proboscis-coelom  {pg\  loops  of  blood-vessels 
lying  in  the  folds,  while  the  cells  covering  them  frequently 
contain  yellow  granules.  It  has  been  supposed  that  these 
granules  indicate  a  glandular  function  for  the  cells,  and  con- 
sequently the  entire  mass  of  folds  has  been  termed  the  pro- 
boscis-gland. 

The  coelomic  cavities  of  the  collar  (Fig.  275,  B  cop)  and 
trunk  are  much  simpler  and  are  paired,  the  cavities  of  the 
right  and  left  sides  being  separated  from  each  other  by  dorsal 
and  ventral  mesenteries.  From  the  dorsal  portion  of  the  an- 
terior end  of  the  trunk-coelom  two  prolongations  extend  for- 
ward into  the  collar,  lying  on  each  side  of  the  dorsal  blood- 
vessel and  forming  the  perihaemal  cavities  in  the  interior  of 
which  are  longitudinal  muscle-fibres.  Two  other  similar  for- 
ward prolongation  of  the  trunk-coelom  lie  between  the  collar 
coelom  and  the  oesophagus,  forming  the  peripharyngeal  cavi- 
ties. 

In  contradistinction  to  what  occurs  in  the  Pterobranchia, 
a  well-developed  blood  system  is  present,  consisting  in  the 
collar  and  trunk  of  a  dorsal  and  ventral  longitudinal  vessel 
with  distinct  muscular  walls  and  lyJug  in  the  mesenteries. 
From  the  dorsal  vessel  branches  pasy  to  the  tonguelike  pro- 
cess of  the  branchial  slits,  and  the  vessel  itself  is  continued 
forward  into  the  proboscis  to  enter  a  space  between  the  pro- 
boscis-vesicle and  the  uotochord  which  is  termed  the  heart. 


ii 


604 


IN  VERTEBRA  TE  MORPUOLOQ  Y. 


The  blood-spaces  iu  the  proboscis-gland  communicate  with 
the  heart,  and  the  dorsal  aud  ventral  vessels  of  the  collar  and 
trunk  are  united  by  a  double  set  of  tine  lacunar  capillaries, 
one  set  being  situated  in  the  body- wall,  and  the  other  in  the 


Fig.  375. — Transvkrse  Sections  through  (A)  the  Proboscis  and  (B)  the 
Branchial  Ukoion  uf  the  Trunk  uf  BalanoglosHun  Kowalewskii  (B  after 


Spenokl). 

cop  —  collar-ccelom. 

g  =  tougue-bar  of  skeleton. 

h  =  heart. 
kd  =  biancbial  valve. 
kh  =  braucbiiil  portion  of  oesophagus. 
ks  =  briiucbiul  septum. 
kp  =  branchial  pore. 


Im  =  longitudinal  muscles. 
nc  =  uotochord. 
nd  —  dorsal  nerve. 
nv  =  ventral  nerve. 

0  =  a'sophagus. 
pc  -    Dioboscis-ca?lom. 
pg  =  proboscis-gland. 


ps  =  proboscis-vesicle, 

wall  of  the  intestine.     The  blood  is  a  colorless  coagidable 
fluid,  apparently  destitute  of  corpuscles. 

In  the  posterior  portion  of  the  proboscis  is  found  a  plate 
of  chitiulike  material  produced  into  two  horns  posteriorly, 
and  frequently'  somewhat  hollowed  out  iu  front.  It  is  evi- 
dently supportive  in  function,  and  forms  the  proboscis-skele- 
ton. In  connection  with  the  branchial  slits  a  similar  chitinous 
skeleton  is  formed  (Fig.  27G)  consisting  of  a  series  of  trans- 
verse bars  placed  over  each  septum  between  adjacent  slits. 
From  the  middle  of  each  bar  a  rod  (really  double)  passes 
down  each  septum  (sb),  and  from  the  extremities  a  bar  (th) 
passes  into  each  of  the  adjacent  tonguelike  valves,  each  valve 
thus  possessing  a  bar  from  the  arch  lying  in  front  of  it  and 
another  from  that  lying  behind  it.     The  septal  bars  aud  the 


■11 


^■feiM 


TYPE  PliOTOCUORDATA. 


606 


1^^^!f^ 


wmm 


tougue  bars  belonging  to  each  arch  are  conuected  by  trans- 
verse bars  (syuapticula,  s)  which  extend  across  the  valves  of 
the  branchial  slits. 

The  digestive  tract  is  practically  a  straight  tube  extending 
from  the  mouth,  situated  on  the  ventral  surface  of  the  neck 
of  the  proboscis,  to  the  terminal  anus.     From  the  dorsal  wall 

of  the  (JL'So})hagus  a  tiuger- 
shaped  diverticulum  arises,  the 
notochord,  which  extends  for- 
wards into  the  proboscis ;  its 
lumen,  in  its  terminal  portion, 
being  practically  obliterated  by 
the  vacuolization  and  enlarge- 
ment of  the  cells  which  line  it 
and  which  are  continuous  with 
the  eudodermal  lining  of  the 
digestive  tract.  In  the  anterior 
portion  of  the  trunk  region  the 
branchial  slits  already  referred 
Pig.  276.-r)iAGHAM  op  the  Bkan-  to  occur,  arising  as  diverticula 
ciirAii  Skeleton  OF  5«^artooio«stt8     «  ii       i  i   i.       i  -• 

(after  spbn«.u.  "'O"^  ^^^^  dorso-lateral  portions 

«  =  syuapticula.  of  the  intestine    (Fig.  275,   B, 

sb  ~  septal  bar.  ]^]i^  ^nd  eventually  opening  to 

tb  =  tougue-bar.  ^^^  exterior  {hp).     More  poste- 

riorly  in  some  species  the  wall  of  the  intestine  is  pouched  out 
into  sacculations  which  have  been  regarded  as  liver-sacs.  In 
Balanoglossus  [Glandiceps)  hacksi  an  accessory  intestine  occurs, 
arising  in  the  middle  of  the  liver  region  from  the  dorsal  sur- 
face of  the  intestine  in  the  form  of  a  tube,  which,  more  poste- 
riorly, opens  again  into  the  intestine.  It  recalls  the  accessory 
intestine  of  certain  Annelids,  Gephyreans  and  Echiuoderms, 
but  is  peculiar  in  being  dorsal  in  position.  In  c  1 1  tin  species 
also  paired  or  unpaired  communications  of  the  iniustine  wit. 
the  exterior  have  been  found,  usually  arising  in  the  livei 
region,  and  opening  upon  the  dorsal  surface ;  their  siguifi- 
cance  is  at  present  unknown. 

A  well-developed  nervous  system  is  present  in  the  form  of 
an  elongated  cord  lying  in  the  mid-dorsal  line  of  the  collar 
region,   with  the  ectoderm  of  which  it  is   in  connection  at 


H 


606 


INVKUrEIiliA  TE  MORPIIOLOO  T. 


V 


either  eud,  though  free  throughout  tlie  greater  portion  of  its 
length.  It  contiiins  in  young  forms  u  central  lumen,  which 
may  be  represented  in  adults  hy  a  series  of  sei)aruted  cavities 
and  which  results  from  its  formation  as  an  invagination  of  the 
ectoderm.  From  this  dorsal  cord  a  plexus  of  nerve-libres 
extends  all  over  the  surface  of  the  body,  lying  in  the  lower 
layers  of  the  ectoderm  and  being  at  certain  regions  specially 
developed  so  as  to  form  nervelike  thickenings.  One  of  these 
surrounds  the  dorsal  and  lateral  surfaces  of  the  base  of  the 
proboscis,  being  })erf orated  by  the  proboscis-pore  ;  another 
occurs  at  the  posterior  edge  of  the  collar;  while  two  others 
occur  in  the  trunk  region,  one  in  the  dorsal  (Fig.  275,  /»*,  nd) 
and  the  other  {iiv)  in  the  ventral  mid-line,  extending  the  entire 
length  of  the  trunk.  No  special  optic,  olfactory,  or  auditory 
organs  seem  to  be  developed. 

The  short  canal  opening  by  the  proboscis-pore  has  been 
regarded  as  excretory,  but  the  assignment  of  such  a  function 
to  it  seems  questionable.  A  similar  function  has  been  as- 
signed to  two  short  tubes  with  folded  ciliated  walls  which 
communicate  internally  with  the  coelom  of  the  collar,  and 
open  to  the  exterior  by  the  collar-pores,  situated,  one  on  each 
side,  on  the  edges  of  the  atrial  folds.  More  definite  informa- 
tion is  required  concerning  these  organs  before  they  can 
finally  be  accepted  as  excretory ;  they  evidently  correspond 
to  the  collar-pores  of  the  Pterobranchia. 

All  the  known  species  of  Balanoglossus  are  bisexual,  the 
reproductive  organs,  ovaries  or  testes,  consisting  of  simple  or 
branched  pouches  situated  in  the  trunk,  beginning  in  the 
brancial  region  and  extending  some  distance  backwards. 
Each  pouch  opens  to  the  exterior  by  a  special  duct,  upon  the 
dorso-lateral  portions  of  the  body. 

Denelnpment  of  the  Enteropneiista. — Some  species  of  Balano- 
glossus (/?.  Kowaletcskii)  develop  directly  without  the  inter- 
vention of  a  larval  stage  in  the  life-history,  but  the  majority 
possess  a  characteristic  free-swimming  larva  known  as  the 
Tornai'ia  (Fig.  277).  It  is  a  barrel-shaped  organism,  bulged 
out  slightly  at  either  pole,  and  possessing  a  locomotor  appa- 
ratus in  the  form  of  somewhat  complicated  bands  of  cilia. 
One  of  these  surrounds  the  posterior  portion  of  the  body  as 


I^JLUqi 


TYi'K  riiorovnuHDA ta. 


007 


■t:^ 


a  simple  circular  biiud,  while  another  ruiiH  over  the  auterior 
portiou  iu  an  exceeiliugly  tortuous  course.  It  may  bo  coii- 
sitlereil  as  cousistiuj;  of  two  portions,  one  pra'onil  iu  position 
and  the  other  postoral,  the  two  meeting,  lunvever,  at  the 
apex  of  the  body  in  an  ectoder- 
mal thickening,  the  apical  plate 
(«),  which  bears  two  eyes. 

The  mouth  (J/)  opens  b}'  a 
short  tesoi)hagus  into  a  capacious  w' 
stomach  (/S'),  separated  by  a  per- 
forated  ])artition  from  the  short 
rectum  (i?),  which  opens  by  the 
termiiial  anus.  In  the  anterior 
portion  of  the  body  is  a  saclike 
structure  (/x;),  united  to  the  apical 
plate    by    a    muscular    band   and 

opening  to  the  exterior  by  a  pore  „  „  .  ^ 
,\  .,  "  .  ,  ,.,,,  .  ,,  "^  ,  /^  .Fig.  277.— TonNAKiA  Lakva  OF 
(/>)  situated  a  little  to  the  left  of  Balanoylo.m,  Koitalew.kii. 
the  mid-dorsal  line.  This  sac  is 
the  proboscis-coelom,  and  the  pore 
the  proboscis-pore,  and  iu  connec- 
tion with  the  sac  is  a  smaller  sac, 
the  so-called  heart,  which  becomes 
the  proboscis-vesicle  of  the  adult. 
At  a  later  stage  of  development 
two  other  pairs  of  coelomic  sacs  [cc  and  tc)  make  their  ap- 
pearance at  the  sides  of  the  stomach  and  give  rise  to  the 
collar  and  trunk  coelom.  The  adult  form  is  acquired  by  the 
gradual  transformation  of  this  larva,  there  being  no  sudden 
metamorphosis. 

The  Affinities  of  the  Hemichordata. — There  seems  little  room  for  doubt 
but  that  the  Pterobratichia  and  the  Enteropneusta  are  closely  related, 
so  many  similar  and  at  the  same  time  peculiar  structures  being  found  in 
both  groups.  Thus,  to  mention  only  some  of  the  more  striking  features, 
both  groups  possess  a  notochord  of  ji  similar  character,  proboscis-pores, 
collar-pores  and  branchial  slits,  and  have  the  body  and  coelom  divided  into 
three  strictly- comparable  regions.  These  remarkable  similarities  can  only 
be  explained  on  the  assumption  of  a  conmion  ancestry  for  both  groups. 

In  many  respects,  too,  the  Enteropneusta  present  similarities  to  the  suc- 
ceeding group,  the  Cephalochorda,  but  a  discussion  of  this  relationship  may 


a  =  apical  plate. 
cc  =  collar-ca'lom. 
M=  mouth. 

p  =  dorsal  pore. 
pc  =  proboBcis-coelom. 

R  =  rectum. 

8  =  stomach. 

tc  =  trunk-coclom. 


I« 


mi^' 


M 


608 


INVEHTEBHATE  MORPHOLOGY. 


W 


be  postponed  for  the  present,  and  iittentioii  called  to  the  suggestive  char- 
aeter  of  the  'Tornaria.  Its  first  describer  took  it  for  an  Echinoderni  larva, 
and  the  majority  of  succeeding  authors  have  been  inclined  to  regard  it  as 
indicating  atlinities  with  that  group.  The  arrangement  of  the  prajoral  and 
l)ostoral  ciliated  bands,  and  the  occurrence  of  tjje  proboscis- pore,  suggest 
tlie  Echiuoderm  larva  without  doubt,  but  it  must  still  be  regarded  as  a 
decidedly  open  question  whether  or  not  these  features  indicate  an  affinity. 
Further  information  is  required  both  in  regard  to  the  ancestry  of  the 
Echinoderms  and  as  to  the  life-histories  of  the  Pterobranciiia,  before  the 
question  can  be  settled. 

Another  line  of  ancestry  must  also  be  mentioned,  namely,  one  which 
leads  back  to  ancestors  common  to  the  Hemichordates  and  the  Prosopygia. 
The  similarities  of  the  Pterobranciiia  to  the  Polyzoa  are  striking,  there 
being  the  same  bending  of  the  intestine,  similar  lophophorelike  tentacular 
structures,  and,  what  is  of  considerable  importance,  a  dorsally  situated 
nervous  system  arising  as  an  invagination  of  the  ectoderm.  A  further 
point  perhaps  of  some  importance  may  also  be  mentioned,  i.e.,  the  occur- 
rence of  three, sections  in  the  body-cavity  of  the  Brachiopoda.  In  following 
out  the  line  of  descent  suggested  by  these  similarities,  we  are,  however, 
quickly  brought  to  a  halt  by  the  uncertainty  connected  with  the  origin  of 
the  Prosopygia,  and  we  are  left  standing  between  two  lines,  one  leading 
back  to  the  Prosopygia  and  the  other  to  the  Echinoderms.  Whether  or  not 
these  two  lines  converged  to  common  ancestors  in  pre-Cambrian  times  can- 
not be  ascertained,  and  the  solution  of  the  problem  must  be  left  to  future 
embryological  investigations. 

II.  Class  Cephalochobda. 

The  class  Cephalochorda  coutaiiis  a  siugle  genus,  Amphi- 
0XU8  {Branchiostoma),  which  is  exclusively  marine  in  habitat, 
being  found  buried  in  an  upright  position  in  the  sand,  the 
anterior  end  of  the  body  alone  showing  at  the  surface. 

The  body  in  all  species  is  elongated  (Fig.  278)  and  some- 
what flattened  from  side  to  side,  and  bears  along  the  mid- 
dorsal  line  an  unpaired  fin,  formed  as  a  fold  of  the  body-wall, 
and  containing  a  cavity  traversed  by  numerous  skeletal  rods 
(see  Fig.  279)  which  serve  as  a  support  for  the  fin.  Poste- 
riorly it  becomes  somewhat  higher,  and  forms  a  caudal  fin 
surrounding  the  posterior  end  of  the  body,  while  on  the  ven- 
tral surface  two  fins  run  forward  a  short  distance,  both  these 
and  the  riaudal  fin  being  sui)ported  by  tin-rays.  Some  dis- 
tance from  the  hind  end  of  the  body  on  the  left  side  of  the 
caudal  fin  is  situated  the  anal  opening  (Fig.  278,  a),  while  iu 


=1 


TYPE  PROTOCUOIWATA. 


609 


front  of  the  anterior  end  of  the  paired  ventral  fin  is  the 
atrial  pore  (p),  which  leads  into  a  cavity  termed  the  atrium 
(6).  The  outer  walls  of  this  cavity  arise  us  folds  oi  the 
sides  of  the  body,  termed  the  epipleural  folds,  which,  iu- 
creasiujj;  in  size,  fuse  together  below  except  at  the  atrial 
pore,  thus  enclosing  the  atrial  cavity  (Fig.  279,  6),  which  is 
lined  throughout  by  ectoderm  and  surrounds  the  sides  and 
ventral  surface  of  the  anterior  two  thirds  of  the  body.  Ante- 
riorly the  cavity  is  closed  by  the  fusion  of  the  foids  with  the 
body-wall  in  the  neighborhood  of  the  larval  mouth,  but  in 
front  of  this  is  a  hood-sha})ed  fold  whicli  arises  iudcpc^ud- 
ently  of  the  atrial  folds  and  forms  the  oral  hood,  at  the  bot- 


^    no  I 

Fig.  278. — Amphioxus  lanceolatua  (after  Bovehi  from  Hertwio). 
a  =  iinus.  TO  =  muscles. 

au  —  e^'c.  n  =  nepbridiuiii. 

0  =  mouth. 
p  =  atrial  pore. 


b  =  atrial  chamber, 
ch  —  iiotochoril. 


g  =  reproduclive  organ. 
I  r=  liver. 


r  =  dorsal  iierve-cord. 
sp  =  branchial  cltft. 


tom  of  which  lies  the  original  mouth,  the  margins  of  the  hood 
surrounding  what  may  be  termed  for  distinction  the  adult 
mouth  (Fig.  278,  o).  These  margins  are  })roduced  into  a 
number  of  tentacular  processes,  termed  cirrlii,  (^ach  one  of 
which  contains  an  axial  su])portive  rod  borne  by  a  skeh»tal 
ring  surrounding  the  adult  mouth,  and  each  has  its  surface 
raised  into  numerous  sensory  papilla'. 

The  ectoderm  is  very  simple  in  its  character,  consisting  of 
a  single  layer  of  cells  resting  below  upon  a  well-developed 
layer  of  connective  tissue.  The  arrangement  of  the  co'lomic 
cavities  is  by  no  means  simple,  however,  and  may  be  best 
understood  by  considering  their  mode  of  develoj)meut.  In  a 
very  young  embryo  a  fold  may  be  seen  extending  along  each 
side  of  the  primitive  digestive  tract  on  its  dorsal  surface.     In 


610 


INVERTEBRATE  MORPHOLOOT 


M' 


time  these  folds  are  gradually  coustricted  off  from  the  intes- 
tiue,  aud  at  the  same  time  are  divided  trausversely  iuto  a 
number  of  sacs  lyiug  one  beliiDd  the  other,  their  number  in- 
creasing as  the  folds  are  separated  from  before  backwards 
from  the  intestine,  until  in  A,  lanceolatits  there  may  be  as 
many  as  sixty-one.  These  sacs  are  the  primitive  mesodermic 
somites,  and  the  cavities  they  contain  are  the  primitive  c(k- 
lomic  cavities.  At  first  entirely  dorsal  in  position,  the  various 
sacs  later  on  extend  ventrally,  those  of  opposite  sides  meeting 
below  the  intestine  ;  and  still  later  the  caviti«s  of  these  ven- 
tral extensions  fuse  to  form  a  continuous  coelom  extending  the 
entire  length  of  the  body  on  the  ventral  surface,  and  +'<>rmiug 
what  is  termed  the  splanchuoccel.  This  becomes  e\  tually 
separated  by  a  layer  of  connective  tissue  from  the  more  dor- 
sal portions  of  the  somites,  which  remain  distinct  from  each 
other  throughout  life  and  are  termed  the  myoccels.  The  future 
history  of  the  two  portions  of  the  mesoderm  thus  formed 
is  very  different.  The  walls  of  the  splanchnocoel  remain 
thin,  and  the  cavity  well  marked  (Fig.  279,  co),  but  in  the 
myocoels  the  cells  forming  the  median  walls  become  converted 
into  longitudinal  muscle-fibres  (m)  which  traverse  the  entire 
length  of  each  mj'occel,  filling  it  almost  completely,  and  are 
inserted  iuto  plates  of  connective  tissue  which  develop  be- 
tween the  various  myocoels  and  separate  them  from  one  an- 
other. At  the  same  time  each  myocoel  becomes  bent,  so  that 
its  dorsal  portion  is  directed  downwards  and  forwards  and 
its  ventral  portion  downwards  and  backwards,  each  muscle- 
plate  having  in  a  longitudinal  section  of  the  body  a  < -shaped 
appearauc^e  and  fitting  into  the  one  in  front  of  it.  When 
the  epipleural  folds  develop,  both  the  splanchnocoel  and 
the  muscle-plates  are  continued  into  them,  the  muscle-plates 
lying  to  the  outer  side  of  the  splanchnocoel,  and  their  fibres 
here  having  for  the  most  part  a  transverse  direction,  in- 
stead of  a  longitudinal  one,  as  iu  their  upper  portions. 
Owing  to  the  myocoels  being  practically  obliterated  by  the 
muscle-plates,  the  coelom  of  the  adult  is  principally  formed 
of  the  splauchuocoel,  but  other  sjiaces  also  occur  which  are 
probably  schizocoelic  in  origin  and  form  various  lacunn3 
throughout  the  body. 


TYPE  PliOTOCHOlWATA. 


611 


A  blood  system  is  present,  probably  communicating  here 
and  there  with  these  lacunae,  and  containing  a  colorless  blood 
in  which  float  numerous  colorless  amoeboid  corpuscles.  The 
system  consists  of  a  dorsal  aorta,  single  throughout  the 
greater  portion  of  its  course,  but  divided  into  two  trunks  over 


Fio.   379.— Transverse   Section  throttgh  the  Branchial   Region  of 

Amphioxus  (after  Lankester  and  Bovebi  from  Hkiitwig). 


a  =  descending  aorta. 

b  =  atrial  chamber. 

c  =  notochord. 
CO  =  coelom. 

e  ■=  bypobrauchial  groove  beneath 
which  is  the  ascending  aorta. 

g  —  reproductive  organ 
hh  =  branchial  arches. 


kd  =  phar^'nx. 

kp  —  branchial  clefts. 

I  =  liver, 
m  =  muscles. 

n  —  nepiiiitlium. 

r  =  nerve-cord. 
sn  =  nerves. 


the  branchial  region  of  the  intestine.  It  sends  off  branches  to 
various  regions  of  the  body  which,  after  breaking  up  into 
capillaries,  unite  again  to  form  the  subiiitestiiial  or  ventral 
vein,  which,  passing  forwards  till  it  reaches  a  point  in  front  of 
the  branchial  region,  becomes  sinuous,  and  gives  off  a  right 
and  left  branch  to  the  lips  of  the  larval  mouth  (the  velum), 
and  a  third  large  vessel  on  the  right  side  which  passes  dor- 


rr 


1^ 


612 


INVERTEBRATE  MORPHOLOGY. 


sally  to  unite  with  the  right  aortic  vessel.  The  blood  which 
passes  from  the  dorsal  aorta  to  the  intestine  is  not,  however, 
returned  directly  to  the  ventral  vein,  but  the  intestinal  capil- 
laries unite  to  form  a  vena  porta  which  passes  to  the  liver 
and  there  breaks  up  into  a  second  set  of  capillaries,  these 
finally  emptying  through  the  hepatic  vein  into  the  ventral 
vessel.  An  hepatic  portal  system,  resembling  that  found  in  the 
Vertebrata,  thus  occurs  in  Amplnoxus.  AVliile  passing  be- 
neath the  branchial  region  of  the  intestine  the  ventral  vein 
gives  off  paired  vessels,  the  branchial  arteries,  opposite  each 
branchial  septum,  and  these  passing  dorsalwards  in  the  sep- 
tum open  into  the  dorsal  aortic  trunks.  There  is  no  definite 
heart,  but  certain  of  the  vessels,  notably  the  vena  porta  and 
branchial  arteries,  seem  to  be  contractile. 

The  notochord  (Figs.  278,  ch,  and  279,  c)  has  a  much  more 
extensive  development  than  in  the  Hemichorda,  since  it 
traverses  the  entire  length  of  the  body.  It  arises  from  the 
dorsal  surface  of  the  digestive  tract,  but  early  loses  all  con- 
nection with  the  intestine ;  and  though  in  early  stages  it  con- 
tains traces  of  a  lumen,  this  quickly  disappears,  the  cells 
becoming  richly  vacuolated,  so  that  the  notochordal  tissue 
assumes  a  characteristic  appearance.  At  either  end  it  is 
pointed,  and  throughout  its  entire  length  it  is  surrounded  by 
a  sheath  of  dense  connective  tissue,  which  is  continuous 
below  Avith  the  partitions  separating  the  splanchuoccel  from 
the  muscle-plates  and  these  from  one  another.  From  each 
side  of  the  dorsal  surface  of  the  sheath  a  longitudinal  lamella 
extends  dorsally,  the  two  lamelhe  enclosing  the  central 
nervous  system  and  being  continued  above  it  as  a  strong 
neural  ridge  (Fig.  279). 

As  has  been  already  stated,  the  adult  mouth  is  formed  by 
the  margins  of  the  oral  hood,  the  original  larval  mouth  lying 
at  the  bottom  of  the  oral  cavity  enclosed  by  the  hood  and 
being  surrounded  by  a  circular  fold  of  tissue  termed  the 
velum.  A  short  tube  leads  from  the  mouth  to  the  branchial 
or  pharyngeal  region  of  the  digestive  tract,  whose  walls  are 
here  perforated  by  numerous  slits  (Fig.  278,  sp)  placing  its 
cavity  in  communication  with  the  atrium  (see  Fig.  279).  In 
the  adult  the  slits  are  elongated  and  are  placed  obliquely  to 


by 

iiid 
Ithe 
liiul 
I  are 
its 
In 
to 


TYPE  PIWTOCUOHDATA. 


013 


the  axes  of  the  body  ;  they  do  not  possess  a  metameric  arrange- 
ment, but  are  much  more  numerous  than  the  muscle-plates  of 
the  branchial  region  of  the  body,  as  many  as  one  hundred 
slits  occurring  in  fully-developed  individuals.  In  the  parti- 
tions between  each  pair  of  slits  of 
the  same  side  of  the  body  skeletal 
rods  occur  which  unite  above  with  a 
longitudinal  bar  lying  above  the 
dorsal  ends  of  the  slits  (Fig.  280). 
A  closer  examination  shows  that 
alternate  bars  differ  in  structure,  a 
condition  due  to  the  fact  that  each 
pair  of  slits  is  primarily  derived 
from  a  single  slit,  which  becomes 
divided  longitudinally  by  the  growth 
from  its  dorsal  edge  of  a  tonguelike 
valve,  which  eventually  fuses  with 
the  ventral  edge  of  the  slit.  The 
condition  of  affairs  then  is  almost 
identical  with  what  obtains  in  Ba- 
lanoglossus,  and,  as  in  that  form,  the 
skeletal  bars  are  to  be  considered  as 
grouped  together  in  threes,  although 
the  two  bars  which  in  Balanoghssus  occur  in  each  tongue  are 
fused  together  (th)  in  Amphioxus,  a  continuity  of  the  vurions 
triads  being  thus  produced.  The  similarity  in  the  arrange- 
ment of  the  branchial  skeleton  in  the  two  forms  may  be  seen 
by  comparing  Fig.  280  with  Fig.  276  (p.  605). 

It  follows  from  this  that  the  actual  number  of  branchial  slits  in 
Amphioxus  is  lialf  the  apparent  number  ;  but  even  with  this  reduction  the 
metamerism  of  the  slits  does  not  agree  with  that  of  the  musch^plates.  In 
the  early  stages  of  development  eight  pairs  of  slits  are  developed  which  do 
correspond  nietamerically  with  the  mesodermic  somites,  but  later  addi- 
tional non-metameric  slits  are  formcul,  and  the  original  ones  are  pushed 
forward  at  the  same  time,  so  tliat  the  metameric  correspondence  is  lost. 

Blood-vessels  occur  both  in  the  septa  between  the  primi- 
tive slits  and  in  the  tongues,  and  the  edges  of  the  slits  are 
ciliated,  so  that  a  current  of  water  enters  by  the  mouth,  passes 
through   the   slits  into  the   atrial  cavity,  and  thence  to  the 


mm 


Fig.  280.  — Diaokam  of  the 
Branchiai,   Skki.kton   of 
Amphioxui  (after  Spenoel). 
s  —  syuapticuhi. 
ab  =  septal  bur. 
tb  =  tongue  bar. 


•A' 


#r' 


w 


614 


IN  VERTEBRA  TE  MORPHOLOQ  7. 


exterior  by  the  atrial  pore.  Along  the  dorsal  and  ventral 
mid-lines  of  the  branchial  region  is  a  distinct  ciliated  groove, 
the  ventral  one  having  projecting  from  its  floor  a  longitudinal 
ridge,  while  ventral  to  it  is  a  chitinous  skeletal  plate  composed 
of  paired  moieties  having  a  metameric  arrangement.  This 
ventral  or  hypopharyngeal  groove  (Fig.  279,  e)  is  termed  the 
endostyle,  and  from  its  anterior  end  a  band  of  ciliated  cells 
passes  dorsally  on  each  lateral  wall  of  the  pharynx  to  unite 
dorsally  with  the  epithelium  of  the  dorsal  or  hyperpharyngeal 
groove. 

From  the  digestive  tract  behind  the  branchial  region  a 
diverticulum,  termed  the  liver  (Fig.  278,  V)  arises,  and  pro- 
jects forwards,  covered  of  course  by  the  body-wall,  into  the 
atrial  cavity  (Fig.  279,  Z),  and  behind  this  the  intestine  passes 
straight  back  to  open  at  the  anus  (Fig.  278,  a),  situated,  as 
already  indicated,  upon  the  left  side  of  the  body,  some  dis- 
tance from  the  posterior  end. 

The  nervous  system  consists  of  a  thick-walled  tube  (Figs. 
278,  and  279,  r)  which  lies  immediately  above  the  notochord 
and  is  enclosed  by  the  connective  tissue  lamellae  which  arise 
from  the  notochord-sheath.  It  extends  throughout  the  entire 
length  of  the  body,  tapering  rather  suddenly  at  either  ex- 
tremity.    Throughout  the  greater  part  of  its  course  the  lumen 


Fig.  281. — Diagram  of  the  Anteriob  Portion  of  the  Nervous  System 

OF  Amphioxim  (after  Hatscheki. 

ch  =  notochord.  N=:  hypophysis. 

1,  2,  3  =  !ire  placed  over  the  three  veutricles. 

is  very  small,  forming  the  central  canal  from  which  a  well- 
marked  cleft,  the  dorsal  fissure,  extends  to  the  dorsal  surface. 
At  the  anterior  end  of  the  tube,  however,  the  lumen  enlarges 
to  form  an  anterior  ventricle  (Fig.  281,  1)  which  has  been 
compared  with  the  anterior  of  the  three  primary  vesicles  of 
the  Vertebrate  brain,  and  behind  this   the  lumen  contracts 


TYPE  PROTOCHORDATA. 


615 


Item 


forming  the  aqueduct  of  Sylvius  of  the  mid  braiu  (2),  while 
behind  this  aguiu  an  expansion  of  the  dorsal  portion  of  the 
dorsal  fissure  forms  a  fossa  rhomboidalis  (3)  similar  to  that  of 
the  Vertebrate  hind-brain,  the  resemblance  to  the  embryonic 
Vertebrate  brain  being  thus  very  marked — a  resemblance 
which  is  increased  by  the  occurrence  of  a  funnel-like  extension 
(N)  of  the  anterior  ventricle  towards  the  dorsal  surface  of  the 
body,  where  it  abuts  upon  a  ciliated  depression  of  the  ecto- 
derm. This  is  the  remains  of  a  communication  of  the  ventricle 
with  the  exterior  which  exists  in  the  embryo,  and  has  been 
compaied  with  the  hypophysis  of  the  Vertebrate  bruin,  the 
difference  of  its  position  in  the  latter  being  due  to  the  flexion 
of  the  bruin  round  the  anterior  end  of  the  notochord. 

From  the  brain  region  of  the  nerve-tube  three  pairs  of 
nerves  are  given  off  (Fig.  281).  the  first  and  second  of  which 
come  from  the  dorsal  portion  of  the  brain,  while  the  third 
pair  on  each  side  is  double,  consisting  of  a  root  arising  fr(;iu 
the  dorsal  surface  oi  the  brain  and  another  arising  from  the 
ventral  side,  a  condition  which  is  repeated  in  the  succeeding 
metamerically-arranged  nerves.  The  dorsal  and  ventral  roots 
never  unite  to  form  a  common  trunk  as  in  the  Vertebrata,  nor 
do  the  dorsal  nerves  bear  a  ganglion,  but  nevertheless  they 
are  sensory  in  function,  while  the  ventral  nerves  are  motor, 
supplying  only  the  musculature  of  the  bod}-. 

Of  sense-organs  the  papilla)  upon  the  cirrhi  and  the 
ciliated  depression  in  connection  with  the  hypophysis  huve 
been  already  mentioned,  the  latter,  on  somewhat  insufficient 
grounds,  having  been  supposed  to  be  olfactory  in  function. 
In  addition  to  these  a  pigment-spot,  which  may  represent 
an  exceedingly  simple  eye,  occurs  upon  the  anterior  end  of 
the  brain,  and  in  the  roof  of  the  oral  cavity  a  patch  of  cells 
occurs,  surrounding  a  depression  lined  with  columnar  cells 
bearing  long  refractive  hairs.  This  last  structure  seems  to 
be  a  sense-organ  of  some  kind,  but  its  exact  function  is  un- 
known. 

Different  structures  have  from  time  to  time  been  con- 
sidered excretory  organs.  In  the  first  place  an  excretory 
function  has  been  assigned  to  a  ciliated  tube  lying  in  the  wall 
of  the  oral  cavity  on  the  left  side  and  communicating  by  a 


WT 


616 


IN VEHTEBliA  TE  MOliPlIOLOG  Y. 


fiiunel  with  the  ca3lom  just  behiud  the  level  of  the  velum. 
Hecoiidly,  although  iu  all  probability  they  are  not  uephridia, 
the  *' browu  cauals"  may  be  here  meutioued.  These  lie  iu 
the  splanchuoccel  at  about  the  level  of  the  tweuty-seveuth 
muscle-plate  iu  A.  lanceolatus,  and  open  by  wide  fuuiiels  iuto 
the  atrium,  though  it  is  uncertain  whether  the  inner  end  Ij'iug 
iu   the   coelom   is   perforated.     Thirdly,   in   the   pharyngeal 


I      ir   s    r       n 

Pig.  283.— Excretouy  Organ  of  Amphioxua  (after  Bovebi). 
ne  =  uephridium.  np  =  nepbridiul  pore. 

nd  =  cephr?<'.;ai  funnels.  «  =  synapticulum. 

2  .=  branchial  septum.  //=  branchial  tongue. 

region  a  number  of  nephridial  canals  have  lately  been  de- 
scribed. They  are  situated  above  the  upper  ends  pf  the 
branchial  slits  (Fig.  282,  wc),  each  opening  into  the  atrium 
opposite  a  tongue-v.alve  {np),  and  from  the  short  tube  which 
passes  inward  from  tliis  openiug  an  anterior  and  a  posterior 
branch  arises,  each  of  which  opens  into  the  coelomic  cavity 
by  a  terminal  funnel.  Between  these  two  funnels  three  or 
four  others  may  occur  (nd),  and  around  the  mouth  of  each 
funnel  are  a  number  of  threadlike  processes  which  end  in 
round  strongly-refractive  cells.  That  these  structures  are 
nephridia  seems  indicated  by  their  relations  to  the  ccelom 
and  furthermore  by  the  fact  that  in  the  neighborhood  of  each 


de- 

the 

lium 

liicli 

erior 

avity 

e  or 

each 

d  in 

\  are 

elom 

each 


TYPE  PliOTOCIIOIWATA. 


617 


of  them  the  branchial  blood-vessels  fonn  a  small  plexus  which 
may  be  regarded  as  a  glomerulus  such  as  occurs  in  connec- 
tion with  the  urinary  tubules  of  the  Vertebrate  kidney. 

Amphioxiis  is  bisexual.  The  reproductive  orgaus,  ovaries 
or  testes  (Figs.  278  and  279,  g),  occur  in  the  epipleural  iolda, 
and  are  arranged  metamerically,  extending  in  A.  hinceoUitus 
from  the  hfteeuth  to  the  thirty-fifth  or  thirty-sixth  metamere. 
They  lie  at  the  level  of  the  junction  of  the  lateral  longitudinal 
and  the  ventral  transverse  muscles,  and  are  contained  within 
a  ccelomic  cavity  which  is  a  portion  of  the  original  niyc^coils 
of  the  segments  to  which  the  reproductive  masses  belong. 
They  lie  on  the  inner  surface  of  the  atrial  folds,  and  are 
covered  on  their  iuuer  surfaces  by  the  thin  body-wall,  through 
which  the  reproductive  elements  break  when  mature,  falling 
into  the  atrium  and  thence  passing  to  the  exterior  through 
the  atrial  pore.     There  are  no  reproductive  ducts. 

Tlie  Affinities  of  AmpMox us. — The  Cephalochorda  have  usually  been 
considered  the  most  primitive  Vertebrates,— as  representing,  in  other  words, 
in  a  more  or  less  modified  condition  the  stem  from  which  the  Vertebrata 
have  descended,— and  there  ai*e  many  points  of  similarity  between  Amphi- 
oxiis and  the  larval  Lampreys  {Atninocetes)  by  which  such  a  view  may  be 
supported.  The  character  and  origin  of  the  notochord  and  nervous 
system,  the  arrangement  of  the  nerves  of  the  latter,  the  lateral  muscle- 
plates,  the  occurrence;  of  an  hepatic  portal  circulation  and  the  character  of 
the  early  stages  of  development  are  all  points  of  similarity  which  seem 
explicable  only  on  the  supposition  of  a  somewhat  close  affinity. 

On  the  other  hand,  resemblances  to  the  Enteropneusta  are  but  slightly 
less  marked.  It  seems  hardly  possible  that  the  marvellous  similarities  in 
the  arrangement  of  the  branchial  skeleton  should  have  been  acquired  in- 
dependently in  the  two  forms,  'ind  the  atrial  folds  of  Amphioxiis  may  be 
regarded  as  the  more  extensively-developed  atrial  folds  of  Balanofjlossiis. 
Amphioxiis  is  undoubtedly  much  more  advanced  along  th(;  Vertebrate  line 
than  Balanoglossus,  and  both  are  probably  more  or  less  widely  divergent 
from  tlie  direct  line,  but  the  general  similarities,  such  as  the  occurrence  of 
branclnal  slits,  of  an  endodermal  notociuird,  and  of  a  dorsally-placed 
central  nervous  system,  which  have  led  to  the  association  of  both  forms  in 
the  type  Protochordata,  speak  strongly  for  a  community  of  descent. 

The  principal  difficulty  in  the  way  of  the  acceptance  of  Balanoijlossus 
as  the  representative  of  the  ancestral  forms  from  which  the  Vertebrates 
are  descended  seems  to  lie  in  the  supposed  necessity  for  deriving  highly- 
organized  metameric  forms,  such  as  the  Vertebrates,  from  more  lowly  but 
also  metameric  ancestors,  and  consequently  most  authors  have  sought  for 


■I  I 


ii'i 


M 


;iiiiii: 


618 


INVERTEBRATE  M0RPU0L007. 


indications  of  Vertebrate  ancestry  among  the  Annelida.  This  difficulty 
depends  upon  the  interpretation  placed  upon  metamerism  and  the  causes 
a.ssij;ned  for  its  origin.  If  the  ideas  regarding  these  points  advocated  in 
preceding  pages  (see  pp.  43  and  217)  of  this  book  be  accepted,  tiie  difficulty 
seems  to  bo  practically  done  away  with,  since  tiiese  ideas  imply  a  possi- 
bility of  the  independent  origin  of  metamerism  in  different  groups.  And 
indeed  it  has  already  been  pointed  out  that  such  an  independent  origin  has 
probably  occurred,  the  metamerism  of  the  Annelids  being  probably  entirely 
unconnected  with  the  metamerism  found  in  the  more  highly-organized 
Platy helminths  (see  p.  317). 

A  full  discussion  of  the  intricate  problem  of  the  origin  of  the  Vertebrata 
would  be  out  of  place  here,  but  one  additional  point  may  be  referred  to. 
Difficulties  have  always  stood  in  the  way  of  an  homology  of  the  Vertebrate 
nervous  system  with  that  of  the  Annelids.  In  the  latter  there  is  a  supra- 
oesophageal  cerebrum  and  a  ventral  chain,  while  in  the  former  the  entire 
central  system  is  dorsal  to  the  digestive  tract.  Various  theories  have  been 
advanced  to  account  for  this  difference,  none  of  which  have,  however, 
proved  entirely  satisfactory.  The  acceptance  of  an  ancestry  leading  back 
to  Hemichordalike  forms  obviates  this  difficulty,  since  in  these  the  slightly 
differentiated  nervous  cord  is  already  entirely  dorsal,  a  future  extension  of 
it  and  a  metameric  arr-'vugement  of  its  elements  and  branches  in  correla- 
tion with  the  metamerism  of  the  mesoderm  bringing  about  the  Vertebrate 
condition.  Furthermore,  the  occurrence  of  a  central  lumen  in  the  nerve- 
cord  and  the  mode  of  its  origin  are  essentially  the  same  in  both  Verte- 
brates and  Ileinichordates,  a  fact  which  in  itself  must  be  given  no  little 
weight  in  the  final  determination  of  the  question. 


' 


m 


Il  !  ill 


III.  Class  Ubochorda. 

The  Urochorda,  also  known  as  the  Tunicata  or  Ascidians, 
are,  like  the  other  Protochordates,  exclusively  marine.  At 
first  sight  they  appear  to  have  little  resemblance  to  such  a 
form  as  Amphioxus,  the  majority  of  them  lacking  in  the  adult 
condition  all  trace  of  a  notochord,  though  a  branchial  region 
of  the  digestive  tract  is  always  present.  In  a  few  adult  forms 
(Appendicularians,  Fig.  285),  and  in  the  larvfe  of  all,  a  well-de- 
veloped notochord  is  present,  however,  situated  in  a  backward 
prolongation  of  the  body,  resembling  in  appearance  and 
structure  the  tail  of  a  young  tadpole — an  arrangement  which 
has  suggested  the  name  applied  to  the  class.  In  the  majority 
of  forms  this  tail  disappears  at  the  close  of  larval  life,  and 
with  it  the  notochord  also  vanishes.  The  name  Tunicate  is 
derived  from   the  fact  that  the  body  is  enclosed  within  an 


TYPE  PROTOCUORDATA. 


619 


and 
licli 
.rity 
and 


external  coat  or  tunic  secreted  by  the  ectoderm  of  the  body 
and  composed,  so  far  as  its  matrix  is  concerned,  of  a  substance 
whicb  is  closely  related  in  its  chemical  characters  to  vege- 
table cellulose. 

Inasmuch  as  considerable  variation  in  form  occurs  in  the 
class,  it  will  be  most  convenient  to  describe  in  detail  what 
may  be  considered  a  typical  Tunicate,  pointing  out  subse- 
quently the  more  important  modifications  which  occur. 

The  simple  Ascidians  are  for  the  most  part  ovate  in  form 
and  usually  attached  at  one  end  to  some  support ;  in  the 
genus  Boltenia,  however,  the  surface  of  attachment  is  at  the 
extremity  of  an  elongated  stalk,  which  at  the  other  end  passes 
into  the  pyriform  body.  The  test  or  tunic  which  encloses 
the  body  presents  usually  towards  the  free  end  of  the  body 
two  openings,  through  one  of  which,  the  branchial  aperture, 
water  passes  into  the  interior  of  the  body  and  is  eventually 
expelled  through  the  second  or  atrial  aperture.  This  test  is 
composed  of  a  matrix  sometimes  almost  homogeneous,  some- 
times fibrillar  {Cynthia),  and  of  varying  consistency,  secreted 
by  the  ectoderm.  It  contains  scattered  through  it  numerous 
cells,  which  have  recently  been  shown  to  be  mesodermal  in 
origin  and  to  migrate  into  the  test  after  it  has  reached  a  cer- 
tain thickness.  They  may  remain  amoeboid  in  shape  or  may 
develop  vacuoles,  becoming  thereby  swollen  into  bladderlike 
structures,  or  may  become  the  seat  of  pigment-depositions, 
or,  finally,  may  secrete  spicules  of  carbonate  of  lime. 

Extending  through  the  test  are  numerous  branching  tubes 
which  communicate  with  the  blood-vessels  of  the  body. 
They  arise  as  outgrowths  of  the  blood-vessels  and  push  the 
body-wall  before  them,  being  therefore  lined  externally  by 
ectoderm,  beneath  which  is  a  layer  of  connective  tissue,  and 
each  is  separated  into  two  compartments  by  a  longitudinal 
partition  which  does  not,  however,  extend  into  the  bulblike 
enlargement  with  which  each  branch  ends,  the  two  compart- 
ments thus  communicating  in  the  bulb  and  allowing  the 
blood  to  circulate. 

The  shape  of  the  body  conforms  to  that  of  the  test,  and 
opposite  the  branchial  and  atrial  apertures  of  the  latter  is 
drawn  out  into  two  tubular  processes,  the  branchial  and  atrial 


Pf 


620 


IN  VEHTEDliA TE  MOliPJIOWU  Y, 


siphons,  in  whose  walls  circular  muscles  are  developed  to 
serve  as  sphincters  of  the  openings.  The  branchial  siphon 
opens  posteriorly  into  the  branchial  region  of  the  digestive 


Fig.  283.— Figure  of  a  Tunicate,  Heterotrema,  removed  from  the  Tept 

(after  Fiedler). 
A  =  atrial  pore.  pe  =  periphaiyugeal  ciliated  band. 

an  =  anus.  «  =  stomach. 

CQ  ■=  cerebral  ganglion.  sn  =  subneural  gland. 

en  —  endostyle.  st  =  branchial  stigma. 

ex  —  excretory  organs.     ,  t  =  testis. 

/  =  intestine.  «d  =  vas  deferens. 

tract,  the  opening  being  known  as  the  mouth,  and  usually 
being  surrounded  by  a  number  of  tentacles  (Fig.  283)  which 
arch  over  it.  The  atrial  siphon,  on  the  other  hand,  does  not 
open  into  the  body  proper  but  into  a  cavity,  lined  probably 


TYPE  PliOTOCUORDATA. 


621 


with  ectoderm,  which  almost  completely  aurrouuds  the  body, 
beiu^  wautiug  only  ulouj^  the  veutral  wall  aud  at  the  extreme 
anterior  aud  posterior  ends.  This  cavity  is  the  atrium  iiud 
its  walls  are  termed  the  uuiutle.  It  is  comparable  to  the 
atrial  cavity  of  Atnphioxus,  but  has  a  somewhat  more  exten- 
sive development  aud  arises  in  the  larva  as  a  pair  of  dorsally- 
situated  iuvagiuatious  of  the  body-wall  which,  j^radually  in- 
creasing in  size,  enclose  the  j^reater  portion  of  the  body,  and 
their  openings,  gradually  approximating,  tiually  fuse  tt)  form 
the  atrial  aperture  (Fig.  283,  ^1),  the  dorsal  partition  between 
them  at  the  same  time  disappearing,  so  that  the  two  cavities 
become  continuous. 

The  external  surface  of  both  the  mantle  and  the  bodv 
proper  is  covered  with  ectoderm,  and  rests  below  upon  a 
layer  of  mesodermal  connective  tissue  which  contains  muscle- 
fibres.  The  ccelomic  cavity  consists  of  a  number  of  lacunar 
spaces  which  have,  especially  in  the  walls  of  the  branchial 
region,  a  more  or  less  definite  arrangement  and  serve  as  blood- 
vessels. In  a  somewhat  distinct  space  near  the  hind  end  of 
the  body  is  situated  a  tubular  heart,  whose  walls  are  formed 
of  a  single  layer  of  cells,  the  inner  ends  of  which  are  con- 
verted into  muscle-fibres.  The  contractions  of  the  heart  are 
wavelike,  starting  from  one  end  and  passing  gradually  though 
rather  quickly  towards  the  other  ;  but  a  remarkable  peculiarity 
is  that  after  a  certain  number  of  beats,  at  each  of  which  the 
contraction-wave  begins  at  one  ena,  its  course  is  reversed, 
and  for  a  similar  number  of  beats  it  begins  at  the  other  end. 
This  change  takes  place  with  a  certain  amount  of  rhythm, 
and  at  each  change  the  course  of  the  blood  t}>  rough  a  portion 
at  least  of  the  body  is  reversed.  At  each  end  the  n^a^l;  opens 
into  a  large  lacuna,  one  of  which  runs  along  the  ventral  ir  id- 
line  of  the  branchial  sac,  while  the  other,  giving  off  lacunar 
branches  to  the  intestine  and  test,  runs  along  the  dorsal  mid- 
line of  the  same  region,  smaller  lacunae,  traversing  the  bran- 
chial bars,  uniting  the  two  vessels.  The  blood-plasma  is 
colorless  and  contains  amoeboid  corpuscles  which  are  also 
usually  colorless,  though  a  few  colored  ones,  resembling  the 
pigmented  cells  of  the  test,  are  fiequently  found. 

The  mouth  opens  into  a  capacious  pharyngeal  or  branchial 


622 


IN  VEHTEBHATE  MORPIIOLOO  Y. 


ii  i' 


sac  whose  walls  are  perforated  by  uunierous  slits  or  pores 
termed  sti^iuata  (Fig.  283,  st),  arranged  in  transverse  or  spiral 
rows.  The  bars  separating  the  stigmata  enclose  lacunae 
which  place  the  ventral  and  dorsal  branchial  lacun^o  in  com- 
munication so  that  the  walls  of  the  sac  are  richl}'  supplied 
with  blood,  opportunities  for  its  aeration  being  provided  by 
currents  of  water  drawn  by  the  cilia  which  border  each  stigma 
through  the  mouth  an<I  out  into  the  atrial  cavity,  whence  it 
esca])es  by  the  atrial  aperture.  The  transverse  bars  which 
separate  the  rows  of  stigmata  are  generally  stouter  than  the 
longitudinal  ones,  and  in  most  species  there  is  a  second  series 
of  longitudinal  bars  lying  on  the  inner  surface  of  the  sac,  less 
numerous  than  the  bars  which  separate  adjacent  stigmata, 
united  with  each  transverse  bar  by  a  short  connecting  branch, 
and  bearing  opposite  oacli  junction  a  hollow  papilla  which 
projects  into  the  cavity  of  the  branchial  sac.  Running  along 
the  entire  ventral  mid-line  of  the  sac  is  a  ciliated  groove  (B^ig. 
283,  En)  bounded  on  each  side  by  a  distinct  longitudinal  ridge. 
This  is  the  endostyle,  comparable  to  that  of  Atnphio.vus,  and 
from  its  anterior  end  a  band  of  ciliated  cells  (p*')  passes  dor- 
sally  on  each  side  of  the  pharyngeal  wall  to  unite  in  the  dorsal 
mid-line.  In  front  of  these  bands  another  pair  running  par- 
allel to  them  is  usually  found,  the  two  pairs  forming  the 
peripharjngeal  ciliated  bands.  From  the  dorsal  point  of 
union  of  the  two  posterior  bands  a  ridge,  the  dorsal  lamina, 
extends  backwards  in  the  dorsal  mid-line  of  the  branchial  sac, 
and  in  many  species  (Fig.  283)  is  produced  into  a  number  of  \n'o- 
cesses  succeeding  one  another  at  intervals,  and  projecting  into 
the  branchial  cavity  ;  these  are  termed  the  dorsal  languets. 

The  remaining  })ortions  of  the  digestive  tract  is  in  the 
simple  Ascidians  generally  situated  in  the  mantle  on  the  left 
side  of  the  ])ody,  owing  to  the  enormous  vlevelopment  of  the 
branchial  sac,  but  in  other  forms  it  constitutes  a  part  of  a 
visceral  mass  lying  immediately  below  the  posterior  end  of 
the  sac.  The  lesophagus,  beginning  at  the  lower  end  of  the 
sac,  forms  a  short  tube  which  opens  into  a  fusiform  stomach, 
from  the  further  end  of  which  the  intestine  (/)  arises.  This 
is  generally  bent  twice  upon  itself,  forming  thus  two  loops, 
and  en«ls  in  a  straight  piece,  the  rectum,  which  opens  by  the 


TYPE  PROTOCIIORDATA. 


623 


he 
}ft 
lie 
a 
lof 
lie 

r 


anus  {an)  into  the  atrial  cavity.  A  thickeuiiig  occurs  along  the 
entire  length  of  the  inner  surface  of  the  intestinal  wall,  form- 
ing the  typhlosole,  and  a  number  of  branched  tubules  open- 
ing into  the  stomach  (.v)  represent  a  so-called  "liver"  or 
digestive  gland. 

The  nervous  S3stem  consists  of  a  single  ganglionic  mass 
(c(i)  lying  on  the  dorsal  side  of  the  body  near  the  anterior 
end  and  giving  off  nerves  both  anteriorly  juid  [K)steri<>rly. 
Immediately  below  it  is  a  glanilulur  structure,  the  subiieural 
gland  (s7i),  from  which  a  duct  passes  forwards  to  open  into 
the  anterior  part  of  the  branchial  sac,  at  the  base  of  a  wtdl- 
markod  papilla  which  may  possibly  be  sensory  in  function. 
The  gland,  from  its  relation  to  the  nervous  system  and  the 
branchial  sac,  has  been  compared  to  the  pituitary  body 
(hypophysis  cerebri)  of  the  Vertebrates. 

Sense-organs  are  but  slightly  developed,  pigment-spots 
situated  in  the  neighborhood  of  the  branchial  and  atrial 
apertures  perhai)s  representing  rudimentary  eyes,  while 
sensory  functions  have  been  attributed  to  the  tentacles,  the 
dorsal  languets,  and  the  pa})illa  at  the  opening  of  the  duct  of 
the  subueural  gland. 

An  excretory  function  has  been  assigned  to  the  subueural 
gland,  but  in  addition  to  this  there  are  found  in  the  visceral 
mass  a  number  of  spherical  bodies  {cr)  without  ducts,  in 
whose  cells  concreti»)ns  of  uric  acid  are  found.  These  setMii 
to  represent  excretory  organs,  the  waste  material  instea*!, 
however,  of  being  passed  out  from  the  body  is  stored  up  in 
the  cells  of  the  organs — a  condition  recalling  what  is  found  in 
some  Echiuoderms  ami,  to  a  certain  (extent,  the  arrangement 
in  the  Ectoproctous  Polyzoa. 

The  Tuuicates  are  for  tlie  most  part  hermaplirodites.  The 
ovary  is  a  ramified  structure  lying  in  the  loop  of  the  intes- 
tine, and  contains  a  cavity  lined  with  a  germinal  epithelium. 
An  oviduct  is  continuous  with  the  ovary  and  leails  towards 
the  atrial  cavity,  opening  into  it  in  close  proximit*  to  the 
anus.  The  testes  (Eig.  '2HIJ,  /)  are  numerous  spherical  bodies, 
also  situated  in  the  visc(>ral  mass,  each  portion  being  provided 
with  a  duct  which  joins  with  others  to  form  the  single  vas 


•r 


624 


INVERTEBRATE  MORPIIOLOO Y. 


deferens  (vcZ),  opeuing  into  the  atrial  cavity  near  the  ojDening 
of  the  oviduct. 

On  account  of  the  larval  characters  being  more  important 

than  the  adult  in  indicating  the 
affinities  of  the  Tunicates  and  in 
justifying  the  term  Urochorda  applied 
to  the  class,  it  seems  convenient  to 
depart  from  the  usual  arrangement 
and  consider  the  development  of  the 
simple  Ascidiaus  here,  before  passing 
on  to  a  description  of  the  various 
orders. 

Development  of  the  Simple  Asci- 
dians. — For  an  account  of  the  early 
development  reference  must  be  made 
to  embryological  text-books,  the 
present  description  being  coutined  to 
the  larva  and  the  changes  it  under- 
goes in  transforming  to  the  adult. 
Suffice  it  to  say  that  the  early  stages 
resemble  very  closely  those  of  Am- 
phio,i'us,  and  they  result  in  the  forma- 
tion of  a  remarkable  structure  usually 
known  as  the  Ascidian  tadpole,  a 
term  which  indicates  its  general  ap- 
pearance. This  larva  is  a  free-swim- 
ming organism  and  consists  (Fig. 
284)  of  an  anterior  somewhat  globular 
portion,  the  body,  and  a  posterior 
flattened  region,  the  tail.  The  entire 
body  is  enclosed  within  a  continuous 
case,  the  test  (c),  which,  in  the  tail 
region,  is  elevated  into  a  dorsal  and 
ventral  ridge,  serving  as  fins.  Upon 
the  anterior  end  of  the  body  are 
papilhe  {<tp)  which  serve  for  fixation 
when  the  larval  life  is  completed,  while  in  the  interior  of 
the  body  region  indications  of  the  various  adult  organs 
may   be   seen.     Certain    interesting    modifications   of  these 


FlO.  284.— DlAGllAM  OF  THE 

Tadpole  Lauva  of  a 
Tunicate. 

a  =  iiuus. 
ao  =  atrial  orifice. 
ap  —  udhesive  papilla. 
at  =  atrial  cavity. 

c  =  cellulose  tPst. 
ee  =  brain. 
en  =  eiidostyle. 

h  =  heart, 
m  —  mouth. 

n  =  nerve 
nc  =  Dotochord. 
ph  —  pharynx. 
sg  =  subueural  gland. 


I 


TYPE  PROTOCUORDATA. 


625 


'IK- 
liar 

ior 

ire 

us 
luil 

ud 

lire 

lou 

of 

Ins 

^se 


are,  however,  to  be  noticed ;  the  atrial  cavity  {at)  is  yet 
quite  small  and  the  anus  («)  may  or  may  not  communicate 
with  it,  while  the  mouth  (m)  is  not  functional,  owing  to  the 
test  being  continuous  over  it.  The  nervous  sj'stem  is  the 
most  remarkable  structure,  however,  consisting  of  a  large  an- 
terior saclike  structure,  the  brain  (ce),  into  the  cavity  of  which 
two  sense-organs  project.  Upon  the  dorsal  wall  is  the  eye, 
consisting  of  a  cup-shaped  group  of  cells  whose  inner  ends 
are  pigmented,  the  hollow  of  the  cup  being  occupied  by  a 
lens  above  which  is  a  membrane,  the  so-called  cornea.  Pro- 
jecting dorsally  from  the  ventral  wall  is  the  otolith,  consist- 
ing of  a  pear-shaped  cell  bearing  a  cap  of  pigment,  its  smaller 
end  extending  between  the  cells  of  the  lower  wall  of  the 
brain,  which  in  its  neighborhood  are  provided  with  stiff  cells 
forming  a  crista  acustica. 

Posteriorly  the  brain  is  prolonged  into  a  thick-walled  tube 
(w)  which  extends  back  almost  to  the  tip  of  the  tail,  to  the 
muscles  of  which  it  sends  off  nerves.  Beneath  the  nerve- 
tube,  throughout  nearly  its  entire  leugth,  lies  a  notochord  {nc) 
which  serves  as  a  skeletal  support  to  the  tjiil,  and  on  each 
side  of  it  is  a  plate  of  longitudinal  muscle-fibres. 

By  means  of  energetic  lateral  movements  of  the  tail  this 
larva  swims  about  for  some  time,  but  when  about  to  trans- 
form itself  into  the  adult  it  fastens  to  some  solid  object  by 
one  of  the  adhesive  papillte.  The  tail  with  its  nervous  sys- 
tem, muscles,  and  notochord  then  undergoes  degeneration 
and  is  completely  absorbed,  the  portion  of  the  test  covering 
it  being  thrown  off,  and  a  rotation  of  the  body  takes  j)lace  so 
that  the  mouth  comes  to  lie  at  the  opposite  end  of  the  body 
from  the  point  of  fixation.  The  branchial  and  atrial  aper- 
tures form,  and  the  anterior  saclike  brain  collapses,  the 
sense-organs  degenerate,  and  the  adult  brain  is  gradually 
developed.  The  extension  of  the  atrium  and  the  develop- 
ment of  additional  stigmata  complete  the  acquisition  of  the 
adult  characteristics. 

It  will  be  seen  from  this,  then,  that  the  larvrei  are  of  great 
importance  in  estimating  the  systematic  position  an<l  the 
affinities  of  the  Urochorda,  and  that  the  adults,  except  in  the 
Appendicidarice,  in  which  the  tail  and  the  free-swimming  habit 


•r 


626 


INVERTEBRATE  MORPHOLOGY. 


1     ; 


are  persistent,  are   to  be  regarded  as   degenerate.     Several 
orders  of  Tuuicates  may  be  recognized. 

1.  Order  Larvacea. 

To  this  order  belongs  the  genus  Appendicvlaria  which  has 
already  been  several  times  mentioned.  It  is  throughout  life 
free-swimming  and  retains  the  larval  tail,  greatly  resembling 
in  general  appearance  a  tadpole  larva.  It  secretes  an  exten- 
sive test  which  is  gelatinous  in  consistency  and  is  but  loosely 
attached  to  the  body,  being  frequently  thrown  off  shortly 
after  its  formation.     The   body  (Fig.  286)  is   comparatively 


Fig.   885.— An   Appendfcularian,    Oikopleura   cophocerca   (after  Fo)  from 


I 


I 


f 


a  =  auus. 

c  =  notocbord. 
d'  =  phiirynx. 
d"  =  stoiuach. 
en  =  eiulostyle. 


Hkrtwi«». 

/=  ciliuted  groove. 
g  =  bridu  with  audi  I  cry  vesicle. 
g'  —  first  guiigliou  of  tail. 
h  —  testis. 
ov  =  ovary. 
«  =  brauchial  cleft. 


small,  the  tail  being  attached  to  its  ventral  surface,  while  its 
posterior  extremity  is  somewhat  enlarged  and  contains  the 
reproductive  organs  {ov  and  h).  The  brauchial  sac  has  but  a 
single  pair  of  stigmata  («)  which  open  to  the  exterior  by  a 
pair  of  fuuuel-like  tubes  situated  behind  the  auus.  This  ar- 
rangement represents  exacliy  a  condition  present  in  the  larvas 
of  other  Tunicates,  two  stigmata  tirst  forming  and  the  atrial 
sac  arising  as  two  separate  invaginations  of  the  body-wall 
into  which  the  primary  stigmata  open,  the  invaginations  only 


■Ml 


TYPE  PHOTOCHORDATA. 


027 


later  fusing  to  form  the  extensive  atrium.  The  eudostyle  {en) 
is  very  short,  and  the  nervous  system  is  constructed  upon  the 
larval  plan,  though  the  brain  (</)  is  not  the  sensory  sac  so 
characteristic  of  the  larva.  It  is  more  ganglionlike  and  sends 
off  branches  to  an  otocyst  lying  in  its  neighborhood,  and  also 
gives  rise  to  a  tubular  prolongation  opening  into  the  terminal 
portion  of  the  branchial  sac.  Pigment-spots,  probably  rep- 
resenting rudimentary  eyes,  are  also  present,  and  from  the 
posterior  end  of  the  brain  a  tubular  nerve-cord  arises  which 
passes  backwards  and  downwards  into  the  tail,  which  it  trav- 
erses, dilating  at  intervals  into  ganglia  ({/').  The  notochord 
(c)  lies  below  the  caudal  nervous  system,  and  the  lateral 
muscles  of  the  tail  show  indications  of  metameric  segmen- 
tation. 


2.  Order  Ascidiacese. 

The  members  of  this  order  present  the  structural  features 
which  have  been  described  as  typical  for  the  class.  Many 
forms  possess  the  power  of  n(m-sexual  reproduction  by  bud- 
ding, colonies  being  thus  produced  some  of  which  may  be 
free-swimming,  the  simple  forms,  however,  being  always  fixed. 
They  differ  from  the  Larvace?e  chiefly  in  the  absence  of  the 
tail  in  the  adult  and  in  the  large  development  of  the  branchial 
sac  and  the  numerous  stigmata. 

Owing  to  the  complexities  produced  by  the  methods  of 
budding  it  is  customary  to  divide  the  order  into  subordinate 
groups. 


^t 


I.  Suborder  Ascidicp  simpUces. 

The  simple  Ascidians  agree  with  the  description  given  as 
typical  and  do  not  require  any  further  notice  here,  except  to 
mention  the  fact  that  there  are  included  within  the  suborder 
a  few  genera  which  reproduce  by  budding,  e.g.  Cht  veil  inn  and 
I*erophora.  The  formation  of  new  individuals  takes  place  in 
these  cases  from  stolonlike  outgrowths  of  the  parent  form, 
and  each  bud  remains  seated  u])on  the  stolon  surrounded  by 
its  own  test.  The  stolon  (Fig.  280)  arises  from  the  lower  [lor- 
tion  of  the  body  of  the  parent  and  ])ushes  before  it  a  portion 


628 


INVERTEBHA  TE  MOliPlIOLOG  7. 


m 


of  the  test ;  the  canity  it  contains  is  continuous  with  the  body- 
coelom  and  is  tlierefore  lined  by  mesoderm,  and  is  divided 

into  two  compartments  by  a  longitudinal 
partition  which  may  be  traced  back  to 
its  origin  from  the  posterior  wall  of  the 
branchial  sac  of  the  original  individual. 
Since  ectodermal  tissue  lies  between  the 
mesoderm  and  the  inner  surface  of  the 
test,  the  stolon  contains  portions  of  all 
three  germ-layers,  and  a  portion  of  each 
passes  into  each  bud  (6)  as  it  arises. 
The  first  indication  of  a  bud  is  a  slight 
wartlike  elevation  of  the  wall  of  the 
stolon  which  increases  in  size,  its  cavity 
being  a  diverticulum  of  the  stolon-ccelom. 
Fig.  286.-  PoiiTioN  of  a  luto  the  elevation  a  process  (en)  of  the 
Sioum  OP  /'^^''^''^'^endodermal  stolon-partition  extends,  and, 

(after   Kowalkwsky   from  _  ^  ^  '    _        ' 

KoRscHEi.T  and  heidkb).    forming    a    hollow    saclike    body,    gives 

rise  to  the  digestive  tract  of  the  bud. 
The  various  layers  give  rise  to  their 
respective  organs  with  one  exception, 
and  that  is  that  the  atrial  walls,  the  man- 
tle, arise  from  the  endodermal  branchial  sac  as  diverticula 
which  unite  together,  the  atrial  cavity  being  thus  lined 
throughout  with  endoderm.  Such  anomalies  are  not  infre- 
quent in  the  Urochorda,  and  indicate  a  necessity  for  further 
study  of  the  nature  of  the  germ-layers  in  these  forms. 

The  simple  non-budding  forms  are  quite  numerous.  Com- 
mon genera  are  iMolgnla,  Cynthia  in  which  the  test  has  a 
leathery  consistency  owing  to  the  fibrillar  character  of  the 
matrix,  and  Boltenia,  a  stalked  form. 


b  =  bud. 
br  =  branch  of  stolou. 
ec  =  ectoderm. 
en  =  eudoderin. 


2.  Suborder  Ascidm  compusitcB. 

All  the  members  of  this  order  reproduce  by  budding  in 
some  form  or  other,  and  diflfer  from  such  forms  as  Chwelli'na 
in  that  all  the  individuals  remain  imbedded  in  a  common  test 
whether  or  not  they  remain  in  organic  connection  with  one 
another.    The  group  seems  to  be  a  somewhat  composite  one, 


TYPE  PliOTOCUOlWATA. 


629 


uud  it  is  probable  that  it  is  au  aggregutiou  of  forms  with  quite 
distinct  derivations.  The  varieties  of  budding  which  are 
found  are  quite  numerous.  In  some  cases  it  closely  resem- 
bles that  of  Clavellina,  some  species  of  Distoplia,  for  example, 
possessing  a  short  stolon,  essentially  similar  to  that  of  the 
former  genus,  from  which  buds  arise  which,  however,  early 
separate  from  the  stolon  and  remain  imbedded  in  the  thick 


Fig.  287.—^.  Young  Solitary  Amnro^cium  dkvkt,opkd  from  Egg:  B,  k 

SLIGHTLY  OLDKK  FoUM,  IN   WHICH   THE  PoSTABDOMKN   IS  8EOMKNTKD  ;    C, 
TUB  Non-SEXUALLY  PKOUUCED  Forms   migrating  towards  TUE  tJtRFACK 
OF  THE  Test  (Hft«r  Kowalbwmky  from  Korschelt  and  Heider). 
a  =  puient  iudividual.  b  =  bud  which  has  reached  the  surface. 

c  =  luigmting  buds. 

test  of  the  parent,  a  repetition  of  this  process  through  several 
generations  leading  to  the  formation  of  massive  colonies. 

In  Amarcedum  the  arrangement  is  a  little  different,  but  can 
readily  be  traced  back  to  the  stolon  form.  The  posterior  end 
of  the  body  in  budding  forms  is  continued  backwards  as  a 
long  postabdomen  (Fig.  '287,  A),  having  a  structure  similar  to 
the  stolon  of  Clavellina,  but  instead  of  giving  off  buds  the 
stolon  segments  into  a  number  of  parts  (Fig.  287,  B)  which, 


630 


INVERTEBRATE  MORPHOLOOY. 


siyi 


separating,  rise  through  the  thick  test  of  the  parent  (Fig.  287^ 
C)  until  they  reacli  the  surface,  and  develop  to  complete  in- 
dividuals, in  which  the  process  may  be  repeated.  Amaroecium 
thus  forms  massive  colonies  consisting  of  a  number  of  quite 
separate  individuals  all  imbedded  in  a  common  test,  and  all 
directly  or  indirectly  the  result  of  the  budding  of  a  single  in- 
dividual developed  by  the  sexual  method. 

In   other   forms,  however,  the  stolon  is  practically  sup- 
pressed and  the  buds  arise  directly  from  the    body  of  the 

b 


|iHl 


Fig.  288.— a  System  op  Six  Individuals  fkom  a  Compound  Colony  of 

BotryllUit  (after  Oka). 
a  =  adult  individual.  ecp  =  eclodernial   processes   extending 

b  =  bud.  into  test  from  each  individual. 

cl  —  commou  cloaca.  m  =  mouth  of  one  of  the  individuals. 

parent.  This  is  the  case  in  Didemnum  and  Tridemnum,  for 
instance,  peculiar  complications  being  also  introduced  into 
the  process.  In  the  latter  form  the  daughter  individual  arises 
as  two  buds  which  later  fuse.  One  arises  from  the  upper  end 
of  the  oesophagus  and  gives  rise  to  the  intestine  and  neigh- 
boring organs  in  the  bud,  while  the  other,  arising  from  the 
branchial  sac,  gives  rise  to  that  structure,  the  atrium,  and 
terminal  portion  of  intestine.  Usu.ally  the  two  buds  arise 
simultaneously,  but  occasionally  one  may  fail,  the  result  be- 
ing the  production  of  half  individuals  which  remain  united 
with  the  parent,  producing  double  monsters  ;  and  since  either 


TYPE  PROTOCUOHDATA. 


631 


{Y  OF 


bud  may  fail,  these  monsters  may  have  either  a  double  bran- 
chial sac,  atrium,  and  rectum,  or  a  double  digestive  tract,  heart, 
and  other  organs  of  the  visceral  mass.  If,  however,  both 
develop  simultaneously,  in  the  course  of  time  the  jjorfect 
daughter  form  separates  from  the  parent.  In  Botryllus  (Fig. 
288)  but  a  single  bud  (6)  arises  from  each  individual  of  the 
colony,  developing  on  one  side  of  the  body  in  the  region  of 
the  branchial  sac,  early  separating  from  the  parent  except 
for  a  slight  stalk  like  union.  After  the  formation  of  the 
daughter  individu.'^s  the  parent  forms  die,  and  so  a  succes 
sion  of  generations  occurs,  the  various  individuals  of  each 
generation  being  arranged  in  a  circle  around  a  common  cloaca 
{cV)  into  which  the  atrial  aperture  of  each  opens. 

The  origin  of  a  colony  of  Botryllus  takes  place  as  follows  :  An  individ- 
ual developing  from  an  egg  fastens  itself  and  gives  rise  to  a  single  bud, 
which  remains  imbedded  in  the  test  of  the  parent,  which,  on  its  part,  dies 
and  disappears.  The  individual  of  this  second  generation  gives  rise  to  two 
individuals  of  the  third  generation  and  then  likewise  dies,  while  each  of 
the  members  of  the  third  generation  gives  rise  to  two  more  members  of  a 
fourth  generation  and  degenerates.  The  four  individuals  so  formed  ar- 
range themselves  so  that  they  radiate  from  a  common  point  at  which  a 
depression  of  the  test  occurs,  forming  the  cloaca.  New  generations  tJieu 
succeed  each  other,  the  parents  of  each  generation  in  tin-n  degenerating, 
and  so  the  colony  extends,  and  some  of  the  individuals  failing  to  form  a 
connection  with  the  original  cloaca  become  centres  for  a  new  radiating 
colony,  still,  however,  imbedded  in  the  common  test. 


¥ 


into 
L'ises 

end 
^igh- 

the 

and 
lirise 
be- 

lited 
Ither 


3.  Suborder  Pyrosomidw. 

In  this  order  there  is  but  a  single  genus,  Pyrosoma,  which 
includes  floating  pelagic  colonies  having  the  shape  of  a  cylin 
der  closed  at  one  end,  the  individuals  composing  the  colonies 
being  enclosed  in  a  common  test  and  arranged  radially  aroui'  ' 
the  central  cavity  or  cloaca  into  which  their  atrial  apertures 
open,  the  branchial  apertures  opening  on  the  exterior  of  the 
cylinder.  Each  individual  resembles  in  structure  a  simple 
Ascidian,  the  principal  diflerence  being  that  the  atrial  aper- 
ture, as  in  Botry/lus,  is  at  the  posterior  end  of  the  body,  and 
that  each  individual  has  the  power  of  reproducing  by  budding 
and  so  assisting  in  the  further  growth  of  the  colony,  the 
parent  forms  not,  however,  degenerating  after  giving  rise  to 


632 


INVEHTKBRATE  MOHPHOLOOY. 


buds,  as  iu  Botrylhis.  The  buds  arise  from  the  branchial 
sacs  behind  the  eudostyle,  and,  on  separatinjj;  from  the  parents, 
place  themselves  between  them  and  the  opening  of  the  com- 
mon cloaca,  so  that  the  oldest  members  of  the  colony  lie  at 
the  closed  end  of  the  cylinder.  On  each  side  of  the  branchial 
sac  of  each  individual  near  the  anterior  end,  or  more  precisely 
near  the  peripharyngeal  ciliated  bauds,  is  a  mass  of  cells 
which  are  brightly  phosphorescent,  the  entire  colony,  which 
may  reach  a  length  of  over  a  metre,  emitting  a  brilliant  light 
when  Swl.nulated. 

The  development  of  Pyrosoma  is  exceedingly  interesting 
inasmuch  as  it  presents  an  alternation  of  generations.    From 


A  B 

Pig.  289.— Lakvai.  Budding  of  Pyrosoma.  A,  embryo  divided  into  the 
cyatbozooid  aud  four  uscidiozouids;  B,  later  stage  showing  the  ascidiozo- 
oids  twisting  to  form  the  circle  of  four  primary  individuals  (after  Kowa- 

LEWSKT). 

cl  =  cloaca.  en  =  endostyle. 

el  =  elseoblast.  h  =  heart. 

n  =  nerve  ganglion  of  ascidiozooid. 

the  embryo  which  develops  from  the  egg  at  a  very  early 
stage  a  stolon  develops  (Fig.  289),  containing  a  prolongation 
of  what  corresponds  to  the  embryonic  branchial  sac  and  also 
of  the  embryonic  mesoderm.  The  embryo  itself  never 
reaches  a  full  development  and  is  termed  the  Cyathozooid, 
serving  to  supply  the  individuals  developed  from  the  stolon 
with  nourishment  until  they  have  reached  a  certain  stage  of 
development.  This  it  is  able  to  do  on  account  of  the  ovum 
being  plentifully  supplied  with  yolk,  which  the  Cyathozooid 
gradually  absorbs.     The  stolon  at  an  early  stage  divides  into 


'mit 


TYPE  PHOTOCUOIWATA. 


633 


four  parts  (Fig.  289,  .1),  which  arrange  themselves  iu  a  circle 
arouud  the  Cyathozooid  (Fig.  289,  B)^  being  euclosed  with  it 
iu  a  common  test,  and  develop  eventually  into  the  four 
primary  individuals  or  Aacidiozooids,  which  occupy  the  closed 
end  of  the  cylindrical  colony.  As  thoy  develop  the  Cyatho- 
zooid degenerates  and  finally  completely  disappears,  the 
cloacal  cavity  of  the  colony  arising,  as  in  Botrj/Uus,  as  a  de- 
pression of  the  test,  which  groAvs  deeper  and  deeper  as  new 
individuals  arise  by  budding. 

Tho  oiitiiv  process  shows  a  luarked  similarity  to  what  occurs  in  Botryl- 
In^i,  the  principjil  differences  being  that  the  Cyathozooid,  which  represents 
the  first  generation  of  Botryllus,  buds  while  yet  in  an  embryonic  condition, 
lind  that  it  alone  degenerates,  the  parents  of  succeeding  generations  per- 
sisting and  forming  parts  of  the  fully-developed  colony.  In  each  Ascidio- 
zooid  there  occure  behind  tho  branchial  sac  a  pair  of  peculiar  masses  of 
cells  termed  the  elveoblast  (Fig.  289,  B,  el),  whose  function  is  uncertain, 
though  it  has  been  suggested  that  they  may  represent  the  rudimentary 
larval  tail. 

3.  Order  Thaliacea. 

The  Thaliacea  are  with  a  single  exception  pelagic  organ- 
isms, and  present  a  life-history  complicated  by  the  occur- 
rence of  an  alternation  of  generations.  In  tlie  genus  Salpa 
(Fig.  290)  a  well-developed  test  is  present,  and  the  muscula- 
ture of  the  mantle  is  arranged  in  bands  which  do  not  (^uite 
surround  the  body  and  furthermore  show  a  tendency  to  unite 
together  on  the  dorsal  surface  of  the  body.  At  one  end  of 
the  body  is  a  large  branchial  aperture  (m)  leading  into  a 
branchial  sac,  in  the  floor  of  which  is  the  endostyle  {en), 
while  iu  its  roof  is  a  ridge,  the  dorsal  lamina,  which  anteriorly 
is  produced  into  a  single  languet  projecting  into  the  interior 
of  the  sac.  On  each  side  of  the  dorsal  lamina  is  a  large 
opening  which  represents  an  enormous  stigma  and  communi- 
cates with  the  atrial  cavity,  whose  aperture  is  situated  at  the 
posterior  end  of  the  body.  The  intestine  and  the  other  viscera 
are  grouped  together  to  form  a  small  mass  termed  the  nucleus 
{nu)  lying  behind  the  branchial  sac  and  ventrall}',  though  in 
some  forms  the  intestine  is  more  elongated  and  projects  some- 
what from  the  nucleus.  The  nervous  system  {n)  has  its  usual 
form  and  position  ;  it  has  in  connection  with  it  three  pigmented 


r 


>9' 


lit 


ii! 


634 


IN  VKHTUBHA  TK  MORPUOLOG  Y. 


spots  probably  represeutiug  eyes,  aud  the  Bubueurul  gluud  is 
preseut  as  usual. 

Each  species  of  Sulpa,  however,  presents  two  distiuct 
forms  (A  and  B),  dift'eriug  iu  shape  aud  iu  the  uumber  of  the 
muscle-bauds  which  are  fouud  iu  the  mautle  aud  having  like- 
wise a  diflereut  origin.  In  the  sexual  form  {A)  reproductive 
organs   are  developed,  the  ovary  usually  containing   but  a 


em 


ma 


Fig.  290.—^,  Salpa  mueronata,  the  SEXUAii  Form,  and  B,  Salpa  democro' 

tica,  THE  Non-Sexual  Form  of  Salpa  democratica'ViucronaUi  (after  Claus). 

c,  cl  =  cloaca.  in  =  brunchial  pore  (mouth). 

cp  —  ciliated  pit.  ma  =  test. 

em  =  embryo.  n  =  uerve-guugliou. 

en  =  eudostyle.  nu  =  nucleus. 

h  —  heart.  at  =  stolon. 

single  ovum.  This  when  fertilized  (em)  is  passed  into  the 
atrial  cavity,  the  follicle-cells  with  which  it  is  surrounded 
forming  an  adhesion  to  the  wall  of  the  cavity,  and  later 
modifying  to  form  a  structure  recalling  the  placenta  of  the 
Mammalian  Vertebrates  by  which  nourishment  is  conveyed 
from  the  parent  to  the  embryo.  As  the  result  of  the  develop- 
ment of  this  ovum  the  non-sexual  form  (5)  is  produced,  which 
is  characterized  not  only  by  its  general  form,  but  also  by  the 
possession  of  a  stolon  (5^)  arising  from  the  branchial  sac  just 


raiKi 


TYPE  riiOTOCUORDATA. 


635 


behind  the  posterior  end  of  the  endostyle.  This  stolon 
eventually  divides  into  a  large  number  of  parts,  each  of  which, 
after  undergoing  certain  somewhat  complicated  shiftiugs  of 
position  on  the  stolon,  develops  into  the  sexual  individual. 
The  terminal  portions  of  the  stolon  with  the  maturer  individ- 
uals from  time  to  time  break  off  and  tiout  about,  forming 
what  are  termed  the  Salpa-chains,  and  the  constituent  individ- 
uals of  the  chain,  the  sexual  individuals,  becoming  mature 
either  while  still  united  with  their  fellows  or  after  separation 
from  them,  start  again  the  life-cycle  which  may  be  represented 
by  the  following  scheme  : 

Ovum  =  Solitary  Salpa Chaiu  Salpa Ovum. 

(Nuii-seximl)  (Sexual) 

In  another  genus  belonging  to  the  order,  DoUohcm  (Fig. 
291),  the  process  is  somewhat  more  complicated.  The  mem- 
bers of  this  genus  are  barrel-shaped  forms,  the  wide  branchiiil 
aperture  being  situated  at  one  end  and  the  atrial  aperture  at 
the  other  ;  the  stigmata  are  fairly  numerous  and  arranged  in 
two  lateral  rows,  and  the  mantle  muscle-bands  form  complete 
circles  around  the  body,  resembling  the  hoops  of  the  barrel. 
From  the  ovum  there  develops  a  peculiarly-constructed  tailed 
embryo,  which,  with  the  loss  of  the  tail,  becomes  converted 
into  the  non-sexual  form  (Fig.  291,  A)  characterized  by  the 
possession  of  nine  circular  muscle  bands,  an  otocyst  {ot)  situ- 
ated some  distance  from  the  nervous  system  on  the  side  of 
the  body,  a  ventral  stolon  (st),  and  a  dorsal  posteriorly- 
directed  process  {dp).  From  the  stolon  a  number  of  buds 
are  produced,  which,  at  an  early  stage,  separate  from  the 
stolon  and  migrate  to  the  dorsal  process  to  which  they  attach 
themselves,  the  migration  being  accomplished,  it  is  said,  by 
means  of  amceboid  cells,  probably  cells  of  the  test,  which 
attach  themselves  in  pairs  to  the  base  of  each  bud  and  serve 
to  convey  it  to  the  dorsal  process.  Upon  this  process  the 
buds  arrange  themselves  in  three  rows,  the  individuals  of  the 
lateral  rows  developing  into  forms  quite  diflfereut  from  those 
resulting  from  the  development  of  the  buds  of  the  median  row. 
The  lateral  buds  when  freely  developed  are  characterized  by 
the  possession  of  a  large  branchial  aperture,  which  occupies 


•" 


636 


INVEHTEBRA TE  MOJiPIIOLOO  Y. 


almost  the  entire  length  of  one  side  of  the  body  and  leads  into 
a  branchial  sac  whose  stigmata  open  directly  to  the  exterior, 
the  atrial  cavity  disappearing  duriug  the  course  of  develop- 
ment. The  intestine  is  well  developed,  but  the  musck>s  are 
but  slightly  indicated,  while  the  reproductive  organs,  rudi- 


ot      en     1,       j^t         J- 


1      g 

Fro.  291.— J,  The  Non-sexual,  and  B,  the  Sexual,  Foum  op  Loliolum 

(after  Ulianin). 

cl  =  cloiica.  »  =  iutestinc'. 

dp  =  dorsal  process.  n  =  nervt'-gauglion. 

en  =  eudoslyle.  ot  =  otocyst. 

g  =  reproductive  organ.  pJi  =  pharynx. 

h  =  heart.  st  =  stolon. 


ments  of  which  were  present  in  the  young  buds,  completely 
atrophy  during  the  process  of  development.  These  buds 
are  incapable  of  leading  a  free  existence,  serving  only  as 
nutritive  and  respiratory  individuals  for  the  median  buds,  as 
well  as  for  the  parent,  whose  digestive  tract  degenerates,  its 
muscle-bands  and  nervous  system  at  the  same  time  under- 


ifSS^S.! 


T Yl'K  riiOTOV/WUDA  TA . 


637 


going  eulargemeut,  so  that  it  serves  eveutiially  as  a  locomotor 
iudiviclual  lor  the  eutire  aggregation  A  iudividuals. 

With  regard  to  the  median  buds  dilierences  of  opinion 
occur.  Certain  of  tliem,  or  according  to  one  authority  all, 
being  set  free,  develop  into  forms  possessing  but  eight  muscle- 
bands,  no  otocyst  or  dorsal  process,  but  having  a  ventral 
median  process  upon  which  buds  are  found.  It  is  in  regard 
to  the  origin  of  these  buds  that  the  dilTerence  of  opinion 
exists.  According  to  one  view  they  are  produced  from  the 
ventral  process  which  is  regarded  as  a  stolon  and  represent  a 
third  generation,  v»hile  according  to  the  other  view  they  are 
certain  members  of  the  median  H'V>  of  the  second  generation, 
the  forms  bearing  them  being  as  it  were  their  sisters  and 
serving  as  nurses  ft)r  them.  Whatever  may  be  their  origin, 
however,  the  buds  eventually  are  transformed  into  sexual 
individuals  (Fig.  291,  /?)  with  which  the  life-cycle  was  com- 
menced. The  two  views  as  to  the  cycle  in  DoJiolum  m-j  be 
schematically  represented  thus : 

Nutritive  iudividimis 


Ovum  —  non-sexual  form 


y 


Ovum  =  non-sexual  form' 


Nurses 

-Nutritive  individuals 


Nutritive  individuals 
Nurses 

-Sexual  forms Ova 

Nutritive  individuals 


-Sexual  forms — Ova 


Mention  should  be  made  of  the  geuus  (Macnemus,  a  deap-sea  form  be- 
longing to  this  '^nLi  vliich  is  apparently  attailicd  by  a  pedicle  to  f  :-eign 
bodies.  The  b,>dy  proper  is  tlattened  aud  disklike,  its  margius  '-Ai:-  pro- 
longed into  -ght  ta|)eriiig  processes.  Tlie  mouth  lies  on  the  surface  of  the 
disk  and  leads  into  a  shallow  branchial  sac.  The  atriimi  is  compaiatively 
hirge,  and  the  intestine  at  il  viscera  are,  us  in  iSalpa,  massed  together  in 
the  form  of  a  imcleus.  N(/thing  is  as  yet  known  as  to  the  life-history  of 
this  form.    ■ 

Affiiiitii's  of  the  Ur.K'fiordd.—Thiiiv  seems  little  :oohi  for  (loiil)t  but 
that  the  Uroehorda  aic  related  to  .1 ////>// /(U7^s■  The  twt>  groups  pre- 
sent too  many  eoramon  slructural  fealm-es  to  -How  of  their  l)eing  re- 
garded as  distincc  types,  but  at  the  same  tiint>  it  is  noticeable  that  the 
relationship  is  through  the  larval  Tunicates  ratlier  than  through  the 
adults.  These  laticr  are  degenerated  forms,  tin*  entire  group  lormiiig  a 
degcMierate  olT.set  from  the  main  line  of  (Solution  repre.Hented  by  the  Proto- 
chordata  and  leading  to  the  Vertel»rata.     The  early  stages  of  development 


638 


INVERTEBRATE  MORPHOLOGY. 


r' 


of  the  simple  Tunicates  (see  text-books  of  embryology)  are  so  very  similar 
to  those  of  Amphioxus  that  it  must  be  concluded  that  the  evolution  of  the 
Urochorda  and  Cephalocliorda  proceeded  for  some  distance  along  similar 
lines,  and  the  general  affinities  of  the  Protochordata  may  possibly  be  indi- 
cated by  a  scheme  thus  . 

Vertebrata 
Cephalochorda 


Hemichorda 


Urochorda 


Ancestral  Protochordata 

Taking  the  larval  Tunicates  as  a  basis  for  comparison,  wo  find  as 
features  coniiuun  to  them  and  Amphioxus  a  dorsal  nervous  system  aris- 
ing as  an  invagination  of  the  ectoderm  and  extending  the  entire  length 
ot  tlie  body  ;  in  the  anterior  portion  the  lumen  of  the  nerve-cord  expands 
to  form  a  l)niiii  which  in  Amphioxits  opens  in  early  stages  to  the  exterior 
anil  in  the  Tunicates  into  the  anterior  portion  of  the  branchial  sac,  i.e. , 
the  ectodernuil  portion,  the  canal  of  communication  in  the  latter  forms 
losing  in  later  stagi^s  its  collection  with  the  brain  and  forming  the  sub- 
neural  gland.  An  atrial  cavity  occurs  in  both,  which,  though  arising  in  a 
soinnwlial  dirt'eri!nt  manner  in  the  two  groups,  nevertheless  seems  quite 
hniuologous,  and  homologies  have  also  been  pointed  out  between  the 
primary  gill-slits.  The  increased  nuiul)er  of  stigmata  and  their  arrange- 
ment in  the  Tunicates  is  a  secondary  character  resulting  probably  from  the 
sessile  existence  ;  and  the  development  of  the  test  and  the  limitation  of  the 
notoeliord  to  the  tail  are  also  jirobably  secondary  cliaraeters.  The  resem- 
blances are  important  ones,  and  wlicui  taken  into  consideration  with  the 
embryonic  development  point  very  strongly  to  a  (flo.se  affinity. 

As  regards  the  relationships  of  \\\v  various  groups  of  Frochorda  to  one 
anotiier  considerable  difference  of  opinion  exists.  The  Appendicularians, 
which  at  first  sight  .seem  to  be  the  most  primitive  of  all  the  ordei's,  prrs-ent 
certain  remarkable  peculiarities,  such  as  the  .separates  openings  of  the  atrial 
cavities  and  the  anus,  and  .some  authors  are  inclined  to  regard  them  not  as 
primitive  forms,  but  as  sexually-mature  larvm  of  sessile  forms  in  which  a 
test  had  already  developed  and  dejfeniMation  far  advanced.  As  regards 
tlie  renuiining  forms  the  simple  Asci<lians  seem  to  be  the  most  primitive, 
the  composite  t'oiins  beinii  derived  from  them  by  ihe  acquisition  of  non- 
sexual reproduction.  The  comjiosite  forms,  however,  seem  really  to 
represent  several  groiq)s  originating  independently,  all  the  members  not 
having  desi^cnded  from  an  ancestral  simple  tortn,  but  .some  frimi  one 
ancestor  and  others  from  another,  and  so  on.     Thi!  Thaliacea,  finally,  have 


TYPE  PROTOCUORDATA. 


639 


probably  been  derived  from  composite  forms,  Pijrosoma  showing  certain 
affinities  in  its  budding  to  Salpa,  the  fact  of  some  individuals  being 
solitary  not  necessarily  indicating  a  primitive  character,  since  in  the  com- 
posite forms  no  organic  union  exists  K'ctween  the  various  individuals  of  the 
colony  when  they  have  reached  maturity,  so  that  the  so-called  colonies  are 
rather  aggregations  than  colonies,  and  the  forms  more  properly  termed 
social  than  colonial  or  even  composite. 

SUBKINGDOM  METAZOA. 
TYPE  PROTOCHOIWATA. 

I.  Class  Hkmichorda.— Body  divided  into  three  distinct  regions  ;  noto- 

chord  a  small  Hngerlike  diverticulum  projecting  forwards 
from  anterior  portion  of  digestive  tract,  with  which  it 
retains  connection. 

1.  Order   Pterobranc/iia.  —  Sessile    colonial    forms,    secreting   a 

"house";  intestine  bent  upon  itself;  collar  region  with 
lophophorelike,  tentacle-bearing  processes.  Cephulodis- 
ciis,  Rhdbdoplenra. 

2.  Order  Entero/meusta. — Free  forms  not  forming  colonies ;   not 

secreting  a  test ;  intestine  straight ;  collar  region  without 
lophophorelike  processes.     Balanoylosims. 

II.  Class  Cepbalochduda. — Free  forms,  not  pelagic  ;  not  secreting  a  test ; 

body  not  divided  into  three  distinct  regions,  with  numer- 
ous metameres ;  notochord  completely  sei)arated  from 
digestive  tract  and  traversing  the  entire  length  of  the 
body.  Aiuphioxua. 
III.  Class  UuocHOHUA.— Sessile  or  free  pelagic  forms;  secreting  a  test; 
body  in  adult  not  divided  into  three  distinct  regions 
and  showing  no  indications  of  metamerism  ;  notochord 
usually  wanting  in  adult,  |)r('senl  in  tail-region  of  larvie 
and  entirely  separate  from  digestive  tract. 

1.  Order  Lurvacea. — lietaining  the  larval  tail  "'ith  notochord  ;  free 

simple  pelagic  forms.     Apiwiidh'ularia. 

2.  Order  Asv.idi<iceii:.—^\m\)\Q  and  sessile  or  colonial  and  occa.sion- 

ally  pehigic  ;  tail  and  notochord  wanting  in  adult. 

1.  Suborder  Aschlia;  siinplices.  —  ii'niiyAii  sessile  forms  or  else 

budding  from  stolons  and  forming  somewhat  straggling 
colonies,  the  various  individuals  not  enclosed  in  a  common 
test.  Simple  forms,  Mi)/(/id(i,  Cynthia,  lioltenia  ;  Co- 
lonial, CliWelUuti,  Pirnplioni. 

2.  Suborder  AhcUHiv.  composiUv. — Colonial  forms,  the  various 

individuals  embedded  in  a  common  test;  if  the  various 
individuals  open  into  a  common  cloaca  the  colony  is  not 
pelagic.  AinaraviutHf  Didemnutn,  Tridemnnm,  Di- 
staplid,  liotnjUns. 


640 


IN  VERTEBRA  TE  MORPUOLOO  Y. 


3.  Suborder  Pi/rosoniida. — Colonial  forms,  the  various  in- 
dividuals imbedded  in  a  common  test,  and  opening  into 
a  common  cloaca ;  pelagic  ;  with  alternation  of  genera* 
tions.  Pyrosoma. 
Order  I'/m/mcra.— Simple  pelagic  forms,  with  alternation  of 
generations.     Salpa,  Doliolum,  Octacnemus. 


1 1 


;-l- 


LITERATURE. 


IIKMICIIOIIDA. 


W.  C,  Mcintosh.  Report  on  C'epfiiilodincns  dodecalophus,  Mcintosh,  a  New 
I'ype  of  the  Polyzoa.  Scient.  Results  of  the  Voyage  of  H.M.S.  Chal- 
lenger, XX,  1887. 

S.  F.  Harmer.  Appendix  to  Report  on  Cephalodiaeua.  Scient.  Results  of  the 
Voyage  «)f  H.M  S.  Challenger,  xx,  lf<87. 

£.  B.  Lankester.  A  Contribution  to  the  Knowledge  of  Rhabdopleura.  Quar- 
terly Journ.  Microscop   Science,  xxiv,  1884. 

G.  H.  Fowler.  The  Morphology  of  Rhabdopleura  Normanni  Allm.  Festschrift 
far  R.  Lcuckart.     Leipzig,  1893. 

A.  Agassii.  The  History  of  Dalanogloasus  and  l^ornaria.  Memoirs  American 
.-Vcad.  Arts  and  Sci.,  ix,  1893. 

W.  Bateson.  Tlie  Early  Stages  in  the  Development  of  Balanoglossus{8p.incert.). 
Quarterly  Journ.  Microscop.  Science,  xxiv,  1884. 

Tlie   Later  Stages  in  the  Development  (f  Bulanoglosaus  Kowalewskii, 

with  a  Suggestion  on  the  Affinities  of  the  Enteio/meusta.     Quarterly  Journ. 
Microscop.  Science,  xxv.  Suppl..  1885;  xxvi,  188(). 

The  Ancestry  of  the  Chordata.     Quarterly  Jouru.  Microscop.  Science, 


XXVI.  1886. 

T.  H.  Morgan.  The  Chrowth  and  Metamorphosis  of  Tornaria.  Journ.  of  Mor- 
phology, V,  1891. 

J.  W.  Spengel.  Die  Enteropneusten.  Fauna  uud  Flora  des  Golfes  von  Neapel. 
Monogr.,  xviii,  1893. 

T.  H.  Morgan.  Tfie  Development  of  Balanoglossus.  Journ.  of  Morphology, 
IX.  1894. 


CEPHALOCIIORDA. 


Mor. 


W.  Bolph.     Unterstichungen  i'tber  den  Bau  des  Amphioxus  lanceolatus. 

phologisches  Jahrbuch,  1876. 
B.  Hatsohek.     Studien  i'tber  Entwicklung  des  Amphioxus.     iLrbeiteu  aus  d. 

Zool.  Inst.  Wien,  iv,  1881. 

E.  Bay  Lankeater.     Contributions  to  the  Knoicledge  of  Amphioxus  lanceolatus, 

Tarrell.    Quarterly  Journ.  Microscop.  Science,  xxix,  1889. 
B.  Lwo£F.     Ueber  Bau  und  Kntwicklunij  der  Chorda  von  Amphioxtis.     Mitth.  a. 

i!    Zooloir.  Station  /,u  Neapel,  ix,  1889 
E  B.  Lankester  and  A.  Willey.     The  Derelopinent  of  the   Atrial   Chamber  of 

Amphioxus.     Quaitcrly  Journ.  Microscop.  Science,  xxxi,  1890. 

F.  E.  Weiss.     Kxcretory  Tubules  in  Amphioxus  lanceolatus.     Quarterly  Journ. 

Microscop.  Science,  xx.xi,  1890. 


TYPE  PROTOCIIORDATA. 


641 


of 


Mor. 

lis  d. 
\aiua, 
th.  a. 
\er  of 
)urn. 


A.  WlUey,     T7te  Later  Larval  Development  of  Ampliioxus.    Quarterly  Journ. 

Microscop.  Science,  xxxii,  1891. 
Th.  Boveri.     Die  Nierenkaniilclien  dea  Amphioxua.    Ein  Beitrag  zur  Pfn/logenie 
lies  Urogenitalayatema  der  Wirbelthiere.     Zoolog.  Jahrblichor,  v,  1892. 

B.  Hatsoliek,     Die  Metumerie  dea  Amphioxua  und  dea  Ainmocetea.     Aiititom. 

Anzeiger,  Ergiliizungs-Heft,  vii,  1892. 
W.  B.  Benham.     The  Structure  of  the  Pharyngeal  Bars  of  Amphioxua.    Quiir- 

terly  Jouru.  Microscop.  Science,  xxxv,  1893. 
E.  B.  Wilson.     Amphioxua  and  the  Moaaic  Theory  of  DeteUypment.    Journal  of 

MorpUology,  viu,  1893. 

uuocnouDA. 

T.  H.  Huxley.  Observationa  on  the  Anatomy  and  Phyaiology  of  Salpa  and 
Pyroaoma.     Philosoph.  Transactions  Royal  Society,  Loinlon,  1851. 

A.  Kowalewsky.  Entwickluitgagesehichte  der  einfachen  Aacidien.  JMenioires 
Acad.  St.  Petersbourg,  7ine  ser.,  x,  1866. 

C.  Knpffer.     Die  Stammeseerwandtachaft  zwiachen  Aacidien  und  Wirbelthieren. 

Archiv  fl\r  mikr.  Anatonue,  vi,  1870. 
A.  Kowalewsky.      Weitere  Studien  iiber  die  Entwicklung  der  einfachen  Aaci' 

dien,     Archiv  fl\r  mikr.  Analoniie.  vii,  1891. 
H.  Fol.     Etudes  aur  lea  Appendiculaires  du  detroit  de  Meaaine.     Memoircs  Soc. 

de  Physique  et  d'Hist.  Nat.  Geneve,  xxi,  1872. 
A.  Oiard.    Recherehea  aur  les  Synaacidiea.   Archives  de  Zool.  exper.  et  generale, 

I,  1872. 
H.  de  Lacaze-Duthiers.     Lea  Aacidiea  simples  des  cdtea  de  France.     Archives  de 

Zool.  exper.  et  gen.,  iir,  1874. 

A.  Kowalewsky.     Ueber  die  Knoapung  der  Aacidien.     Archiv.  fUr  mikr.  Aua- 

tomie.  x,  1874. 
W.  0.  Herdman.     Beport  on  the  Tunicata.     Scientific  Residts  of  the  Voyage  of 

HM.S.  Challenger,  vi.  1882;  xiv.  1886;  xxvn,  1888. 
L.  Soale.     Becherchea  aur  lea  Aacidiea  aimplea  dea  cotea  de  Provence.    Annales 

Musee  d'Hist.  Nat.  Marseille,  ii.  1884-5. 

B.  Ulianin.     Doliolum.     Fauna  und  Flora  des  Qolfes  von  Neapel.     Monogr., 

X,  1884. 
E.  van  Beneden  et  C.  Julin.     Tje  ayati t,.e  nerveux  central  dea  Aacidiea  adultea  et 

sea  Rapports  avec  celui  dea  larves  Urodelea.     Archives  de  Biol  ,  v,  1884. 
Becherchea  sur  ia  Morphologie  dec  Tuniciera.     Archives  de  Biol.,  vi, 

1887. 

C.  Maurice.     Etude  monographtque  d'une  Eaphe  d'Aacidie  composee  {Fragaroi- 

dea  aurantiacum  n.  ap.).     Archives  de  Biol.,  viii,  1888. 
0.  Seeliger.     Ziir  Entwiekbi ngageschichte  der  Pyroaomen,     J^naische  Zeitschr. 

filr  Naturvviss.,  xxiii,  1889. 
A.  Oka.      Ueber  die  Knoapung  der  BotryUiden.     Zntschr.  fl^i"  wisseusch.  Zoo- 

logie,  MV,  1892. 
A.  Willey.     Studies  on  the  l*rotorh&rdata.     Quarterly  .lourn.  IMicroscop  Scl- 

eiicc,  XXXIV,  189;i :  xxxv,  189;{. 
W.  K.  Brooks.      The  Genus  Salpa :    a  Monogr  iph.     With  a   Supplementary 

Paper  by  M.    M.    Melcalf.     Memoirs    Biolog.    Labor.    Johns    Hopkins 

Univ.,     II.  1893. 


(it  i" 


INDEX  OF  PROPER  NAMES. 

Group  na Je?rirHr ':i;;!rrr^^^^^^^   "•  ^^^  so,i^,,„a.., 

roman  type.  '^'•"^'^'  ««"«••««  "ames  in  italics,  and  popular  names  in 

ACANTHOCEPIIAT  \,  179.  188 

Acanthometra,  20,  39 
AcAuiNA.  458,  458 
Acervularla,  17,  38 
Achtheres,  3D7,  423 
Ackula,  322 
Acineta,  36,  89 
ActHd'a,  364 
AccKLA,  132,  169 

ACOTYLEA,  139,  169 
ACUASPKUA,  97 

Acliuian,  114 
Actinometra,  542,  593 
Actinommu,  39 
Actinophrys,  17,  39 
ActinoHph(trium,  20,  89 
Acn.EATA,  527 
^*>/«,  415,  424 
^/iW/«,  315,  364 
■yfJolosoma,  219,  251 
■^-h^quorea,  8(5.  no 
^■Eftclnui.  506.  526 
Af/alma,  92,  116 
^•I/A  /<(««,  450,  458 
Aglaoplieiiia,  «7,  ng 
Affrioii,  506.  526 
Aiptami,  113.  117 
Alciopr,  212,  251 
Alcippe,  399.  423 
Alcyonahi^;,  108,  117 
AlcyomUa,  261,  274 
Alcyonidium ,  2fjl,  274 
Aleyoniuin,  \m,  117 
Alj.oiocosi.a,  133,  169 
Alp/ieus,  412,  424 

6^ 


Amaroscium,  689,  689 
Amwba,  15,  88 
Amphineuka,  284,  863 
Amphioxus,  608,  639 
Ampiiipoda,  416,  424 
Amphijiorus,  167,  170 
Amphitrite,  213,  251 
Amphiura,  564.  592 
Ampullaria,  308.  364 
AnuOolid,  515.  527 
AiMchiKtii,  218,  251 
Anulges,  453.  459 
AiuiHa,  510,  .527 
Ancfiorella,  397,  428 
Aneltifimo.  402 
Anisoi'oua,  4];!,  424 
Annkmua,  202,  251 
Anodon,  33!),  365 
Anomia,  330,  365 
Antcdo/i,  542,  592 
AxTnoMKurs^:,  87,  116 
Antii(»7,<).\.  104,  117 
Antipatharuf,  lU,  U7 
Ants.  518 
Awtliin-d,  414,  424 
Annrida,  .loy,  5ag 
Ariiii».+:.  r»10 
■4p///;v,  .■»2T 
vl/;/j»,  .-)1H,  ,527 
Aplynia,  ;!13.  364 
Ai'oDA,  .",»»; 

Apj)€iidi\'ucnia,  626,  680 
ApsindiH,  414,  424 
Aptekygota   501,  526 

^;>M«.  387.  423 


ii 


,  f    tfr 


:?  J 


"I 


644 


HfniSX  OF  PHOPER  NAMES. 


AltACHNiDA.  435 

AuANK.«,  448,  458 
Avbucia,  580,  593 
Area,  330  365 
Arcella,  US,  38 

AuCillANNKLlDA,  211,  351 

ArchigeteH,  161,  170 
Ariciu,  209,  251 
Areuicola,  209,  251 
Argiojw,  271,  274 
Argonauta,  350,  365 
Arguliis,  307,  423 
^m«,  316,  364 
Arinadillidium,  414,  424 
Artemia,  386 
AuTiiuopouA,  368,  523 
AuTiiitosTitACA,  413,  424 
Aacaris,  48,  177,  182 
Abciuiace/K,  627,  630 
AsciDi^  composite:,  627,  639 
Ahcidi^  BiMi'iiicKS,  627,  630 
Asddians,  617 
Aacopodaria,  256,  274 

ASCOTHOUACIDA,  403 

AseUua,  414,  424 
Aspergilluin,  329 
Aspidiotus,  510,  527 
Asplanchna,  180,  200 
Asteruia,  556,  502 
Aaterina,  552,  592 

ASTKHOIUEA,  0.52,  593 

Aathenoaoina,  570,  593 
Aatraiigia,  114,  117 
AatropecteH,  556,  593 
Aatrophyton,  561,  592 
AtalaiUa,  300.  364 
^<«j;.  453.  4.50 
Atropoa,  500,  ,526 
yl<<««.  451,  458 
Aurelia,  101,  117 

AUTOKI-AGKLtiATA,  28,  39 

Autolijtua,  213,  251 

Balanogloaaus,  601,  639 
Balanua,  400,  423 
Barnacles,  398 
Basommatophoka,  317,  364 
Bddlura,  136,  170 


Beach -Qea,  416 
Bees,  518 
Beetles,  512 
Belemnitea,  360 
Beloatoma,  510,  537 
i?tvw,  121,  126 
Bipalium,  136,  170 
Z/j/'f/Mfl,  412,  424 
Biastoids,  550 
Bolina,  124.  126 
Bolteniu,  628,  639 
Bombua,  518,  527 
Bunellia,  241,  253 
Book-scorpion,  444 
Bopyrua,  415,  434 
Bothriocephalua,  153,  170 
Botryllua,  631,  630 
Bntchinua,  513,  527 
Braehionua,  189,  200 
Bkachiopoda,  268,  274 
Bkachydka,  412,  424 
Branchellion,  228,  252 
BuANCHiopoDA,  385,  433 
Bi'unchiostoma,  608 
Branchipua,  386,  428 
Branchiuka,  397,  433 
Brininga,  552,  503 
Bristle-tails,  .501 
Brillle-stars,  561 
BuYozoA,  255 
Bugs,  510 
Bugula,  262,  274 
/?«««,  313,  364 
i»«<A//.'»,  443,  458 

Caddis- flies,  5l5 
Cndulus,  322,  364 
Culamocrinua,  .544,  593 
Calanus,  3!»6,  423 
Calcakea,  73,  115 
CaUgun,  306,  423 
CalUnectes,  412,  424 
Caloptenus,  504,  5^0 
Calmoma,  513,  5'27 
Calyptrwa,  307,  364 
Cambarus,  412,  424 
Campanulakia;,  85,  116 
Campodea,  502,  526 


IHDEX  OF  PROPER  NAMES. 


646 


Camponotua,  518,  527 
Campy laspia,  408,  424 
Canctr,  412,  424 
Canthoeamptua,  81)6,  423 
Capitella,  206,  251 
Caprella,  416,  424 
Carabus,  513.  527 
Caravellu,  »2,  116 
Carinaria,  809,  364 
Carinella,  106,  170 
Carpocapsa,  516,  527 
Caryophyllivm,  152,  176 
Cecidomyitt,  60 
Centipedes,  484 
Cephai.ociiokda,  608,  639 
Cephalodiacus,  597,  639 
Cephalopoda,  340,  865 
Cephalothrix,  169 
CeraUum,  30,  39 
Cercomonas,  81,  89 
Cerebratulm,  116,  170 
Cekiantiik^:,  110,  117 
Cerianthus,  110,  117 
CE8TODA,  152,  170 
Cfes<?/«i,  124,  126 
Cetochilua.  396.  423 
Chtrtobmncliiis.  218,  251 
Chwtoderma,  285,  364 
Chfftognster,  222,  251 

ClI^TOGNATHA,  186,  200 

Cho'tonotus,  196,  200 
Cha:topoda,  204.  251 
Vhiirybdea,  101,  117 
Chelifer,  448.  458 
Chernes,  444.  458 
Chilodon,  84,  39 
CniLooNATiiA,  482 
CiiiLopODA,  484,  525 
Cim-osToMATA,  262,  274 
Chirodota,  585.  598 
C//^•to/i,  289,  364 
Chiionellns,  288.  364 
Chlamydomonas,  31,  39 
ChondracnntJius.  397,  423 
Chryaojm,  514,  527 
Ciiiarfrt.  510,  527 
Cicindela,  518,  527 
(7i(?«m,  580,  593 


CiLlATA,  38.  39 
CiituiiiPKDiA.  398,  423 
C'LAUOCEHA,  38H,  423 
C'luin,  339 
ClathruUm,  18,  39 
CY«ra.  87.  116 
Cldvelliita,  627.  639 
Clepniue,  236.  252 
67to/<«,  74,  115 
6Wow«,  314,  804 
Clmociunpa,  516,  527 
Clymenellu,  204 
Clypeaaler,  581.  598 

CLYPEA8TK01DEA,  581,  593 

C^^<««,  513,  527 
Cniuauia,  76,  116 
CoCCIDiE,  510 
CoccioiA,  24 
Coccinella,  512,  527 
Cockroach,  504 
Vodomja,  28,  39 

CtELENTEKA,  68,  115 

CoetojHanu,  125 
CoLEOPTEItA,  512,  527 
COLLEMBOLA,  503,  526 

CoUosaendeia,  464 
Colpidium,  37,  39 
Colpodn,  36,  39 
Convoluta,  132,  169 
CoPEPODA,  398,  423 
Coiul,  114 
Corallinm,  108,  117 
C0KNACU8P0N01A.  73,  115 
Cm'ophium,  417,  424 
COHUODKNTIA,  507,  526 
Coryctvua,  396,  423 
Corydalia,  514,  527 
Coryne,Sl,\\(i 

CoTYT.EA,  189,  170 

Crabs,  412 
Criiiua,  271,  374 
C:ra.vtish.  412 
Crickets,  504 
CitiNoiDEA,  541,  598 
Crisia,  274 
Criatatella,  260,  274 
cuustacea,  368 
Cryptopentameka,  527 


ill 


] 


i' 


I'j 


646 


INDEX  OF  PROPER  NAMES. 


Cryptophialus,  309,  428 
Chyi'totetuameua,  537 
Vteniza,  453,  45H 
Cteitodnliia,  58,  333 
CTKNoriiuiiA,  130,  136 
Ctenoplana,  135 
Ctknobtomata,  363,  374 
cuiiomedus^:.  101, 117 
Cucumariit,  585,  5U3 
Vulex,  530,  538 
CUMACKA.  407.  434 
Ciinina,  96,  116 
Cunoctantha,  84,  116 
CuiicuLioNiDi*:,  513,  627 
CuspuUirm,  'A'6'd,  365 
t\i/tiiiiU8,  416,  434 
Cyanea,  101,  116 
CyrliiH.  339.  365 
Cyclopa,  396,  433 
Cydoatoma,  308,  364 
CYCL08Tt)MATA,  363,  374 
Cymbuliopsts,  314,  364 
Cymothoa,  415,  434 
Cynijw,  51 H,  538 
Cynthia,  638,  639 
Cypliophthalmus,  447,  458 
Cyprtia.  307,  364 
Cypridina,  391,  433 
C:v;)m,  391,  433 
Cystofi<aoellata,  30,  89 
Cystoida,  550 
CV</'fcve,  391,  433 

Dnctylopiua,  510,  527 
Daphnia,  388,  433 
Daudebardia,  316.  364 
Decapooa  (CepliJilopoda),  359,  365 
Decapoda  ((^riistiicea).  410,  434 
ZJejWft,  585,  593 
Demodcx,  453.  458 
Deudroccelum,  136,  170 
Dendrogaater.  403,  433  , 

Dentalinm,  332,  364 
Dermnleichua,  453,  458 
Deumaptera,  504,  5;36 
7;cro,  318,  351 
Desmosticha,  580,  593 
Diadema,  580,  593 


Duipheromera,  505,  526 
Diaatylia,  408.  434 
DiHUAN(  itiA.  359,  366 
Die  ye  ma,  64 
DicYEMii)^:,  64 
Didnunum,  630,  680 
Difflugia,  16,  38 
Dinobryon,  38,  39 

DiNOFLAOEI-IiATA.  30,  89 

JJinophilua,  198,  300 
IHopatra,  312,  351 
DiuTocAUDiA,  305,  864 
Diphyea,  93,  116 
Z>{>);au;,  506,  526 
DiPLopoDA,  483,  536 
Di/ilozoon,  147 
DiPNEL'MONES,  453,  458 
Diporpa,  147 
DiPTEiiA,  519,  528 
DiscoMEUuSi*:,  101,  117 
Diacoaomu,  114.  117 
Diatuplui,  639.  689 
DiSTOMEiE,  147.  170 
Diatomum,  147,  170 
Lochmiua,  177,  183 
Dolioluvi,  635,  640 
Douderaia,  887,  364 
Z>o?7«,  315,  364 
Doryphora,  513.  527 
Dragou-tlies,  506 

Earwigs,  504 
EcAKuiNES.  269,  374 
Echinarachniua,  581,  593 
Echinocucumis,  5.  ;: 
ECIIKNOUEUA,  184,  200 
Echimdcrea,  184,  300 
ErillNOUEHMA.  531 

EciiiNoiuEA,  570,  593 
Echinorhynchna,  180,  188 
Eciiii'UEi*:,  340.  253 
Echinrna,  340,  253 
EcroPKOCTA.  357,  374 

EuUIOPHTHAIiMATA,  418 

Edicardaia,  117 
Edwauusia;.  109,  117 
Elasipoua,  585,  598 
ELATEIilDiE.  513,  537 


INDEX  OF  PROPER  NAMES. 


647 


Mlpidia,  S85.  598 
Kndopiiocta,  266,  274 
Enmtella,  880.  805 

ENTEUOi'NKUSTA.  601,  689 

Kntomostuaca,  885,  428 
Eutoniacus,  415,  424 
Epeira.  450,  458 
Ephemera,  504,  526 
Ephemeridm,  505,  526 
Ephydatiti,  78,  115 
Ergmilm,  8U6,  428 
EUKANTIA,  211,  251 
Enperelta,  75,  115 
EMeria,  387,  428 
Eucope,  86,  116 
EucoPEPODA.  896,  428 
Eu(kndrium,  93,  116 
Eitglena.  28,  89 
Euglypha,  16,  88 

EULAMELLIUKANCHIA,  889,  866 
EUNEMATODA,   174,  182 

Eupagurvs,  411,  424 
Euphuusia,  406,  424 
Eupleetella.  74,  115 
EuKYALiUA,  569,  592 
Eurylepta,  189,  170 
Euri/fMiuropun,  483,  525 

ElIKYPTElUDiK,  432 

Enrypterus,  483 
EUHYSTOMEiK,  125,  126 
Euscorpius,  443,  458 
Eiispongia,  73,  115 
Evadne,  888,  423 

FacelUna,  315,  364 
i'Y^anrt,  177,  182 
FiLiuuANCiiiA,  389,  365 
Fiona.  811 
Fissurella,  305,  364 
Flagellata,  28,  39 
Flies,  519 

Floacularia,  189,  200 
Flustra,  261,  274 

FOKAMINIFEKA,  15,  38 

Forficula,  504,  526 
Formtea,  518,  527 
Fredericella,  260,  274 
Fungia,  114,  117 


GaUodea,  445,  458 
Oamasua,  453,  459 
Gammarua,  416,  424 
Gahteuopoua,  293,  864 
Gaateropteron,  318 
Gecareinm,  412,  424 
Gelaaimua,  412,  424 
OEOMETItlDiK.  517,  527 
Geophilua,  484,  525 
Gepiiykea,  287,  252 
Geryonia,  85,  ^16 
Gibboi'eUum,  447,  458 
Globigerina,  17,  38 
Glomeris,  483,  525 

GNATIIUUUELI.IDi«,  236,  251 

Gonactinia,  111,  117 
Gonodactylua,  409,  424 
GonoUptua,  448,  458 
GOKDIACEA,  178,  188 
Gordiua,  178,  188 
Gorgonia,  108,  117 
Grantia,  73,  115 
Grasshoppers.  504 
GUKOARINIDA,  24,  89 
Gromia,  16,  38 
Gryllotnlpii,  504,  526 
Gryllna,  504,  526 
«M«<?/r,  l:i«,  170 
Gymnol/Kmata,  261,  274 
Gymnosomata,  311,  364 
Gyrinita,  513,  527 
Gyrodactylua,  147,  170 

ILrmenteria,  236 
Ilalcampa,  118,  117 
Ilalecium,  87,  116 
Ilaliotia,  805,  364 
Ilalisarca.  74,  115 
Ifalobdtea,  510,  527 
Ilalorypris,  891,  423 
llalodrilm,  227 
IhtrpacUcHs,  396,  428 
Ilnrjmlm,  518,  527 
Ha  r vest-men .  448 
Hiirvt'st-mite,  453 
Harvest-spider,  447 
Ileliopoim,  109 
Ilelioaphffra,  19,  39 


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23  WIST  Ki)AIN  STRUT 

WHSTIR,N.Y.  14SS0 

(716)«7a^903 


648 


INDEX  OF  PROPER  NAMES. 


Heuozoa,  17,  89 
Helix,  316.  364 
Hemichouda,  596,  639 
Hemipteka,  510,  537 
Hermil  Crab,  411 
llemne,  307,  351 
lleterodera,  176,  182 
Heteuomeua,  537 
Jletero nereis,  316 
Heteuopoda,  309.  364 
Heteuotuicha,  39 

HEXACTINIiE,  113,  117 

Hexarthra,  194.  200 
Hippa,  411,  434 
HiuuoiNBA,  238,  251 
Hirudo,  236,  251 
Holopus,  541,  592 
Ilolothuria,  585,  593 
Holothuruidba,  584,  598 
holotkicha,  39 
Homarua,  413,  434 
HOMOPTERA,  510,  537 
HOPLONEMEUTINI,  167,  170 

Horseshoe  Crab,  488 
Hyalospongi/E.  74,  115 
Hydra,  58,  83,  116 
Hydrachna,  453.  459 
Hydraetinia,  58.  87,  116 
HYDUARIiE,  83,  11 G 

Hydkocorallin^,  89, 116 
IIydromedus^,  78,  116 
Hydrometra,  510,  527 
Hydrophilus,  513,  527 
Hymenopiera,  517,  527 
Hyocrinua,  542,  592 
Hypotricha,  39 

lantMna,  307.  364 
Ma,  401,  438 
Ichneumon,  538 
lehthydium,  196,  200 
Ichthyobdella,  228,  253 
Idotea,  415,  424 
Idyia,  125,  126 
Infusoria,  33,  39 
Insecta.  487,  526 
Im,  108,  117 
hopoda,  414,  424 


lulm,  483,  525 
Ixodes,  453,  459 

Janm,ZU,  864 

King-crab.  428 
Kolga,  589 

Labia,  526 

Lacinularia,  189,  200 
Lamellibranchia,  826 
Lampyria,  513,  527 
Larvacea.  636,  639 
Laura,  403,  433 
Leiolnmum,  448,  458 
Lepaa,  399.  433 
Lepidonottta,  313,  251 
Lepidoptera,  515,  537 
Lepiama,  503,  536 
Leptodiacua,  30,  39 
Leptodora,  418 
Leptogorgta,  108,  117 
Lbptomedus^,  85, 116 
Leptoplana,  139,  170 
Leptostraca,  404.  428 
Lemma,  397,  433 
Leucosolenia,  73,  115 
LibellHla,  506,  536 
Libinia,  413,  424 
Ligula,  153,  170 
Limacina,  314,  364 
Limapontia,  315,  364 
Zmao!,  316,  364 
Limnadia,  387,  423 
Limneea,  316,  364 
Limnetia,  387,  423 
Limulua,  427 
Lingula,  271,  274 
Liotheum,  509.  536 
Liriope,  85,  116 
Lithobius,  485,  525 
Lobster,  412 
Zoii\70.  359,  365 
Lophopiia,  261,  274 
Loxoaoma,  256,  274 
Lucernaria,  100. 116 
Lucifer,  411,  424 
ZwtWa,  553,  593 


INDEX  OF  PROPER  NAMES. 


649 


LUMBRICOMOKPHA,  251 

Lumbricus,  2^3.  251 
Lycosa,  452,  458 
LysiopeUilum,  483,  525 
Lysioaquilla,  409,  424 
Lytta,  513,  527 

Macrobdella,  236,  253 
Macrobiotus,  467 
MACnOLEPlDOPTEUA,  516,  527 
Machuba,  411,  424 
Madrepora,  114,  117 
Malacobdella,  167,  170 
Malacobdellina,  167,  170 
Malacodekmata,  114,  117 
Malacopoda.  475 
Malacostraca,  403,  423 
Mallophaga,  509 
Margelis,  87,  116 
May-fly,  505 
Melicerta,  189,  200 
Mellita,  581,  593 
Meloe,  513,  527 
Melolontha,  512,  527 
MelopJiagus,  520,  528 
Membranipora,  261,  274 
Mermis,  178.  183 
Mertensia,  124,  126 
Meaostoma,  135,  169 
Mesozoa,  63 
Metazoa,  41 
Metridium,  114,  117 
Microgaster,  518,  528 
Microgromia,  21,  39 

MiCHOLEPIDOPTERA,  516,  527 

Microstoma,  58, 135,  140,  169 
Miliola,  16,  38 
MUlepora,  89,  116 
Millipedes,  482 
Mites,  453 

Mnemiopsts,  124,  136 
Modiolaria,  339,  365 
JJfotna,  388,  423 
3/ojm,  583,  593 
Molgula,  628,  630 
MOLLUSCA,  276 
Molpadia,  593 
ifo/ia«,  28,  39 


Monoatomum,  147,  170 
MONOTUCAKDSA,  306,  364 
Mo  not  us,  134,  169 
MiiUertu.  585,  593 
vVwsca,  .')20,  5:i8 
Mussel,  339 
Mya,  339,  36? 
My  gale,  451,  458 
Myriapoda,  480,  525 
Myrmeleon,  514,  527 
i/^'J'M,  406,  424 
Mytilus,  339,  365 
Myxosporidia,  26,  39 
Myzostome^,  244,  252 
Myzoatomum,  244,  252 

Naidomorpha,  251 

Nais,  227,  251 
Narcomeduatp,  84,  116 
Natica,  307,  364 
Nautilus,  357,  365 
Nebalia,  405,  423 
Neerophorus,  512,  527 
Nemathelminthes,  172,  183 
Nematoda,  173,  182 
Nemertina,  162,  170 
Neocrinida,  551 
Neomenia,  286,  364 
Nephelis,  236,  251 
iVcrm,  212,  251 
Neritina,  305,  364 
Neuroptera,  514,  527 
Noctiluca,  30,  39 
Nodosarin,  17,  38 
Nothrua.  453,  459 
NotodelpJiys,  396,  428 
Notomcta,  510,  527 
Nucula,  339,  365 
NuniBUANCHiA,  315,  364 

06f?ijrt,  86,  116 
Obiaium,  444,  458 
Octaenemus,  637,  639 
OcTOPODA,  359,  365 
Octopus,  359,  365 
Oculina,  114 
Oeypoda,  412,  424 
Odonata,  506.  526 


650 


INDEX  OF  PROPER  NAMES. 


OLiGOoaaiTA,  218,  251 
OinmaatrepJiea,  359,  865 
Onchidium,  816,  364 
Oniacus,  414,  424 
Onychophora,  475 
Opalina,  35,  89 
Ophiactia,  566,  592 
Ophioderma.  561,  592 
Ophiolepis,  567,  592 
Ophiomyxa,  561,  592 
Ophiothrix,  564,  592 
Ophiura,  592 
Ophiurida,  569,  592 
Ophiuroidea,  561,  592 
Opilio,  448,  458 
Opisthobranchia,  810,  864 
Oraetis,  111,  117 
Orgyia,  516,  527 
Oribates.  453,  459 
Orthonectida,  65 
Orthoptera,  504,  526 

OSTRACODA,  891,  423 
Oslrea,  889,  365 
Oxyuris,  177,  182 
Oyster,  889 

Palmmon,  412 
Palcemonetea,  412,  424 

PAIiiEOCRINIDA,  551 

Pal^onemertini,  166,  170 
Palinurus,  421 
Paludicella,  261,  374 
Paludina,  307,  364 
PalytJioa,  112.  117 
Pandarua,  396,  423 
Panorpa,  514,  527 
Panorpata,  514,  527 
Papilio,  527 
Paratnoscium,  35,  39 
Patella,  805,  864 
Pauropoda,  481,  525 
Pauropua,  482,  525 
Pec^en,  339,  365 
Pedalion,  195,  200 
Pedata,  593 
Pedicellina,  256,  274 
Pediculus,  510,  527 
PspiPALPi,  446,  458 


Pelagia,  103.  117 
Pelecypoda,  326,  365 
Pelmatozoa,  551 
Pemphigua,  511,  627 
Penaua,  419,  424 
Penella,  397,  423 
Pennaria,  87,  116 
Pennatula,  109,  117 
Pentacrinua,  542,  592 
Pentamera,  527 

PENTASTOMIDiE,  461 

Pentaatomum,  461 
PerichcBta.  218,  251 
Peripatua,  474,  525 
Periplaneta,  504,  526 
Peritricha,  39 
Pcrto,  507,  526 
Peromedus^,  101,  116 
Perophora,  627,  639 
Petalosticha,  582,  593 
Phagocata,  136,  170 
Phalangida,  447,  458 
Phalangium,  448,  458 
Phascolion,  242,  252 
Phaacoloaoma,  242,  253 
PMlichthya,  897,  428 
Philodina,  189,  200 
Pfdceothripa,  510,  526 
P/ioia«,  339,  365 
Phoronid^,  247,  252 
Phoronia,  247,  252 
Plioxichilidium,  464 
Phryganea,  515,  527 
Phrynua,  446,  458 

PHYLACTOLiEMATA,  261,  274 

PJiyllirhoe,  315,  364 
Phyllopoda.  385,  423 
Phymanthua,  114,  117 
P/i^sa,  816,  864 
Physapoda,  509 
Phytoptua,  454,  458 
P/erw,  516,  527 
Pinnotherea,  412.  424 
Piacicola,  236,  252 
Plagioatoma,  133,  169 
Planaria,  136,  170 
Planoeem,  136,  170 
Planorbia,  316,  864 


INDEX  OF  PROPER  NAMES. 


661 


Platyhelminthes,  127,  169 
Platyonychm,  412,  424 
Plecopteka,  507,  526 
Pleurobrachia,  134,  126 
Pleurobi'anchma,  312 
Pleurobranchus,  313,  364 
Pleurophyllidia,  315,  364 
Pleurotomaria,  305,  364 
Pneumoderma,  314,  364 
Podura,  503,  535 
Porcellana,  413,  424 
Porcellio,  414,  424 
POLYCH^TA,  204,  251 
POLYCLADEA,  138,  170 

Polydesmus,  483,  525 
Polygordius,  211,  251 
Polyophthalmu8,  209,  251 
Polyphemus,  388,  423 
POLYPLACOPHORA,  288,  364 
POLYBTOMa*,  147,  170 
Polystomum,  147,  170 
PoLYZOA,  255,  274 
Pontobdella,  236,  252 
PORIPKRA,  69,  115 
Poroapora,  25 
Porpita,  91,  116 
Portuguese  Man-of-war,  92 
Priapulus,  243,  252 
Proctotinirtes,  518,  528 
Proneomenia,  285,  364 
ProrJiynchus,  135,  169 
Prosobranchia,  303,  364 
Prosopygia,  254 

ProT ACTINIA,  111.  117 

Proteolepas,  402,  433 
Protobranchia,  338,  365 
Protochordata,  596 
Protodrilua,  211 
Protozoa,  13,  38 
protrachkata,  474,  525 
Protula,  215 

PSEUDOLAMELLIBRANCHIA,  339,  365 
PSEUDOSCORPIONIDA,  443,  458 

PsociD.*:,  509 
Psolus,  584,  593 
Pterobranchia,  597,  689 
Pteromalus,  518,  528 
Pteropoda,  313,  364 


Pterotrachea,  309,  364 
Pterygota.  504,  526 
Pulex,  520,  528 

PULMONATA,  316,  364 

Pycnogonida,  463 
Pyralid.*:,  516,  527 
Pyrosoma,  631,  640 
Pyrosomid^,  631,  640 

Radiolaria,  18,  39 
Ranatra,  510,  527 
Razor-shell,  339 
Renilla,  108,  117 
Rhabditis,  176 
Rhabdocosla,  134,  169 
Rhabdopieura,  597,  639 
Rhegmatodes,  86,  116 
Rhizocephala,  402 
RJikocrinua,  542,  592 
Rhizopoda,  14,  38 
Rhizostomid^,  101 
Rhopalodina,  584,  593 
Rhopalonema,  85,  116 
Rhopalura,  66 

Rhynchobdellid^,  236,  251 
Rhynchonella,  272,  274 
Rhynchota,  510,  527 
Rotalia,  17,  38 
RoUfera,  189,  300 

Sabella,  21  !s,  251 
SaccuUna,  402,  423 
Sagitta,  186,  200 
Siilenia,  570,  593 
Salpa,  633,  640 
Saud-dollar,  581 
Saperda,  512,  527 
Sapphirina,  396,  423 
Sarcoptea,  453,  459 
Sarcosporidia,  27,  39 
Scalaria,  308,  364 
Scallop,  339 
Scalpellum,  400,  428 
Sc'APHopoDA,  322,  364 

SCHIZONEMERTINI,  166,  170 

ScHizopoDA,  406,  424 

SCLERODERMATA,   114,  117 

Scolopendra,  484,  525 


652 


INDEX  OF  PlWPEli  NAMES. 


Scolopendrella,  486,  525 
Scorpion,  441 
ScoupioNiDA,  441,  458 
Scrupocellaria,  261,  374 
Scutigera,  485,  525 
Scyllarus,  421 

SCYPHOMEDUS^,  97,  116 

8cytophoru8,  111,  117 
Sea  Auemoues,  114 
Seu- lilies,  541 
Sea-urcliius,  570 
Sedentaria,  212,  251 
Segestria,  452,  458 
Selandria,  519,  528 
Sepia,  359,  365 
Septibkanchia,  339,  365 
Sergestes,  411,  424 
Serpula,  213,  251 
Sertularia,  86,  116 
Ship-worm,  339 
Shrimp,  412 
SicUi,  388,  433 
Siphonodentalium,  822,  364 
SlPHONOPHOR^,  91,  116 

Sipunculacea,  341,  253 
Sipunculua,  343,  353 
Smella,  407,  434 
Sitaris,  513 
Slavina,  333 
Solarium.  308,  364 
SOT.ENOOASTRES,  385,  864 
SoLiFUQ^,  444,  458 
Solpuga,  445,  458 
Sow-bug,  414 
Spadella,  186,  300 
Spatangm,  583,  593 
SphcBroma,  415,  434 
SpJwerozoon,  19,  39 
.Sphyranura,  147, 170 
Spiculisponqi^,  74,  115 
Spider,  448 
Spirula,  369.  865 
Spotidylua,  835 
Sponges,  69 
Spongilla,  78,  115 
Sporozoa,  24,  89 
Spring-tails,  508 
Squilla,  409,  424 


Staphylinid^,  512,  587 
Starfish,  552 

STAUROMEDUSiE,  100,  116 

Stentor,  35,  39 
Sternaspis,  243 
Stomatopoda,  408,  434 
Stomolophus,  101,  117 
Stone-flies,  507 
Strombus,  307,  364 
Strongylocentrotus,  580,  593 
Strongylosoma,  484,  525 
Stylaater,  90,  116 
Styliola,  314,  364 
Stylochus,  141,  170 
Stylommatophora,  318,  864 
Stylonychia,  39 
SucTORiA,  35,  39 
Sun-animalcule,  17 
Syllis,  212,  251 
Symphyla,  486,  526 
Synapta,  633,  640 
Synccelidium,  136,  170 

Tabanus,  530,  538 
T«5Bnm,  153,  170 
Tanais,  414,  434 
Tanysitylum,  463 
Tardigrada,  466 
TeaWa,  113,  117 
Tectibranchia.  313,  364 
Tegenaria,  450,  458 
Telea,  517,  537 
Tentacclata,  134.  186 
Terebella,  313,  351 
Terebrantia,  537 
Terebratulina,  371,  874 
Teredo,  339,  365 
Termea,  508,  536 
Termites,  507 
Tessera,  100,  116 
TESTir\RDiNEs,  269,  374 
Tetrabranchia,  357,  865 
Tetranyclius,  454,  459 
Tetrapneumonbb.  458,  458 
Telrastemma.  167.  170 
Textularia,  17,  38 
Thalamma,  341,  268 
T/ialaaaianthtu,  114, 117 


INDEX  OF  PROPER  NAMES. 


653 


TJialassieolla,  18,  39 
Thaliacea,  633,  640 
Thauniatocrinus,  543,  593 
Thkcasomata,  314,  364 
ThelyphonuH,  446,  458 
Theridimi,  450,  458 
Thokacostkaca,  406,  424 
Thrip8,  510,  536 
Thyone,  587.  593 
Thysanopteua.  509,  536 
Thysanozoon,  139,  170 
Thysanura,  501.  526 
Ticks,  453 
Tinea,  516.  537 
Tracheata,  469 
Trachydermon,  389,  364 

TRACHYMEDUSiE,  85,  116 

Trematoda,  143,  170 
Tremoctojma,  359,  365 
TricenopJun'ua,  153,  170 
Triehaater,  569,  593 
Trichina,  176,  182 
Trichoeephalna,  177, 182 
Triehodectes,  509,  536 
Triclioplax,  63 
Trichoptera,  515,  537 
Tricladea,  136,  169 
Tristomum,  147,  170 
Trochosphmra,  194,  300 
Troclim,  305,  364 
Tromhidium,  453,  459 
lubipora,  108,  117 
Tuhularia,  89,  116 
TUBULARI^,  87,  116 
TUNICATA,  617 


TURRELLARIA,  130,  169 

Turbo,  305,  364 
Tyroglyphua,  453,  459 

Unio,  339,  365 
Urnatella,  256,  274 
Urocuorda,  617,  639 

Vaginula,  316,  364 
Vnmpyrella,  23,  39 
Vanesaa,  517,  527 
Fe^cWa,  92,  116 
Venus,  339,  365 
Venus'  girdle,  121 
Vermilia,  209 
Fe«j9a,  518 
Volcox,  29,  32,  39 
Vortex,  135,  169 
Voi-ticella.  33,  39 

Waldheimia,  271,  274 
Walking  Stick.  504 
Wasps,  518 

Wheel-animalcule,  189 
White  ants.  507 
Wood  louse,  414 

XiPHOSURA,  437 

Toldia,  839,  365 

ZOANTHE^,  113,  117 
Zoanthus,  113,  117 

ZOOXANTHELL^,  20 

Zoroaster,  553,  593 


INDEX  OF  SUBJECTS. 


Accessory  intestine,  Cephalocorda, 
605;  Echinoidea,  579;  Gephyrea, 
239;  PolycLseta,  207 

Aconous  eyes,  473 

Actinotrocha,  249 

adrectal  gland,  305 

adliesive  cells,  123,  131 

alar  muscles,  470 

albumiuiparous  gland,  312 

alternation  of  generations,  60;  Cesto- 
da,  158;  Hydromedusje,  96;  Nema- 
toda,  176;  Scypliouiedusae,  108; 
Trenialoda,  148;  Urocliorda,  683 

aniitosis,  9 

anietabolic  insects,  499 

amoeboid  motion,  15 
ampbiaster,  11 
ampbidiscs,  76 
ampulla,  537 
antennary  gland,  383 
apical  plate,  213,  607 
arcbenteron,  54 
arcbicerebrum,  379 
Aristotle's  lantern,  578 
artbrobrancbia,  410 
ascidiozooid,  682 
Ascon,  70 
aster,  7 

atrium,  596;  Cepbalochorda,  612;  En- 
teropueusta,    601;     Pterobranchia, 
599,  600;  Urocborda,  622 
atrium  (genital),  184 
auriculae,  573 
Auricularia,  590 
avicularia,  262 
axial  sinus,  588 

Basement-membrane,  127 
biogenetic  law,  143 


Bipinnaria,  559 

bivium.  572,  584 

blastocoj],  62 

blastopore,  54 

blustuia,  52 

blood-vascular  system,  Arachnida,  437 
Cepbalocborda,  611;  Cephalopwla, 
346;  Cbaetopoda,  206,  220;  Crusta 
cea,  376;  Enteropneusta,  603;  Gepb 
yrea,  238;  Hirudinea,  230;  Molluscu 
278;  Nemertinu,  165;  Pboronidse 
247;  Tracheata,  470;  Urocborda 
621;  Xipbosura,  429 

Bojanus.  organ  of,  837 

Brachiolaria,  559 

brancbial  beart,  347 

brancbial  skeleton,  Cepbalocborda, 
613;  Enteropneusta,  609 

brown  body,  267 

brown  canal,  616 

bursa  copulatrix.  Nematoda,  174,  182; 
Tracbeata.  497;  Turbellaria,  136, 
138 

byssus  gland,  329 

Calamistrum,  449 
calcar,  193 

calciferous  glands,  220 
calyptoblastic,  86 
caryolympb,  6 
caryoplasm,  5 
cell,  4, 

cell-division,  9 
cellulose,  30,  619 
cenogenetic,  143 
central  capsule,  19 
centrolecilbal,  53 
centroaome.  7,  51 
cepbulization,  369. 

655 


656 


INDEX  OF  SUBJECra. 


cfialu,  315 

Cercaria,  150 

cerci,  489 

chumbercd  organ,  546 

chela,  373 

cheliceias  429,  435 

chiitHtuucurism,  296 

ohilaria,  430 

chloragoguc'-cells,  219.  238 

cLloropliyll,  Fliigelluta,  29,    30;  Hy- 

drariae,  83;  lufusoiia,  35;  Porifeia, 

73 
cburdotoual  organ,  495 
chromatin,  6 
chroiuosonie,  11 
chrysalis,  600 
cilia,  33 
cilia-plates,  121 
cirrus,  146,  155 
cirri,  Cephalochorda,  609;  Criuoidea, 

542;  Myzostome«B,  244;  Polychaeta, 

204,  205 
clavulse,  674 
clitellum,  219,  238 
cloaca,  Nematoda,  175,  179;  Rotifera, 

192;  Urochorda,  631 
cnidocil,  77 
cnidoblast,  77 
coelenteron,  77 
ccBlom,  57 
coeiienchynie,  108 
coenosarc,  79 
Canurtis.  158 
colony-formation,  5, 8;  Anihozoa,  108, 

111,  112,  114;Flagellata,29;  Hydro- 

meduscB,  78,  85,  87,   91;    Polyzoa, 

255;    Rhizopoda,    21;     Urochorda, 

628 
columella,  90,  107 
complemental  males,  401 
conjugation,  24,  25,  31,  87 
contractile  vacuole,  15 
coral! um,  89 
cormus,  41 
costae,  107 
coxal   glands,  Arachnida,  441;    Xi- 

phosura,  432 
cribellum,  449 


crural  glands,  Insecta,  502;   Myriop- 

oila,  485.  487;  Protracheuta,  479. 
crystulliue  style,  833 
cteiiidium,  278 
Cuvieriau  organs,  588 
cyathozooid,  632 
CypJionautes,  264 
Cysticercoid,  158 
CyslicercMS,  158 
cytode, 8 
cytolympb,  4 
cytoplasm,  4 

Daclylozoid,  90 

delaminutiou,  55 

Desor's  larva,  167 

deutovuni.  456 

development,  Acantbocepbala,  182 
Acarina,  456;  Asteroidea,  559 
Brachiopoda,  272;  Coplialopoda, 
36a ;  Cestoda,  157;  Crinoidea,  551 
Crustacea,  417;  Ecbinoidea,  583 
Euteropneusta,  605;  Gasteropoda, 
819;  Gephyrea,  242;  Hirudinea 
237;  Holothuroidea,  590;  Hydro 
medusoe,  92;  lusecta,  521;  Nema 
toda,  176;  Nemertina,  167;  OH 
gochseta,  225;  Ophiuroidea.  570 
Pelecypoda,  339;  PentaslomidoB; 
463;  Phorouidae,  249;  Polycbaeta; 
213;  Polyzoa,  263;  Porifera,  74 
Pycnogouida,  466;  Scapbopoda 
824;  Scypbomedusae.  103;  Trema 
toda,  148;  Turbellaria,  140;  Uro 
chorda,  624;  Xiphosura,  432 

digestive  gland,  Arachnida,  437; 
Brachiopoda,  271:  Crustacea,  378; 
MoUusca,  280;  Rutit'era,  192; 
Xiphosura,  430 

digestive  system,  Ampbineura,  280; 
Arachnida.  437;  Brachiopoda,  270; 
Cephalocorda,  612;  Cephalopoda, 
348;  ChflBtogiiatha,  187;  Chajtopo- 
da,  206,  220;  Crustacea,  377;  Di- 
nophilus,  198;  Ecbiuodera,  185; 
Echiiioderma,  5o9:  Euteropneusta, 
605;  Gasteropoda,  300;  Gastrotricba, 
196;    Gephyrea.    238;    Hirudinea, 


INDEX  OF  SUBJECTS. 


657 


ita, 
la, 
en, 


231;  Mollusca,  379;  Myzosloiueui, 
244;  Neinatoda,  174;  Neinertina, 
163;  Pelocypoda,  333;  Peulastomi- 
daj,  461;  Fhoronidse.  347;  Polyzoa, 
255;  Pycuogonida,  465;  Kolifera, 
191;  Scapliupuda,  333;  Tardigmda, 
467;  Tracheata,  471;  Tremutmla. 
144;  Turbcllaria.  133,  135,  136.  138; 
Urocborda,  633;  Xipliosura,  430 

dimorphism,  sexual,  193,    199,    341, 
395,  496 

dimorphism,  seasonal,  501. 

dissepiment,  107,  187,  202,  270 

dissogony,  123 

divLsion  of  labor,  85,  87,  91    (see  also 
polymorphism) 

doeoglossate  dentition,  306 

dorsal  organ,  546 

dorsal  pore,  219 

Echinococcus,  158 
ectocyst,  255 
ectoderm,  54 
ectoplasm,  3 
elseoblast,  633 
elytra,  490 
embole,  54 
eucystment,  22,  36 
endocyst,  255 
eudoderm,  54 
endopbragmal  system,  875 
endoplasm,  3 
endopodite,  373 
eudosteruite,  429,  457 
endostyle,   Cephalocorda,   614,    Uro- 
cborda, 622 
enterocoel,  57 
ephippium,  391 
Ephyra,  103 
eplbole.  54 
epipleural  folds,  609 
epipodite,  373 
epipodium,  291 
epistome,  247,  260.  597 
epitbelio-miisculiir  cells,  80 
Eh-ichthua,  432 
euconous  eyes,  473 
excretory    system,    Acantbocepbala, 


181;  Amphiueura,  387,  393;  Aracb- 
nida,  437;  Brachiopoda.  373;  (-'epbu- 
locorda,  615;  Ceplialopoda,  353; 
Cestoda,  155;  Clitelopoda,  310,  'i'^i'i; 
Crustacea,  38;{;  Dinajthilitx,  199; 
Ecbinodera,  185;  Euteropiieusia, 
605;  Gasteropoda,  30'2;  Gnstciotri- 
cha,  196;  Gephyrea.  339;  Hinidiuea, 
235;  Alollusca,  283;  Myzosloineuj, 
245 ;  Nematoda,  175;  Nemertiiia, 
164;  Pelecypoda,  337;  Phorouiiluj, 
249;  Platyhelmiutlies.  139;  Poly- 
zoa, 357,  260;  Pterobniiicliia,  599, 
600;  Kotifera,  193;  S(apbop<)(ia.334; 
Tracheata,  474;  Tri'inaloda,  145; 
Turbcllaria,  135,  138;  Urocborda, 
633;  Xipliosura,  433 

exopodite,  373 

eyes— Ampbineura,  392  ;  Arachnida, 
438;  Asteroidea,  559;  Cepbalochor- 
da,  615;  Cephalopoda,  351 ;  Climto- 
gnatha,  188;  Cbaetopoda,  208;  Crus- 
tacea, 380;  Gasteropoda,  301,  318; 
Uirudinea,  234;  Hydromedusa?,  83; 
Nemertina,  164;  Pelecypoda,  335  ; 
Pycnogonida,  465;  Rolifera,  192; 
ScyphomedusjB,  99;  Tracheata,  472; 
Turbcllaria,  131;  Urocborda,  625; 
Xiphosura,  431 

Fascioles,  574 
fat-body,  492 
flagellum,  28 
tlame-cell,  129 
follicle-cells,  46 
fossa  rhomboidalis,  615 
funiculus,  259 

Qasterozooid,  89 
gastrula,  53 
gemmation,  22 
gcmmules,  75 
genital  burste,  561 
germ -cell,  44 
germ-layer,  54 
OlocJiidium,  339 
Goette's  larva,  141 
gonopolyp,  85 


658 


INDEX  OF  SUBJECTS. 


gonotheca,  86 
green-gland,  888 
gymnoblastic,  87 

HeemocyaQin,  278,  877,  429 

haemoglobin,  877,  588 

Iioemolynipb,  206 

bal teres,  520 

bead-kidnuy,  214,  222 

beart— Ampbiueura,  289;  Arachnida, 
487;  Cepbulopoda,  846;  Crustacea, 
876;  Gasteropoda,  298;  Pelecypoda, 
882  ;  Pycnogouida,  465;  Tracbeata, 
470;  Urochorda,  621;  Xiphosura, 
429 

Heetocotylus,  856 

hemimetiibolic  insects,  500 

bermapbroditism,  44 

heterogony,  60.  148,  498 

bistoiysis,  456 

bolometabolic  insects,  600 

book-gland,  462 

hydranth.  79 

bydrocnulus,  79 

bydrocoel,  535 

bydrorbizu,  79 

bydrotheca,  79 

bypermetamorpbosis,  513 

hypodermis,  174 

bypopbysis  cerebri,  615,  623 

bypostorae,  79 

Imago,  500 
immigintion,  55 
individuality,  41 
ink-bag,  849 

interteutacular  organ,  260 
invagination,  54 

Karyokinesis,  9 
Keber's  organ,  337 

Lacunar  system,  538 

languets,  622 

lateral -line  organs,  210,  222 

Laurer's  canal,  146 

lemnisci,  181 

Leucon,  71 


liniu,  6 
liver,  614 

lopbopbore,  247,  254 
luug-bookH,  486,  457 

Madreporiform  tubercle,  586 

madreporite,  536 

malpigbian  tubules  —  Ampbipoda, 
417;  Aracbnida,  487;  Tracbeata, 
474 

mantle,  268.  276,  621 

manubrium,  81 

mastax,  191 

Medusa,  77,  80 ;  Craspedote,  81 ; 
Gymuopbtbalmatous,  82 ;  Ocel- 
late,  82,  89;  Vesiculate,  82 

megaloestbetes,  292 

Megalopo,  422 

mestudoderni,  68,  182 

mesenterial  filaments,  99.  105 

mesentery,  57,  104,  179, 187,  206,  270 

mesoblasts,  57,  214,  225,  287 

mesoderm,  56 

mesogloja,  68 

mesopodium,  296 

mesotborax,  488 

metagenesis,  60 

metamerc,  41 

metamerism,  48 

metamorpbosis  of  insects,  499 

Metanaupliua,  418 

metapodium,  296 

metatborax,  488 

Metazoea,  421 

micrtestbeles,  292 

micronucleus,  35,  %n 

microsomes,  4 

mitosis,  9 

Morren's  gland.  220 

MUller's  larva,  141 

muscular  system  —  Acanthocepbala, 
180;  Ampbineura.  286,  289;  Antbo- 
zoa,  166;  Bracbiopoda,  270;  Cepba- 
locborda,  610 ;  Cepbalopoda,  348; 
Cestoda,  154;  ChaBtognatba,  187; 
Chaetopoda,  205,  219;  Crustacea, 
375;  Dinophilus,  198;  Ecbinodera, 
185;  Ecbiuoderma,  585;  Gasteropo- 


n 
n 
ni 


ni 
ni 
ni 
^l 

0( 

ol 


INDEX  OF  SUBJECTS. 


659 


da,  208;  Gastrotricba,  196;  Hirudi- 
neii,  220;  lusecta,  492;  Neinatoda, 
174.  178;  Pelecypoda.  882;  Tra- 
cheiita,  460 

myoea'l,  610 

myophuues,  35 

Naupliua,  417 

uectoctilyx,  01 

Needbuin's  pouch,  855 

nciimtocyst,  77 

nupbi-idiii  (see  Excretory  System) 

nephroblasts,  226,  287 

nervous  system  —  Acuutlioccphala, 
181;  Ampbiueura,  287.200;  Aruch- 
uida,  487 ;  Biaciiiopodti,  271  ;  Ce- 
pbalocborda.  614 ;  Cepbalopoda, 
850  ;  Cestoda,  155  ;  Cbtetoguatba, 
187  ;  Chuetopoda,  208,  221 ;  Crusta- 
cea. 378;  Cteiiopbom,  124;  Dinophi- 
his,  109;  Ecbinodera,  185;  Ecbino- 
derma,  539  ;  Enteropneusta,  605  ; 
Gast'-opf  la,  300;  Gastrotricba,  197; 
Gepbyroa,  239;  Hhudiuea.  232;  lly- 
dromedvissB,  80;  Mollusca,  281;  My- 
zostome8B.244;  Ncmatoda,  175.  179; 
Neniertiiia,  163  ;  Pelecypoda,  334; 
Pentastoniido),  461;  Pborouidoe,247; 
Platybelminthes,  128;  Polyzoa,  257; 
Porifera,  73 ;  Pterobrancbia,  599, 
600  ;  Pycnogonida,  465  ;  Uotifera, 
192  ;  Scapbopoda,  323  ;  Tracbeata, 
471  ;  Trematoda,  144;  Tuibellaria, 
131,  132,  137;  Urocborda,  623;  Xi- 
pbosura,  430 

neuroblasts,  226,  237 

nidameutal  gland,  312,  355 

notochord  —  Cepbalocborda,  612  ; 
Enteropneusta,  603;  Pterobrancbia, 
599,  600;  Urocborda,  625 

nuclein,  3 

nucleolus,  6 

nucleus,  5 

nympb,  456 

Odontoblasts,  280 

olfactory  organ — Cephalopoda,  853; 
Cbsetognatha.  188  ;  Mollusk,  282  ; 


Scypbomeduso;,    100 ;    Tracbeata, 
472;  Xipbosura,  432 

omniatidium  —  Cbtclopoda,  209 ; 
Crustacea.  381;  Insecta,  472;  Pele- 
cypoda, 387;  Xipbosura,  431 

(Mucia,  263 

ootyp,  146 

operculum— Gasteropoda,  296;  Poly- 
zoa, 262;  Scorpiouida,  442;  Xipbo- 
sura, 420 

organ,  41 

organs  of  Bojanus,  837 ;  of  Cuvier, 
588;  of  Stewart,  576 

ortboueurisu),  810 

osculum,  69 

ospbradium,  288 

otocysts  —  Cba'topoda,  209  ;  Crusta- 
cea, 383  ;  Ci,  ii  >pbora,  122  ;  Holo- 
thuroidea,  589  ,  HydromedusoB,  82, 
84,  85,  86;  Moliusca,  283  ;  Scypbo- 
meduso- 9;  Tuibellaria,  131,  182, 
134;  Urocborda,  625 

ovary,  44 

ovicell,  268 

yvum,  44  ;  Fertilization  of,  49  ;  Ma- 
turation of,  46;  Segmentation  of 
51 

Psedogeuesis,  60,  499 

pali,  107 

palpi,  205 

parapodia,  204,  818 

paratrocb,  213 

pareucbynia.  12)3 

Parencbyuiella,  55 

parthenogenesis,  60,  498 

paxillae,  553 

pectines,  442 

pedicellariae,  574 

pedipalps,  435 

Pentactcea,  591 

pereiopod,  410 

pericanliul  glands.  298,  887,  345 

pericardium,  278,  437 

yierisaro,  79 

peritoneal  cells,  205 

phaospbero,  439 

phosphorescence,  —  Crustacea,    382; 


11, 


660 


INDEX  OF  SUBJECTS. 


Cystotiugellata,   31;    Insecta,    493; 

Urochorda,  633 
phragiuocoue,  360 
Phyllosoma,  431 
PilhUum,  168 
piuiiules,  543 
plasomi',  48 
plastiu,  3 
pleopod,  410 
pleurobraucbia,  410 
Pluleus,  570 
pueuniatophore,  91 
podobraucliia,  410 
polar  bodies,  46,  49 
Poliau  vesicle,  536 
polyp,  76,  78 
polypide,  355 
polymorphism,  —  Alcyonariae,     109; 

Insecta,    497,    500;     Polyzoa,   363 

(see  also  division  of  labor) 
polyspermy,  50 
portal  system,  613 
proboscis,  —  Acautbocepbala,       180; 

Gasteropoda,     300;     Myzostomeaj, 

344;  Nemertina,  163 
proglottid,  154 
proOstracou.  360 
propodiiim,  396 
prosopyle,  71 
prostoniiuni,  318 
prothorax,  488 
protoplasm.  3 
protopodite,  573 
prototrocb,  313 
Frotozoen,  419 
pseiidotiluriii,  3fS 
psc'udonavicella,  35 
pseudopodium,  14 
ptenoglossate  dentition,  308 
pupa,  500 

liacbiglossate  dentition,  307 

rndiila,  379 

liedia,  149 

regeneration,  59 

reproduction, — Flagellata,  31;  Infuso- 
ria, 36;  Metazou,  43;  Myxosporidia, 
27;  Rbizopoda,  30;  Sporozoa,  35 


reproduction, — by  budding,  33,  58, 
71,  83,  96,  114,  315,  356,  366,  637; 
by  conjugation,  34,  35,  31,  37;  by 
division.  31,  36,  58,  114,  337;  by 
spore  -  formation,  33,  35,  81,  36; 
sexual,  44 

reproductive  system, — Acautbocepb- 
ala, 181;  Ampbineura,  387;  An- 
tbozoa,  105;  Aracbnida,  441;  Bracb- 
iopoda,  373;  Cephalochorda,  617; 
Cephalopoda,  354;  Cestoda,  155; 
Clia3toguatha,  188;  Cbselopoda, 
311,  333;  Crustacea,  884;  Cteno- 
pliora,  133;  Dinophilm,  199;  Eclii- 
nodera,  185;  Echiiiodernia,  540; 
Gasteropoda,  303,  3o.),  311,  318; 
Gastrotricba,  197;  Gephyrca,  3i0; 
Hirudinea,  335;  Hydromedusa',  83, 
85,  86;  Myzostomeae,  346;  Nema- 
toda,  175,  179;  ^tnicrtina,  166; 
Pelecypoda,  337;  Peniaslouiidie, 
463;  Platybelmintbes,  139;  Poly- 
zoa, 357,  360;  PterobnincbiM,  600; 
Pycnogonida,  466;  Rotifcra,  193; 
Scypbomedusse,  98;  Tracbeata,  474; 
Trematoda,  146;  Turbellaria,  133, 
134,  135,  187,  139;  Urochorda,  633; 
Xipbosura,  433 

repugnatorial  glands,  483 

respiratory  system. — Annelida,  304; 
Aracbnida,  436;  Asteroidea,  554; 
Cephalochorda,  613;  Cephalopoda, 
343;  Crustacea,  375;  Ecbinoidea, 
576;  Enteropneusta,  601;  Gaster- 
opoda, 397,  317;  Mollusca,  378; 
Pelecypoda,  339;  Pterobrauchia, 
599,  600;  Trncheala,  470;  Uro- 
chorda, 633;  Xipbosura,  439 

respiratory  trees,  340,  588 

Rhabditis,  131 

rhipidoglossate  dentition,  306 

rostellum,  154 

Sacculi,  548 

salivary  ghmds,  380,  498 

scaphognathite,  410 

schizocoel,  57 

scolex,  158 


INDEX  OF  SUBJECTS. 


661 


Scyplioatoma,  103 
seasonal  dimorphism,  501 
seiiiiles,  574 
seplii,  107 
suUL',  204,  218 
seta-sacs,  204 

sexual  dimorphism,  193,  199,  241, 
395,  496 

shell,— Aiuphiiieura,  J89;  Brachiop- 
oda,  269;  Cephalopoda,  343,  357, 
360;  Gasteropoda,  303.  316;  Pele- 
cypoda,  337;  Scaphopoda,  332 

shell-gland,  383 

siphon,  304.  337,  493,  579 

siphouoglyplie,  106 

sipuncle,  358 

skeleton,— Cephalochorda,  613;  Ente- 

ropiieusta,  609 
somatic  cells,  44 
somatic  mesoderm,  206 
spermatid,  48 
sperniato(;jte,  48 
spermatogenesis,  48 
spermatogone,  48 
spermatophore,  355 
spermatozoa,  44,  47 
sphairidia,  574 
spinning  glands,  449 
splanchnic  mesoderm,  206 
sphi'.ichnoccel,  610 
spongiolin,  73 
sporocyst,  149 
statobliist,  261 
Sterrula,  55 
Stewart,  organs  of,  576 
stigma,  29 

stigmata,- Arachnida,      436;       Tra- 

clieata,  470;  Urochorda,  623 
stomal  oda'um.  105 
stomodffium,  213 


stone-canal,  536 
strobila,  104,  154 
subneiiral  gland,  623 
Sycon,  71 
symbiosis,  30,  83 
syncerebrnm,  379 

Ticnioglossate  dentition,  307 

tapetum  hicidum,  336,  440 

telolecithal,  53 

lelson,  369 

testis,  44 

thorax,  488 

Tiedemann's  vesicles,  557 

tissue,  41 

Toi'iiaria,  606 

toxiglossaie  dentition,  308 

trachea),— Arachnida,   436;    Isopoda, 

414;  Trachcala,  470 
tracheal  branchia},  491 
trichocyst,  35 
trivium,  573,  584 
Trochophore,  313 
trophopolyp,  85,  91 
tube-feet,  537 
tympanal  organ,  496 
typhlosole,  230,  623 

Veliger,  320 
velum,  81,  320,  612 
ventral  plate,  326 
vibracula.  263 
vitellarium,  130.  155,  193 

Water-vascular  system,  535 
wax-glands,  490 
wings,  489.  523 

Zoea,  420 
zcHBcium,  355 
zooxanthella),  20 


}, 


